Common on many plants and easily recognized, powdery mildew is a fungal disease found throughout the United States. It is caused by a variety of closely related fungal species, each with a limited host range. (The fungi attacking your roses are unlikely to spread to your lilacs). Low soil moisture combined with high humidity levels at the plant surface favors this disease.

Symptoms usually appear later in the growing season on outdoor plants. Powdery mildew starts on young leaves as raised blister-like areas that cause leaves to curl, exposing the lower leaf surface. Infected leaves become covered with a white to gray powdery growth, usually on the upper surface; unopened flower buds may be white with mildew and may never open. Leaves of severely infected plants turn brown and drop. The disease prefers young, succulent growth; mature leaves are usually not affected.

Fungal spores overwinter inside leaf buds and other plant debris. Wind, water and insects transmit the spores to other nearby plants. Zucchini, beans, cucumbers, squash, pumpkins, tomatoes, roses and zinnia are especially susceptible.

Contents

Treatment

  1. Plant resistant cultivars in sunny locations whenever possible.
  2. Prune or stake plants to improve air circulation. Make sure to disinfect your pruning tools (one part bleach to 4 parts water) after each cut.
  3. Remove diseased foliage from the plant and clean up fallen debris on the ground.
  4. Use a thick layer of mulch or organic compost to cover the soil after you have raked and cleaned it well. Mulch will prevent the disease spores from splashing back up onto the leaves.
  5. Milk sprays, made with 40% milk and 60% water, are an effective home remedy for use on a wide range of plants. For best results, spray plant leaves as a preventative measure every 10-14 days.
  6. Wash foliage occasionally to disrupt the daily spore-releasing cycle. Neem oil and PM Wash, used on a 7 day schedule, will prevent fungal attack on plants grown indoors.
  7. Water in the morning, so plants have a chance to dry during the day. Drip irrigation and soaker hoses will help keep the foliage dry.
  8. Use a slow-release, organic fertilizer on crops and avoid excess nitrogen. Soft, leafy, new growth is most susceptible.
  9. Destroy all plant debris after harvest (see Fall Garden Cleanup). Do NOT compost.

If disease symptoms are observed, treat plants with one of the following approved organic fungicides:

  • Apply sulfur or copper-based fungicides to prevent infection of susceptible plants. For best results, apply early or at first sign of disease. Spray all plant parts thoroughly and repeat at 7-10 day intervals up to the day of harvest.
  • Green Cure Fungicide contains a patented formula of potassium bicarbonate — commonly used in food products — that kills many plant diseases on contact and provides up to 2 weeks of residual protection. At first sign of disease, mix 1-2 Tbsp/ gallon of water and apply to all exposed surfaces of the plant. Monterey® BI-CARB is a similar product containing micro-encapsulated potassium bicarbonate as the active ingredient. Mix 4 tsps in 2 gallons of water to thoroughly cover foliage.
  • Effectively treat fungal diseases with SERENADE Garden. This broad spectrum bio-fungicide uses a patented strain of Bacillus subtilis that is approved for organic gardening. Best of all, it’s safe to use — you can treat and pick crops the same day!
  • SNS 244 and Zero Tolerance Herbal Fungicide are made from 100% pure, food-grade ingredients that work fast to kill existing plant diseases and prevent new fungal problems from starting.
  • Indoor growers may want to consider using a Sulfur Burner/ Vaporizer which turns sulfur prills into a fine dust and changes the pH of leaf surfaces. Fungal spores and mold can’t get established on this plant coating. As an added benefit, studies have shown that this dust will eliminate spider mites.

Growing your own veggies and herbs is always a challenge, but nothing gets me more frustrated than when I have to battle powdery mildew.

It’s one of the most common diseases that will afflict your plants, especially if you’ve been gardening for a while. It’s caused by many different types of fungus that cause white, powder-like spots to appear on the leaves and stems of your plants.

If you leave it unchecked, it can quickly affect the health of your plants, reducing their vegetative growth and inhibiting their ability to fruit or flower.

Fortunately, you can treat it in a variety of ways, which I’ll cover below.

First: What Is Powdery Mildew?

Powdery mildew completely covering this leaf. It will spread quickly to the rest of the plant if left unchecked. Photo by Pollinator

When people refer to “powdery mildew”, they could mean many different types of fungus-related plant diseases, However, the one that is suspected to have caused the rest of the other fungus’ to develop is Erysiphe cichoracearum, a fungus that primarily attacks squash plants.

Regardless of the specific type of fungus that afflicts your plants, they all act in a similar way. The fungus will spread over the vegetation of your plants and prevent photosynthesis and the plant’s ability to utilize nutrients. Without catching it early, the damage may be too far along to stop, and you will have to remove the plant from your garden completely.

How To Identify Powdery Mildew

The most obvious sign of this disease is the distinct powdery, white dust that covers a plant’s leaves. These start out small, but grow in size as the disease progresses.

The difference between downy mildew and powdery mildew. source

It can be easy to confuse powdery mildew with a similar disease, downy mildew. To tell which type of mildew you have, look at where it appears on the plant. Downy mildew only grows on the underside of plant leaves, whereas powdery mildew will grow anywhere. Downy mildew also lacks the powder-like appearance:

Symptoms of Powdery Mildew

As the disease progresses, the small white spots begin to form a root-like structure that enters the plant’s leaves and saps them of nutrition. Leaves will become starved and begin to turn yellow.

If the disease continues to progress, leaves will turn brown and die. This is a bad enough problem on its own, but the dying leaves also open your plant up to sun damage, a malformation of buds and fruit, as well as potentially failing to fruit altogether.

How To Prevent and Treat Powdery Mildew

Complete prevention of powdery mildew outbreaks can be difficult. The spores of the fungus are carried through the air on gusts of wind. The spores then fall on plant surfaces and begin to reproduce if the conditions are right. In moist conditions, powdery mildew may not be a problem. However, other types of plant diseases can grow in damp conditions.

The best ways to prevent powdery mildew are:

  • Choose plants that are resistant to powdery mildew
  • Avoid planting vulnerable varieties in the shade
  • Manage aphid problems, as they can carry the spores into your garden
  • Provide moisture to leaves on a regular basis
  • Remove dried or diseased plant matter immediately upon seeing it
  • Use a variety of home or professional treatments if your plants have a serious mildew problem

Best Products to Treat Powdery Mildew

A variety of commercial products are available to cure powdery mildew. Many of these are fungicides that contain copper to kill off the powdery mildew spores.

However, a number of other treatments can provide good results at lower cost, such as:

Common baking soda that you keep in your kitchen for cooking and baking can be used to prevent the spread of powdery mildew in your garden. Just dilute one tablespoon of baking soda in one gallon of water.

Add 1/3 teaspoon of dish-washing liquid to help the solution stick to the leaf surfaces of the plant. Do not save leftover solution. Make a new solution each time you provide treatment for the plants.

Potassium bicarbonate is a powdery compound that has a number of uses in food processing, in medicinal products and for wine-making. This compound can also be used in solution as a treatment for powdery mildew problems on garden plants.

The advantage of using potassium bicarbonate is that the compound is effective against powdery mildew that is already established, instead of a preventative measure.

Common mouthwash that you have in your medicine cabinet or on your sink can be used to treat powdery mildew in your garden. The mouthwash should be ethanol-based and should be mixed one part mouthwash to three parts water.

Because mouthwash is formulated to kill germs, it can be used as a powdery mildew spray to eliminate the spores that will continue to reproduce and damage your plants.

Common household vinegar can also be diluted and used a powdery mildew treatment. Mix four tablespoons of vinegar in one gallon of water and spray onto the plants every three days. The solution can be used safely for gardens with edible fruits & vegetables.

However, vinegar is an acidic substance and repeated use can negatively impact the condition of your plants.

Sulfur and sulfur spray have been used for many years to prevent and eliminate molds on plants. It can be used to treat powdery mildew and a variety of other plant diseases. It can be found at your local garden center or plant nursery.

Some products include both sulfur and lime, which is thought to be even more effective against powdery mildew. However, these compounds can burn delicate plant tissue, so use them only as directed and space applications a sufficient amount of time apart to avoid harming the plants.

Milk has been recommended for powdery mildew for generations, but only now is the science behind it being investigated. A study found that a 10% solution of milk was as effective against the mold as other methods of treatment. Milk is an inexpensive and completely organic way to fight this plant disease and can be used safely in any planting area.

Powdery mildew spores require a hot, dry environment in order for the spores to spread and proliferate. When conditions are moist, the spores cannot multiply. Keeping your garden plants slightly moistened will help to prevent the spread of powdery mildew spores that are carried on the wind.

**Note: Many other plant diseases thrive in a damp environment, so using water is not a long-term solution.

Neem oil is an extract made from the fruit and seeds of the neem tree, which is native to India. It has been used as an insecticide and anti-fungal for thousands of years, and it is still useful today as an organic compound to eliminate garden pests.

Neem oil is used to remove powdery mildew by reducing the ability of the spores to reproduce. It’s often recommended as a garden spray against plant insects, but it is also effective as a powdery mildew spray.

However, it may be more effective as a preventative than as a treatment when the problem has already occurred.

Although powdery mildew can be a stubborn and frustrating problem, good gardening practices can reduce the likelihood of developing this troublesome plant disease.

Try a few of the treatments above to see which works best for your garden.

The Green Thumbs Behind This Article:
Kevin Espiritu
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How to Manage Pests

Pests in Gardens and Landscapes

Powdery Mildew on Vegetables

Revised 11/08

In this Guideline:



  • Identification and damage
  • Life cycle
  • Management
  • About Pest Notes
  • Publication
  • Glossary

Powdery mildew on melon leaves.

Sugar pea foliage damaged by powdery mildew, Erysiphe polygoni.

Powdery mildew causes irregular yellow blotches on tomato leaves.

Brownish spots on pea pod from powdery mildew infection.

Powdery mildew is a common disease on many types of plants. There are many different species of powdery mildew fungi (e.g., Erysiphe spp., Sphaerotheca spp.) and each species only attacks specific plants. A wide variety of vegetable crops are affected by powdery mildews, including artichoke, beans, beets, carrot, cucumber, eggplant, lettuce, melons, parsnips, peas, peppers, pumpkins, radicchio, radishes, squash, tomatillo, tomatoes, and turnips. Powdery mildews generally do not require moist conditions to establish and grow, and normally do well under warm conditions; thus they are more prevalent than many other leaf-infecting diseases under California’s dry summer conditions.

IDENTIFICATION AND DAMAGE

Powdery mildew first appears as white, powdery spots that may form on both surfaces of leaves, on shoots, and sometimes on flowers and fruit. These spots gradually spread over a large area of the leaves and stems. An exception is one of the powdery mildews that affects artichokes, onions, peppers, and tomatoes: it produces yellow patches on leaves but little powdery growth.

Leaves infected with powdery mildew may gradually turn completely yellow, die, and fall off, which may expose fruit to sunburn. On some plants, powdery mildew may cause the leaves to twist, buckle, or otherwise distort. Powdery mildew fungal growth does not usually grow on vegetable fruits, although pea pods may get brownish spots. Severely infected plants may have reduced yields, shortened production times, and fruit that has little flavor.

LIFE CYCLE

All powdery mildew fungi require living plant tissue to grow. Year-round availability of crop or weed hosts is important for the survival of some powdery mildew fungi. Special resting spores are produced, allowing overwinter survival of the species that causes the disease in cucurbits, lettuce, peas, and certain other crops.

Most powdery mildew fungi grow as thin layers of mycelium (fungal tissue) on the surface of the affected plant part. Spores, which are the primary means of dispersal, make up the bulk of the white, powdery growth visible on the plant’s surface and are produced in chains that can be seen with a hand lens; in contrast, spores of downy mildew grow on branched stalks that look like tiny trees.

Powdery mildew spores are carried by wind to new hosts. Although humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, spores of some powdery mildew fungi are killed and germination is inhibited by water on plant surfaces for extended periods. Moderate temperatures (60° to 80°F) and shady conditions generally are the most favorable for powdery mildew development. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight.

MANAGEMENT

The best method of control is prevention. Planting resistant vegetable varieties when available, or avoiding the most susceptible varieties, planting in the full sun, and following good cultural practices will adequately control powdery mildew in many cases (Table 1). However, very susceptible vegetables such as cucurbits (cucumber, melons, squash, and pumpkins) may require fungicide treatment. Several least-toxic fungicides are available but must be applied no later than the first sign of disease.

Resistant Varieties

In some cases, varieties resistant to powdery mildew may be available. If available, plant resistant varieties of cantaloupe, cole crops, cucumber, melons, peas, pumpkins, and squash. If you plant more susceptible varieties, you may need to take control measures.

Table 1. Host Plants and Control Measures for Powdery Mildew Species.

Hosts Fungus species Controls
cucumbers, endive, lettuce, melons, potato, pumpkin, squash Erysiphe cichoracearum resistant varieties of lettuce, cucumber; water sprays; fungicides if necessary on squash and pumpkin
broccoli, Brussels sprouts, cauliflower, and other cole crops; radicchio, radishes, turnips Erysiphe cruciferarum not usually required
tomatoes Erysiphe lycopersici fungicides if necessary
peas Erysiphe pisi resistant varieties; sprinkler irrigation
carrots, parsley, parsnips Erysiphe heraclei tolerant varieties
beets Erysiphe polygoni tolerant varieties
artichoke, eggplant, peppers, tomatillo, tomatoes Leveillula taurica rarely required; fungicides if necessary
beans, black-eyed peas, cucurbits, okra Sphaerotheca fuliginea resistant varieties for some; fungicides if necessary
Cultural Practices

Plant in sunny areas as much as possible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow-release fertilizer. Overhead sprinkling may help reduce powdery mildew because spores are washed off the plant. However, overhead sprinklers are not usually recommended as a control method in vegetables because their use may contribute to other pest problems.

Fungicide Application

In some situations, especially in the production of susceptible cucurbits, fungicides may be needed. Fungicides function as protectants, eradicants, or both. A protectant fungicide prevents new infections from occurring whereas an eradicant can kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Use eradicants at the earliest signs of the disease. Once mildew growth is extensive, control with any fungicide becomes more difficult. The products listed here are for home garden use. Commercial growers should consult the UC Pest Management Guidelines.

Fungicides

Several least-toxic fungicides are available, including horticultural oils, neem oil, jojoba oil, sulfur, and the biological fungicide Serenade. With the exception of the oils, these materials are primarily preventive. Oils work best as eradicants but also have some protectant activity.

Oils

To eradicate mild to moderate powdery mildew infections, use a horticultural oil such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil, or one of the plant-based oils such as neem oil or jojoba oil (e.g., E-rase). Be careful, however, to never apply an oil spray within 2 weeks of a sulfur spray or plants may be injured. Also, oils should never be applied when temperatures are above 90°F or to drought-stressed plants. Some plants may be more sensitive than others, however, and the interval required between sulfur and oil sprays may be even longer; always consult the fungicide label for any special precautions.

Sulfur

Sulfur products have been used to manage powdery mildew for centuries but are only effective when applied before disease symptoms appear. The best sulfur products to use for powdery mildew control in gardens are wettable sulfurs that are specially formulated with surfactants similar to those in dishwashing detergent (e.g., Safer Garden Fungicide) However, sulfur can be damaging to some squash and melon varieties. To avoid injuring any plant, do not apply sulfur when air temperature is near or over 90°F and do not apply it within 2 weeks of an oil spray. Other sulfur products, such as sulfur dust, are much more difficult to use, irritating to skin and eyes, and limited in terms of the plants they can safely be used on. Copper is also available to control powdery mildew but is not very effective.

Biological Fungicides

Biological fungicides (such as Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on the plant, destroys fungal pathogens. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew from infecting the plant. While this product functions to kill the powdery mildew organism and is nontoxic to people, pets, and beneficial insects, it has not proven to be as effective as the oils or sulfur in controlling this disease.

How to Use

Apply protectant fungicides, such as wettable sulfur, to susceptible plants before or in the earliest stages of disease development. The protectant fungicides are only effective on contact, so applications must provide thorough coverage of all susceptible plant parts. As plants grow and produce new tissue, additional applications may be necessary at 7- to 10-day intervals as long as conditions are conducive to disease growth.

If mild to moderate powdery mildew symptoms are present, the horticultural oils and plant-based oils such as neem oil and jojoba oil can be used to reduce or eliminate the infection.

WARNING ON THE USE OF PESTICIDES

McCain, A. H. 1994. Powdery Mildew. HortScript #3, Univ. Calif. Coop. Ext. Marin County.

PUBLICATION INFORMATION

Pest Notes: Powdery Mildew on Vegetables

UC ANR Publication 7406

Produced by University of California Statewide IPM Program

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Powdery mildew can present a serious problem in your vegetable garden especially for things like cucumbers. Powdery mildew is actually a fungus which forms as a gray or white powder and it leaves your plants susceptible to deformities and death.

You will probably notice it first and foremost on the leaves. It can be very problematic in damp areas with high levels of humidity or parts of your garden which are shaded. One of the biggest issues that you will find is that once it starts to spread on your plant leaves it is incredibly difficult to control. It is for this reason that you must take proactive measures to prevent it from forming in the first place. This is even more applicable if you have dealt with powdery mildew issues in the past.

How to prevent powdery mildew on you cucumbers

The best thing you can do first and foremost is to plant mold and mildew resistant breeds. If you’re going to plant cucumbers look for the diva cucumber. Obviously if you are not planting a brand-new vegetable garden it might be too late because you might already have a different variety. But in the future if you have the option always go with the powdery mildew resistant option.

Where to plant

When you plant your cucumbers make sure you put them in full sunlight. These powdery mildew spores will have a difficult time surviving if you put the plant in direct sunlight. A lot of people might plant the cucumbers in pots and move them around the yard, sometimes moving them underneath the patio cover when there is heavy rain. But you always have to put it back out in the direct sun to dry it out and prevent the spores from growing.

Another tip for planting is to make sure you give your plants adequate space. Cucumbers need a lot of space because they need a lot of air circulation. If you plant them too close together then mildew spores can spread very easily and very quickly from one plant to another.

If you are planting different types of vegetables in and around your cucumbers, always disinfect your tools before you use them on the cucumbers. If, for example, you have a hearty summer squash it might have mildew spores on it that you don’t see with the naked eye.

If you do not disinfect the tools you are using for the summer squash and then you simply start working on the cucumbers, you could accidentally be passing the spores from one plant to another. In order to disinfect your tools always use a mixture with full strength vinegar or one part bleach for every nine parts water.

Watering

You should always avoid watering your cucumbers in the evening. If you are somebody who works all day and will typically tend it to your garden after work, try and change your routine at least for the cucumbers so that they are watered in the morning. If you water them at night the leaves will stay damp overnight especially if the temperature drops substantially in this can increase the powdery mildew.

By watering your plants in the morning you give the leaves an opportunity to dry out before the sun goes down. When you are watering, water the soil around the plant rather than watering on top of the plant. If you can avoid getting the leaves wet, it will prevent mildew.

If you start to see the signs of powdery mildew, which will manifest as small white or gray spots on your cucumber leaves, make sure you remove the infected leaves and put them in the trash. It is best not to put them in your compost pile.

Homemade sprays

You can treat the cucumber plant with homemade sprays comprised of baking soda and milk. One of the fastest and most cost effective ways to prevent and control powdery mildew is the application of a mixture containing baking soda and milk. You have to spray the mixture on top and on the bottom parts of every leaf to increase the pH level on the surface. Increasing the pH levels inhibits the ability of the fungi spores to survive.

It remains unclear how the milk impacts the spores but for whatever reason spring milk on the plant rather than water results in a significant decrease in the amount of mildew by 90%. Some reports stipulate that milk boosts the immune system of the plant so that it is better able to fight off mildew and disease.

You can also integrate liquid dish detergent similar to homemade pest control methods. You can take a regular sprayer and fill it with ½ quart of milk, 3 teaspoons of baking soda, and one drop of liquid dish detergent. Mix this together and spray the top and bottom sides of any infected leaves.

You will want to reapply the solution once every week. Make the solution from scratch each time you do it, even if there is some left over.

If you regularly have issues with powdery mildew on your cucumbers you might want to start spraying this mixture long before you actually see the powdery mildew. Using this spray before the symptoms appear is critical in preventing it from forming in the first place.

Once again, powdery mildew is incredibly difficult to control once it has manifested. Not only will it slowly spread to all of the cucumbers but it will jump to nearby plants. Certain plants might be less susceptible to the impact and damage than others but it is nonetheless not something you want to risk.

Once the cucumber plants have become sturdy enough you can always start applying the solution once a week especially if the weather is turning, if you accidentally watered in the evening for a few days, or anything else. Preventative care is important not just for people but for your plants.

Mold on the soil

Many people grow vegetables indoors using pots if they don’t have the space for a full garden. But you might notice that there is a dusty looking mold on top of the soil for your indoor cucumber. If you see white mold growing on the surface of your potting soil is probably a harmless fungus. It is important not to mix up the harmless fungus on the soil with the harmful mildew on the leaves.

The mildew on the leaves will compromise the health of the plant and any fruit it produces. This is clearly visible as singular spots appearing at random all over the leaves of your plant and eventually on the cucumbers themselves if any have been produced.

By comparison, mold on your potting soil could cover the entire surface. This type of fungus is not a problem for the plant itself but it is indicative of a different problem. If you notice the dusty looking mold on the soil, it might be indicative of poor drainage, contaminated potting soil, or over-watering.

It is important to be cognizant of such fungus because, if you do not take steps to rectify the issue that is causing it, it could eventually lead to an environment in which mold or mildew is cultivated.

Cleaning the mold

If you see this on your indoor cucumber plant you can simply scrape the mold away from the top of the soil. After that, put the pot in a well ventilated area. You want to allow the soil an opportunity to dry out so obviously do not continue to water it until it has dried. If the soil is still soggy you should report your plant being careful not to disturb the roots.

Before using the same pot, so the pots in a solution containing one part bleach for nine parts water. Let it soak for 10 minutes and then scrub it clean with dish detergent and warm water.

You do not want soggy soil because not only does it result in this top layer of white fungus but it could lead to serious issues like root rot. If your cucumbers start to rot the roots it will work its way through the entire plant and compromise any and all vegetable production. A good rule of thumb when it comes to watering is to place your finger gently inside the soil about 1 inch in depth.

If the soil is soaking wet when you pull your finger out then it obviously doesn’t need more water. Never over-water the soil or this can compromise the health of your cucumbers.

You might also consider moving their indoor location. Regardless of whether you have them in pots on your balcony, your patio, or inside, place them in an area where they will get full sunlight if at all possible. If issues still persist you can always add drainage holes to the bottom of whatever pot you are using, or add pebbles to the bottom of the pot before you replant the cucumber.

Photo by uccsbiology on Foter.com / CC BY-NC-SA

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Powdery mildew, common on zinnias and other crops in mid-summer, starts with just a few spots on the lower leaves, as shown in this photo. Without treatment, it will spread to shoots and buds and eventually kill the plant.

By Lynn Byczynski

As warm, dry weather settles in this month, be alert for the development of powdery mildew on crops. Powdery mildew is a common summer problem on many types of vegetables and cut flowers. But most powdery mildew can be avoided or cured with inexpensive, homemade remedies that have been proven to work as well as or better than commercial fungicides.
Several species of fungus cause powdery mildew, each affecting different groups of plants. The main species is Erysiphe cichoracearum, which affects composites, including:
•Vegetables – cucurbits, cucumbers, endive, lettuce, melons, parsley, poumpkins, and potatoes.
•Flowers – chrysanthemum, begonia, dahlia, phlox, sunflower and zinnia.
Nine other species of powdery mildew fungus affect cole crops, peas, eggplant, pepper, tomato, strawberries, beans, black-eyed peas, grapes, and tree fruits.
Identification
The first thing to know about powdery mildew is that it is quite different from downy mildew, despite some similarities in appearance. Both produce light-colored masses of spores on foliage. Downy mildew develops spores only on the undersides of leaves, whereas powdery mildew will appear on both sides of leaves as well as shoots, buds and sometimes flowers. Moreover, downy mildew is a disease that appears in cool, wet conditions and is generally stopped by warm, dry, windy weather. Powdery mildew thrives when foliage is dry and the weather is warm; wind spreads the spores to other plants. In fact, powdery mildew spores can’t germinate or grow when foliage is wet, so overhead watering is sometimes recommended as a preventative on highly susceptible crops.
Powdery mildew spores overwinter on perennial crops such as grapes, raspberries, strawberries and fruit trees, or in plant debris left from last year. When conditions are just right, this year’s growth can be affected and the disease spreads quickly. The optimum situation for the development of the disease is cool, humid nights followed by hot, dry days.
If left unchecked, powdery mildew will cause leaves to turn yellow, die and fall off, leaving fruits and vegetables exposed to sunburn and making cut flowers unmarketable. Preventative action in susceptible crops and regular scouting can prevent a catastrophic outbreak of this disease. Following are some of the remedies and products available for preventing powdery mildew Some of these recipes may be acceptable for organic production, but you should check with your certifying agency if in doubt. Commercial products should have the approval of OMRI, which can be verified at www.omri.org. Much of the advice about how to mix these sprays is from the Bio-Integral Resource Center, whose contact information is at the end of the article.
Milk
The newest organic control for powdery mildew is milk. In 1999, Brazilian scientist Wagner Bettiol reported excellent control of the fungus on greenhouse-grown zucchini using fresh cow’s milk diluted with water to a 10% solution. An Australian researcher, Peter Crisp, experimented with milk on roses and wine grapes – which get powdery mildew from different organisms. Crisp found that in most cases the 10% milk solution worked as well as the leading synthetic fungicide and as well as sulfur.
According to an article in Science News, the results have been proven in the field by grape growers. But there is some concern than using any product, even milk, repeatedly may allow the fungal organisms to develop resistance. So the current recommendation is to spray for powdery mildew every week, but alternate between remedies. And there are plenty of other recipes from which to choose.
Baking soda
Sodium bicarbonate – the same stuff you use for making biscuits or deodorizing the refrigerator – is highly effective against powdery mildew. Its effectiveness is not understood precisely, but it is thought to be a case of induced resistance – that is, the baking soda causes the plant to produce some compound to defend itself against pathogens.
Baking soda is particularly effective when paired with a horticultural oil. To mix your own solution, for each gallon of water in your sprayer, add 1 Tablespoon baking soda and 2.5 Tablespoons of horticultural oil. This makes a 0.5% concentration of bicarbonate, the maximum recommended for control of powdery mildew on roses; other species may tolerate greater concentrations, but you should test for phytotoxicity before spraying large areas.
Baking soda has a few drawbacks: First, it must be sprayed every week to protect new growth on the plant. It also can build up in soil when used in drought-stressed areas where only drip irrigation is used. Increased bicarbonate in soil can lead to removal of calcium and magnesium, and prevent the absorption of iron and lead to iron chlorosis. (These risks appear to be small in most farm situations.)
Garlic
Yet another kitchen remedy is garlic extracts, which can be made by blending two bulbs (not cloves!) of fresh garlic in a quart of water with a few drops of liquid soap. The liquid should be strained through cheesecloth to remove solids and then refrigerated. That concentrate should be diluted 1:10 with water before spraying. That provides a concentration of 25-50 parts per million of the active compound allicin, which will help prevent germination of powdery mildew spores. Once the spores are active, though, a concentration of 300 to 500 ppm is needed to cure powdery mildew.
Compost tea
The antifungal properties of compost tea are by now well known. Many organic growers, especially in the rainy Northwest part of the country, have had great results and are advocates for the benefits of compost tea. Several companies sell equipment for making compost tea, and some growers just mix one part of finished compost with six parts of water and let it soak for a week, then strain and dilute with water until it’s the color of tea.
Oils and anti-transpirants
Oils alone can be used to control powdery mildew. Vegetable seed oils such as canola oil can be used, at a rate of 2.5 to 3 Tablespoons per gallon of water, with the addition of a quarter-teaspoon of liquid soap to emulsify the oil. Most commercial horticultural oils already have an emulsifier added, so additional soap is not needed. Soap itself has been found to control powdery mildew, but it can cause phytotoxicity.
Neem oil is labeled for control of powdery mildew, rust, blackspot, botrytis, downy mildew and other diseases. Spraying with 2.5 Tablespoons per gallon of water every 7 to 14 days is recommended.
Mint oil (Fungastop) and rosemary oil (Sporan) are now being marketed as fungicides. Cinnamaldehyde (Cinnamite) also has been effective at controlling powdery mildew.
Antitranspirants are sprays that are used to prevent water loss from plant foliage. They have been found to also protect against several foliar diseases, including downy mildew, powdery mildew, and blackspot. One study showed that the antitranspirants Wilt Pruf and Vapor Guard, which are widely available at garden centers, protected roses from powdery mildew for 30 days. According to BIRC, antitranspirants are nonspecific against pathogens, so the fungus is not likely to develop resistance. However, they do reduce plant photosynthesis and should be used only during sunny weather and they need to be reapplied to protect new growth.
Copper and sulfur
A traditional treatment for plant diseases involves spraying sulfur and copper on highly susceptible plants. Several products are available for use by organic growers, but they are considered restricted, which means they can only be used if other management practices have failed. Both copper and sulfur can irritate skin and mucous membranes, so breathing protection should be worn.
Bacteria
Bacillus subtilis and Bacillus pumilis are two bacteria that have been found effective against powdery mildew and other diseases. Several commercial products containing these bacteria are now available: Serenade for home gardeners and Rhapsody for commercial growers. Sonata is specifically for control of powdery mildew in roses.

Lynn Byczynski is the founder of Growing for Market magazine and the author of The Flower Farmer: An Organic Grower’s Guide to Raising and Selling Cut Flowers.
Please subscribe! Growing for Market is published 10 times a year and features practical, how-to articles about growing and selling produce and flowers. With a Full Access subscription, you will also have access to thousands of articles in our archive. to learn about print and online subscriptions.

What is Powdery Mildew?

Powdery mildew (PM or WPM) is one of the most common fungal diseases that affects cannabis.

The species of powdery mildews that infect cannabis are in fact different from the species that infect other plants. There are at least two unique species that infect cannabis.

MacPartland et al. reported the species L.taurica and P.macularis (formerly known as S.macularis). These species are obligate biotrophs, which means they need a host to grow.

The P.macularis species found growing on cannabis are nearly 98% identical to the P.macularis species that grows on hops, according to research done by Medicinal Genomics.

Organic Fungicides to Stop Powdery Mildew

Overview: A lot of organic fungicide products for powdery mildew are simply potassium bicarbonate and a sticker agent, plus a flashy label on the bottle. Save money on overpriced liquid formulations by mixing it yourself using potassium bicarbonate powder and an organic wetting agent.

Application: Make a 0.5–2.0% solution (5–20 grams per liter) of potassium bicarbonate in water. Spray directly onto pm infected spots.

Cannabis Guru Ed Rosenthol prefers to mix an ounce of potassium bicarbonate in a gallon of water together along with 1.5 cups of milk.

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Bacillus Subtilis

Overview: Bacillus subtilis is a naturally occurring strain of bacteria that can be found in soils and in the gastrointestinal tract of humans. When bacillus subtilis is applied to powdery mildew, the bacteria feed on the powdery mildew as a nutrition source, until the infection is gone and then the bacteria die off.

Application: Use 30 ml (~6 tsp) of Cease per gallon of water (non-chlorinated) for foliar sprays. Alternatively, Serenade also contains Bacillus subtilis, and comes premixed into a spray bottle. Multiple applications of bacilus subtilis may be necessary as residual powdery mildew spores from the environment may settle onto plants after the first round of bacteria dies off.

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Streptomyces lydicus

Overview: Streptomyces lydicus is the bacterium species WYEC 108 from the genus Streptomycess. The naturally-occurring bacteria is used in horticulture to cure a number of diseases, including powdery mildew. Streptomyces lydicus works by establishing itself onto the plant surfaces, and attacks the powdery mildew fungus at it’s binding site.

Actinovate contains Streptomyces lydicus as the active ingredient, is 100% organic, and labeled as SAFE for people, pets and the environment.

Application: Mix 1-2 teaspoons of Actinovate per gallon of non-chlorinated water, plus a sticker (optional). Spray onto mildew infected areas of the plant. The dilution ratio can be increased for heavy pm infections.

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GreenCure

Overview: One of the most popular organic powdery mildew killers, GreenCure is a potassium bicarbonate-based broad spectrum foliar fungicide that has performed as well or better than chemical fungicides in over 200 university studies. Like other potassium bicarbonate fungicides, Green Cure works by causing an immediate dehydration of the spores and destruction of the cell walls.

Application: Mix up to 2 tablespoons of GreenCure per gallon of water, spayed on plants to come directly into contact with the powdery mildew. Repeat application weekly for 3 weeks along with preventative measures.

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Plant Oil Blends

Overview: Blends of organic plant oils such as garlic oil (sulfur), cottonseed oil, corn oil and others help to control powdery mildew and other pests and diseasese, especially when combined with potassium bicarbonate as FoxFarm does with their Bush Doctor Force of Nature fungicide.

Application: One quart bottle makes 25 gallons of foliar solution. Don’t spray buds the garlic will stink.

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Apple Cider Vinegar

Overview: The acetic acid contained in vinegar helps to rid plants of powdery mildew infections. It should be noted that although apple cider vinegar is strong enough to stop most moderate-level PM infections, it’s not quite the best solution for eliminating a well-developed outbreak of powdery mildew.

Application: Mix 2 teaspoons of organic apple cider vinegar into a quart of water – be careful not to overdo it, high concentrations of vinegar can burn plants.

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Hydrogen Peroxide (H202)

H2O2 is hydrogen and oxygen, same as water but with an extra oxygen atom. A 3% wash of hydrogen peroxide is used by growers like Jorge Cervantes to clean off powdery mildew on freshly harvested cannabis buds. The buds are submerged in the H2O2 bath for a brief period and washed afterwards with pure water, then hung and dried off with fans.

This cleaning procedure is a good alternative to bringing pm infected buds into the drying room. Yet there has been speculative criticism of using hydrogen peroxide to wash cannabis buds, due to the susceptibility of certain terpenes to oxidization, such as the terpene limonene.

Other Ways To Kill Powdery Mildew

High Alkaline PH Water

Changing the surface PH of cannabis leaves is a technique used to create an uninhabitable environment for powdery mildew. Using reverse osmosis water with a PH of 8 is perhaps the safest solution for spraying on cannabis buds during mid-to-late flowering cycle, still the effectiveness of this method varies.

Baking Soda

One of the most popular powdery mildew solutions is to use baking soda (sodium bicarbonate) mixed with water as a foliar spray on plants. Baking soda works to change the surface PH of the leaf to inhibit powdery mildew growth. Although this may have limited benefit, studies have shown sodium bicarbonate to be less effective than potassium bicarbonate.

Milk

Milk has shown to help remedy powdery mildew in some trials but not others. It’s speculated that the benefit is due to the probiotic lactobacillus. However, beware of spraying flowering plants with milk, as the proteins will create an suitable environment for gray mold to grow. Low-fat skim milk is preferred, as the fats present in milk can turn rancid.

Neem Oil

Years ago we used to recommend Neem Oil for powdery mildew prevention and control during a plant’s vegetative growth cycle.

Not anymore though, due to the various discussions linking the chemical Azadirachtin, an active ingredient found in neem oil, to a condition known as cannabis hyperemesis syndrome.

Furthermore, neem oil makes buds oily and smelly, which is another reason to avoid it altogether, especially during the cannabis plant’s flowering cycle.

UVC Light

Shortwave UV radiation (UVC) kills powdery mildew. There are a few UVC lights marketed to cannabis growers, however certain amounts of UVC radiation is harmful to both humans and to plants. UVC lights must kept at minimal intensity or use protective shields to prevent overexposure. The sun produces UVC radiation, but is absorbed in the ozone layer. For more information read this UVC hazards sheet.

Of the UVC methods available, manually applying UVC lights to PM infected plant surfaces creates the most dangerous liability. Furthermore, there’s a fine line between eliminating the existing powdery mildew with UVC and causing DNA damage in plants.

The more sensible UVC solution to powdery mildew is purely preventative, in protected germicidal air filtration units. UVC air filtration can be installed in heating, air conditioning and ventilation systems, to disinfect the air while also avoiding direct exposure to plants and humans.

Sulfur Burners

Elemental sulfur can be vaporized in sulfur burners to prevent and control powdery mildew. When used improperly or at too high a concentration, sulfur causes leaf damage to plants.

In best practice, avoid sulfur vaporization during the plant’s flowering cycle. There are many aggrievances about the harsh effects sulfur has on the taste and flavor of buds.

Sulfur burners are hazardous to humans, precautions need to be taken if this method is to be used. Read this safety sheet for more info.

Powdery Mildew Prevention

Along with application of organic fungicides, here are some tips to help prevent further powdery mildew outbreaks in cannabis.

Clean and Quarantine

Important in the prevention of powdery mildew is the quarantine of infections and elimination of residual spores, which can hide on surfaces in the growroom/garden. Also suspect are any means of reintroducing the mildew in the grow room via infected clones, clothing, ect.

Indoor growers will want to wipe all surfaces, ventilation, fans ect. top-to-bottom with a bleach and water solution to kill any spores present. Ozone generators can also be used to sterilize grow rooms, although beware these are dangerous at high concentrations.

Climate Control

Lowering humidity increasing airflow, and clean air filtration all play a role in the prevention/control of powdery mildew.

While it’s often recommended for indoor growers to keep their grow rooms at a relative humidity of 45% or lower, there is anecdotal evidence among growers that the species of powdery mildew that infects cannabis has developed resistance to low humidity levels.

Further research is needed, but one thing is certain – powdery mildew thrives in stale air, humid environments.

Trichoderma

Presence of the fungi Trichoderma in a plant’s rhizosphere may play a crucial role in the prevention of powdery mildew. While there have been no studies directly related to the powdery mildew species that grow on cannabis, peer-reviewed studies of powdery mildew on other plants have shown that application of the species T. harzianum in soil resulted in a 75–90% reduction of powdery mildew infection on the leaves.

Researchers speculate that the enzymes Trichoderma produces helps the plant to resistant powdery mildew, among other infections.

Trichoderma can be found in products such as Soil Blast on Amazon. However, note that over-application of Trichoderma may create problems with another beneficial fungi for plants, mycorrhizal fungi.

Silicon (Si)

Silica (silicon dioxide) is created when silicon comes into contact with oxygen. Relating to powdery mildew, silica helps strengthen the epidermal layer of plants, forming fortified areas that may help to protect plants from powdery mildew infections. Note that silica is naturally alkaline and will raise the PH level of a nutrient solution.

Popular liquid formulations of Silica include Armor Si by General Hydroponics and BulletProof by Cutting Edge.

Methods of increasing Si in the soil or root medium include diatomaceous earth, greensand, pyrophyllite clay, and high-silica fertilizers such as Pro-TeKt.

Is Powdery Mildew Systemic?

One of the most commonly debated issues among cannabis growers concerning powdery mildew is whether or not it is “systemic” to the plant.

The leading researchers of powdery mildew in cannabis, Medicinal Genomics, aren’t quite sure of the answer yet. Read their response to this question here- medicinalgenomics.com/powdery-mildew-systemic

In any case, the term systemic may not be the right word to use.

Is Powdery Mildew Dangerous to Smoke?

There isn’t enough evidence to define any dangers of smoking powdery mildew – It isn’t considered a human pathogen, and at worst may cause allergenic symptoms. The main gripe about powdery mildew on buds is the nasty appearance it creates.

This is not the case for another mold on cannabis, Botrytis cinerea, which clearly should not be smoked as it has links to causing Aspergillosis.

On the subject of making extraction with powdery mildew infected buds, it’s probable that ethanol-based extraction would burn off most of the spores. Still, we can’t endorse smoking any derivatives of powdery mildew infected buds until further research is done.

How does Powdery Mildew Infection Start?

The infection starts when a spore (conidium) lands on a leaf. Once the spore germinates, it quickly develops an appressorium, which is a growth structure that attaches the spore to the plant tissue. The fungi then pierces the plant’s dermal tissue with a sort of taproot, called a haustorium.

The haustorium is used to absorb nutrients from inside the plant cell, which causes weakening of the leaf and reduced photosynthesis ability. The haustorium sucks up plant nutrients and transferres them to the fungus, weakening the leaf and slowing growth.

How does Powdery Mildew Spread?

Once the powdery mildew infection has started, a mycelium network is quickly developed throughout the plant.

Spores of both species are spread airborne. The P.macularis spores also migrate with moving water such as drops of water falling from leaf to leaf, or blown by the wind to other plants.

Hardy spores and spore structures, called cleistothecium, can survive though long periods and overwintering.

Research has shown roughly 20-40 dpi (days post infection) from the start of a spore infection to when it’s produces spores of it’s own.

Where does Powdery Mildew Grow?

Powdery mildew spores germinate in slightly acidic conditions with temperatures ranging from 18-24°C. Once germinated the powdery mildew fungi can withstand a wider range of climates. Infectivity of P. macularis conidia is greatly reduced at temperatures ≥30°C.

These are much of the same conditions plants prefer, which makes fighting off powdery mildew all the more of a challenge.

Stagnant air, low-light greenhouses and indoor grow rooms without proper ventilation are easy places for powdery mildew to reproduce. Plants that have been overcrowded grown Sea of Green or SCROG style may be problematic, as they trap in moisture.

There isn’t much evidence to suggest powdery mildews hopping from species to species.

What does Powdery Mildew do to Cannabis?

Powdery mildew hinders ability of a plant to collect nutrients though it’s leaves (photosynthesis). As PM spreads though the cannabis plant, the leaves unable to collect nutrients bring the plant into a slow stunted growth stasis.

PM spreads quickly though cannabis, smothering it in white mildew – many times in just a few days. Left untreated, powdery mildew will effectively ruin a bud harvest.

While bringing back a matured plant covered in PM can be close to impossible, getting rid of it on a young plant is easy enough. Eliminating an outbreak is more probable the earlier on it’s spotted.

How to Apply Foliar Spray Fungicide

A simple garden pump sprayer like this on Amazon is all you need to do the job. Even water bottle sprayers can be used, however it’s important that the spray gets applied to both the top and undersides of infected leaves, along with stems and stalks.

While foliar treating plants, it may be helpful to cover up the base of plants to avoid fungicide drip into the root-zone. Although these powdery mildew remedies are organic, they still can be harmful to beneficial microorganisms in the soil.

Most remedies require more than one application to really be effective. Moisture may wash the foliar spray off – even morning dew on plants can remove foliar applications from leaves. Perhaps the best time to spray outdoors is before noon on a dry, sunny day. Indoor growers usually spray before lights out.

Using an organic wetting agent (sometimes called a spreader-sticker) helps the spray stay on the foliage longer.

Breeding For PM Resistance

Certain strains of cannabis have natural resistances to powdery mildew infection. What cannabis breeders can do is work with these genetic lines by selective breeding to further increase the resistance in strains.

Visit our Mold Resistant Strains homepage for some real good strains. Some of the best producing strains from our favorite’s grown outdoors on the Big Island of Hawaii. High mold resistance.

I recommend you check out the Shaping Fire podcast ep. 28, where I learned much of the information needed to write this article.

You may enjoy further learning about powdery mildew though these links

Hemp Diseases and Pests: Management and Biological Control () medicinalgenomics.com/powdery-mildew newcropsorganics.ces.ncsu.edu/2010/07/the-cornell-formula-fungicide-an-example-of-why-you-need-to-check-out-your-internet-information-sources/ merryjane.com/health/the-curious-case-of-cannnabis-hyperemesis-syndrome apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-93-3-0281

Other references are linked in article

If you have any questions, experience, or comments about powdery mildew on cannabis please comment below. Disclaimer: We do not promote or undertake in illegal activities.

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Floriculture and Ornamental Nurseries

Powdery Mildew

Pathogens: Erysiphe spp., Leveillula (= Oidiopsis) taurica, Oidium sp., Sphaerotheca spp.

(Reviewed 3/09, updated 3/09)

In this Guideline:

  • Symptoms
  • Comments on the disease
  • Management
  • Publication
  • Glossary

SYMPTOMS AND SIGNS

Powdery mildew is the name given to diseases resulting from infection by fungi that produce a white, powdery growth on the surfaces of leaves and sometimes other plant parts. Leaves may yellow, then brown and die. Infected tissues may be distorted and misshapen.

COMMENTS ON THE DISEASE

There are many kinds of powdery mildew fungi, and most are highly specialized. For example, the powdery mildew that infects squash plants will infect other plants in the cucurbit family but will not infect roses, and the powdery mildew from roses will not attack zinnias and vice versa, although the fungus that infects zinnias also infects many other members of the composite family. Powdery mildew fungi are obligate parasites; that is, they can grow only on living plant tissue. When the mildew-infected plant part dies, so does the mildew unless cleistothecia (resting stages of the fungus) are formed.

Most powdery mildew fungi grow over the surface of the leaf, sending short food-absorbing projections (haustoria) into the epidermal cells. The fungi produce masses of spores (conidia), which become airborne and spread to other plants. Powdery mildew spores are unique in that they require no external moisture for germination; most other fungi require free water in the form of dew, guttation, rain, or water from overhead irrigation for germination and infection or growth whereas the conidia of powdery mildew (except those that infect grasses) die in water. Spores may be dispersed, however, by splashing water.

The fungus survives in the absence of susceptible host tissues by forming a sexual stage (cleistothecium) resistant to drying and other adverse environmental conditions. With many perennial plants, such as rose, the fungus survives as mycelium in dormant buds or actively on plant tissues. Powdery mildews are particularly severe in semiarid regions, such as most of California, and are less troublesome in high rainfall areas.

Powdery mildews are favored by warm days and cool nights and moderate temperatures (68° to 86°F). At leaf temperatures above 90°F, some mildew spores and colonies (infections) are killed. Shade or low light intensities also favor powdery mildew fungi. Greenhouse conditions are often ideal for development of the disease.

MANAGEMENT

The best control is through the use of resistant cultivars. However little attention has been paid to development of resistant cultivars of flower crops. Because high relative humidity (greater than 95%) favors some powdery mildew fungi, increased air movement around the plants in the greenhouse tends to reduce infection potential in these mildews.

In general, there are two types of fungicidal control: eradication of existing infections and protection of healthy tissues. In practice, some products provide both protection and eradication, especially when good wetting of the plant is achieved. To achieve good wetting, some of these products may require the addition of surfactants.

The fungus has developed resistance to some of these fungicides. Rotate the different fungicides to help slow down the development of fungal strains that are resistant to the fungicides. Plants that have been treated with antitranspirants are less likely to develop powdery mildew infections.

Common name Amount to Use R.E.I.+
(trade name) (hours)
When choosing a fungicide, consider the general properties of the fungicide as well as information relating to environmental impact.
PROTECTANTS (Must be applied to healthy tissues before infection takes place)
A. WETTABLE SULFUR# 3 lb /100 gal water 24
MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
COMMENTS: Use a wetting agent. Effectiveness of sulfur increases with increasing temperature, but the likelihood of plant injury increases also. Plant damage may result if sulfur is applied at temperatures exceeding 90°F. Some plants, such as melons, are sensitive to sulfur. Sulfur can be applied as a dust or as a spray. Repeated applications are generally necessary to protect new growth and also to renew deposits removed by rain or irrigation.
B. MYCLOBUTANIL
(Hoist) 40WSP 4 oz/100 gal water 24
MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant of rusts or powdery mildew on carnations, crepe myrtle, gerbera, roses, and snapdragons.
C. AZOXYSTROBIN
(Heritage) 1–4 oz/100 gal water 4
MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
COMMENTS: Acts as a protectant but has some eradicant properties. A locally systemic fungicide that is an eradicant and protectant against some powdery mildews.
D. FENARIMOL
(Rubigan) AS 3–10 fl oz/100 gal water 12
MODE OF ACTION: A DMI (Group 3)1 pyrimidine fungicide.
COMMENTS: Apply on a 10- to 14-day interval. A systemic fungicide used for prevention or eradication of powdery mildew on roses and field and container-grown ornamentals.
E. TRIADIMEFON
(Strike, Bayleton) Label rates 12
MODE OF ACTION: A DMI (Group 3)1 triazole fungicide.
COMMENTS: A long-lasting systemic fungicide that provides for general control of some powdery mildews, some rusts, and leaf blight and spots in greenhouses and commercial nurseries.
F. THIOPHANATE-METHYL
(FungoFlo, Cleary’s 3336F, etc.) Label rates 12
MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
COMMENTS: Not as effective against powdery mildew as other materials.
G. PROPICONAZOLE
(Banner Maxx) 5 fl oz/100 gal water 24
MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
COMMENTS: A preventive fungicide.
H. POTASSIUM BICARBONATE#
(Kaligreen) 1–1.5 lb/half-acre 4
MODE OF ACTION: An inorganic salt.
COMMENTS: Primarily a protectant but it eradicates some existing infections with thorough coverage. Apply in 75–100 gal water/half-acre at first signs of infection. Thorough coverage is essential for good protection. Labeled for use on roses, field ornamentals, and greenhouse ornamentals; make no more than 8 applications/season.
ERADICANTS
A. STYLET OIL#
(Organic JMS Stylet Oil) 1 oz/gal water 4
MODE OF ACTION: A contact fungicide with smothering and barrier effects.
COMMENTS: A good eradicant for mild to moderate powdery mildew infections; oils work best as eradicants but also have some protectant activity. Registered for use on chrysanthemum, diffenbachia, philodendron, poinsettia, and roses. May be phytotoxic, especially on greenhouse roses. Do not apply to plants suffering from heat or moisture stress. Never apply any oil within 2 weeks of a sulfur spray or plants may be injured.
B. NEEM OIL#
(Triact) 70 Label rates 4
MODE OF ACTION: Unknown.
COMMENTS: A broad-spectrum botanical pesticide derived from the neem tree that is effective against various fungal diseases including black spot on roses, powdery mildew, downy mildew, anthracnose, and leaf spot. Registered for landscape and nursery ornamentals; oils work best as eradicants but also have some protectant activity. When using as a protectant, apply on a 14-day schedule; as an eradicant, apply on a 7-day schedule. Never apply any oil within 2 weeks of a sulfur spray or plants may be injured.
C. PIPERALIN
(Pipron) 84.4 Ec 4–8 fl oz/100 gal water 12
MODE OF ACTION: An amine (Group 5)1 piperidine fungicide.
COMMENTS: Requires thorough coverage. For use in greenhouses only. A foliar spray that eradicates powdery mildew on rose, lilac, dahlia, phlox, zinnia, chrysanthemum, and catalpa.
D. LIME SULFUR 28%# 1.5–3 pt/100 gal water 48
MODE OF ACTION: A multi-site contact (Group M2)1 inorganic fungicide.
COMMENTS: Primarily an eradicant but has some protectant properties. Plant damage may result if applied when temperatures exceed 80°F. Not as effective against powdery mildew as other materials. Not for use in greenhouses.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
# Acceptable for use on organically grown ornamentals.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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Powdery Mildew Treatment Indoors: How To Get Rid Of Powdery Mildew On Houseplants

It’s not talcum powder and it’s not flour. That white chalky stuff on your plants is powdery mildew and it needs to be dealt with as the fungus spreads easily. Read on to learn how to get rid of powdery mildew on your indoor plants.

Powdery Mildew on Houseplants

Powdery mildew on houseplants is a fungal disease. Initially, it produces circular powdery white spots on the foliage of plants. As the disease spreads, the entire plant material can be affected with the fluffy white fungus. Over time parts of the plant will succumb to the disease and die. It is very contagious and once one part is affected, it will infect the rest of the plant if not checked.

The fungus can affect plants outdoors, but indoor powdery mildew is more common due to conditions. The indoor powdery mildew requires temperatures around 70 F. (21 C.). It occurs when there is poor air circulation, low light, and unlike outdoor powdery mildew, thrives in drier conditions.

The mycelium formed from the fungal spores is the source of the fluffy stuff on the plant parts. The spores spread in the air

and when water splashes on plants. Powdery mildew control is essential in the home due to this aggressive, contagious state.

How to Get Rid of Powdery Mildew

The white substance rubs off easily with your fingers or a cloth. Don’t mist plants. Prevent the foliage from getting wet when watering. Keep plants spaced to enhance air flow or use a small fan to circulate the air.

Once one plant shows signs of infection, isolate it to prevent the spread of the fungus. Pinch off the affected areas and discard. Common plants affected by indoor powdery mildew are:

  • Begonia
  • African violet
  • Kalanchoe
  • Ivy
  • Jade

If powdery mildew on houseplants is present on all specimens and cultural control is not effective, advance to chemical control. Powdery mildew treatment indoors may be achieved with common household ingredients.

Water the plants well from under the foliage, then apply a spray of 1 tablespoon baking soda, 1/2 teaspoon liquid soap and 1 gallon of water. You may also add 1 tablespoon of horticultural oil to help the mixture adhere to the fungus. Apply to the top and bottom of the foliage to get all the fungal areas. Using this powdery mildew control indoors is safe and non-toxic and effective on some, but not all, species of plants.

Another organic method to try is a milk spray. Use organic milk that is free of hormones and preservatives. Mix one part organic milk with nine parts water and spray once per week on all surfaces of the plant. Provide adequate ventilation while the spray dries on the foliage to prevent mold.

Fungicides for Powdery Mildew on Houseplants

When all else fails, use a household fungicide to kill the spores and prevent the spread of indoor powdery mildew. There is some risk of toxicity in any preparation you purchase so read the label carefully and apply as the product is intended. It’s best to apply any fungicidal spray outside to prevent drift of the particles in your home.

The use of neem oil as a fungicide for powdery mildew on houseplants can also be used.

By Melinda Myers / Special to the Herald

Spots and patches of a white or gray talcum powder-like substance on your plant means powdery mildew infected your plant.

This is one of the most widespread fungal diseases and attacks a wide range of plants. You may see mildew on a variety of trees, shrubs, flowers, vegetables and lawn grasses. Don’t despair — you can reduce the risk of this disease with proper plant selection, maintenance and, if needed, organic intervention.

Which plants are common victims of powdery mildew in the Northwest? Unfortunately, Rick Peterson, Great Plant Picks’ education and events manager, said there’s a long list of them.

“A wide range of native and exotic plants get powdery mildew in the Puget Sound region,” said Peterson, who is a master gardener. “Many older varieties of hybrid tea roses, phlox, apple and pear trees, cucumbers, squash, pumpkins, deciduous azaleas and our native big-leaf maple (fall victim), to name just a few.”

Powdery mildew is most common during hot dry weather. Wet foliage does not increase the risk of this disease, but high humidity does. You’ll typically see more mildew problems in crowded plantings, damp and shady locations as well as areas with poor air circulation.

Powdery mildew, like other diseases, occurs when the fungal organism and susceptible plants are present together and the environmental conditions are right for the disease to occur. Remove one of these factors and you eliminate the disease. You can’t change the weather but there are some things you can do to reduce the risk of powdery mildew.

Avoid purchasing mildew susceptible plants. Instead select disease-resistant varieties whenever possible. Fortunately, many new varieties of phlox, bee balm, lilacs and other mildew-resistant plants are now available at your favorite garden center.

Further decrease the risk by growing plants at the recommended spacing in the preferred amount of sunlight. Giving plants room to reach full size ensures they will receive sufficient sunlight and air circulation, thus reducing the risk of this and other diseases. Your plants will be healthier and better looking when they have space to show off their beauty.

Thin susceptible varieties of perennial plantings like phlox and bee balm in spring as new growth emerges. Removing one fourth of the stems increases light penetration and airflow reducing the risk of powdery mildew. Grow vine crops like cucumbers and squash on trellises and fences to accomplish the same results. Support large fruits of squash and melons with cloth slings anchored to the trellis.

Avoid excessive nitrogen fertilization that encourages lush succulent growth that is more susceptible to this and other diseases and many insect problems. Consider using low-nitrogen, slow-release fertilizers that promote slow, steady growth above and below ground.

If mildew continues to be a problem and is impacting the health and beauty of your plants, you may decide to intervene. Lightweight horticultural oils trap the fungal spores on the plant preventing it from spreading. Select certified-organic products that are approved for organic gardeners.

Always read and follow label directions of all chemicals, organic or synthetic, before applying to any plant. Thorough coverage of the upper and lower leaf surfaces and stems will improve your success rate.

Check plant tags, purchase wisely and adjust plant maintenance to reduce powdery mildew problems in your landscape. A bit of prevention and proper care go a long way to reducing the time spent maintaining healthy, productive and beautiful gardens and landscapes.

Melinda Myers has written more than 20 gardening books, including “Small Space Gardening.” She hosts The Great Courses “How to Grow Anything” DVD series and the nationally syndicated Melinda’s Garden Moment TV and radio segments. Myers is a columnist and contributing editor for Birds & Blooms magazine. Her website is www.melindamyers.com.

Powdery Mildew

A downy white fungal growth, powdery mildew attacks a wide variety of trees and shrubs as well as ornamental plants, indoors or out. Although any tree can get this common disease, the ones that are most commonly affected are oak, maple, dogwood, magnolia, catalpa, and crabapple. Powdery mildew consists of millions of tiny fungal spores that can be spread by the wind to other parts of a tree and on to new hosts. Although the fungus loves humidity and moisture, it can grow on both dry and damp leaves.

Powdery Mildew Signs and Symptoms

Appearing as white or grayish powder-like patches on the surface of leaves and stems, powdery mildew develops mostly on newer growths. Although the fungal spores often look like someone sprinkled baby powder on foliage, the disease can also appear as blotchy, felt-like mats or cobweb-like formations.

Disease symptoms increase when cooler weather is paired with high humidity, and these signs occur more frequently on areas of a tree that are shaded and lack good air flow. As this mildew spreads, leaves begin to yellow and wilt, and eventually, the entire branch dies. Advanced symptoms of an infection also include distorted leaves, premature leaf drop, blemishes on fruit, and buds that won’t open.

Preventing and Treating Powdery Mildew

There are many varieties of powdery mildew, but prevention and treatment methods are very similar.

To prevent conditions in which powdery mildew can thrive, don’t plant trees in extremely shaded areas. Opt for locations with good air flow, sunlight, and plenty of growing room. If necessary, prune trees to provide better air circulation.

Proper sanitation is important to reduce the spread of infection. Immediately remove and dispose of infected leaves or branches that have dropped. Don’t use them for composting or mulching. And be sure to clean and disinfect tools after pruning before you use them again.

Fungicides can be used to control infestations, though many cases of powdery mildew do not require this drastic measure. Because the mildew spreads rapidly, fungicides must be applied at the first sign of the disease’s symptoms. Be sure to prune out severely infected branches first. Sulfur can be used as a spray or dust, but can cause other kinds of damage to the foliage—especially in high temperatures–if not applied carefully.

If you need assistance identifying, treating, and preventing powdery mildew and other tree diseases, contact Elite Tree Care at 610-935-2279.

Need Help with Powdery Mildew?

Call Elite Tree Care today at 610-935-2279 and let’s talk about how we can help you with Powdery Mildew and other Pennsylvania tree diseases.

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Management of Powdery Mildew in Beans1

Qingren Wang, Shouan Zhang, and Teresa Olczyk2

Powdery mildew, caused by the fungal organism Erysiphe polygoni, is one of the most commonly occurring diseases on many types of beans. Green beans, pole bean, long bean, Italian bean, and snow pea crops are all susceptible to powdery mildew in tropical and subtropical climates. Although the causal organism rarely causes extensive damage, significant yield losses (up to 69%) were reported in Columbia County when infection occurred in dry beans prior to flowering (Schwarts et al. 2005). Accurately identifying this disease and immediately taking action for control are critical to effectively prevent spread of powdery mildew in order to reduce significant losses of yield and quality.

Powdery mildew can affect all above-ground parts of bean plants. Initial symptoms appear as small and white talcum-like spots (Figure 1), which most commonly are seen on the upper surface of leaves. These spots increase in size and run together to form a whitish, powdery growth, gradually spread over a large area of the leaves (Figures 2, 3, and 4), and can spread even farther to the stems. As the symptoms develop, infected leaves may gradually curl downward, from pale yellow or brown (Figure 5), die, and fall off. Under severe conditions, the entire leaves and plants could be covered by white cottony mycelial growth of the fungus (Figure 6). Symptoms on infected leaves may vary with bean varieties, but powdery mildew may cause the leaves to be twisted, buckled, or distorted. The powdery mildew fungus usually does not grow on bean pods except pea pods (Davis et al. n.d.). However, powdery mildew spots can develop on snap bean pods (Pernezny and Stall 2005). The development of powdery mildew symptoms is not often observed on pole bean in Miami-Dade County, but it is apparent on both Italian bean and long bean (dark green type) (Figures 7 and 8) grown under the same conditions. Severely infested plants may have reduced yields, shortened production periods, and even completely die (Figure 8). Severe symptoms of powdery mildew infection can also be seen in snap beens when humidity is high. Severe symptoms of powdery mildew infection can also be seen in snap beens when humidity is high. Severe symptoms of powdery mildew infection can also be seen in snap beens when humidity is high (Figure 9).

Figure 1.

Early stage of powdery mildew development on Italian bean.

Credit:

Qingren Wang, UF/IFAS

Figure 2.

Powdery mildew development on pole bean.

Credit:

Qingren Wang, UF/IFAS

Figure 3.

Powdery mildew on snap bean.

Credit:

Qingren Wang, UF/IFAS

Figure 4.

Powdery mildew on long bean.

Credit:

Qingren Wang, UF/IFAS

Figure 5.

Severe development of powdery mildew on Italian bean.

Credit:

Qingren Wang, UF/IFAS

Figure 6.

Development of powdery mildew on pole bean plants without full sunlight.

Credit:

Qingren Wang, UF/IFAS

Figure 7.

Powdery mildew development on Italian bean pod.

Credit:

Qingren Wang, UF/IFAS

Figure 8.

Powdery mildew development on long bean pods.

Credit:

Qingren Wang, UF/IFAS

Figure 9.

Severe infection of powdery mildew on snap beans.

Credit:

Qingren Wang, UF/IFAS

Infection and Spread

The fungus of powdery mildew grows as thin layers of mycelium (fungal tissue) on the surface of the affected leaves. Spores, which are the primary means of dispersal, make up the bulk of the visible white, powdery growth. Powdery mildew spores can be easily carried by wind to new hosts. The spores can germinate and infect beans in the absence of free water. Powdery mildew growth generally does not require moist conditions (Davis et al. n.d.); however, increased humidity can increase the severity of the disease (Pernezny and Stall 2005). Moderate temperatures (60°F to 80°F) and shady conditions generally are the most favorable environmental factors for the development of powdery mildew (Figure 6). Strong and direct sunlight with high temperatures (over 90°F) can noticeably reduce the incidence of the disease.

Management Practices

Preventive Practices

Prevention is the optimal method for controlling powdery mildew. Growing resistant varieties or cultivars of beans wherever available, planting in full sun, and following good cultural practices can adequately control powdery mildew in most cases.

Cultural Practices

Plant in sunny areas as much as possible and provide good air circulation. Overhead watering may help reduce powdery mildew, because spores might be washed off the plants. However, such a measure may contribute to other pest management concerns.

Fungicide Application

In vegetable fields, the least-toxic fungicides may be used to prevent, protect, or eradicate powdery mildew. Apply protective fungicides to highly susceptible varieties before the disease appears, and utilize eradicative fungicides at the earliest signs of the disease. Once powdery mildew growth becomes prevalent, control with any fungicide can be difficult. Below are recommendations for fungicides for control of powdery mildew on beans. Make sure to follow the application instructions on the labels.

Pernezney K., and W. M. Stall. 2005. Powdery Mildew of Vegetables. PP-14. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Tables

Table 1.

Fungicide recommendations for powdery mildew control on beans

Fungicide

(active ingredients)

Fungicide Group

REI* (hours)

OMRI-listed**

Remarks

Actinogrow,

Actinovate

(Streptomyces lydicus WYEC 108)

1 h

Yes

Armicarb 100 Fungicide

(potassium bicarbonate)

4 h

No

Kaligreen Fungicide (potassium bicarbonate)

4 h

Yes

Milstop (potassium bicarbonate)

1 h

Yes

Confine Extra,

Kphite 7LP AG

(phosphorous acid)

4 h

No

Double Nickel LC

(Bacillus amyloliquefaciens strain D747)

4 h

Yes

Fungi-Phite,

Fosphite Fungicide,

Rampart

(potassium phosphite)

4 h

No

Oxidate

(hydrogen dioxide)

1 h (for enclosed areas)

No

Regalia SC

(extract of Reynoutria sachalinensis)

P

4 h

Yes

Sonata,

Sonata Fungicide

(Bacillus pumilus strain QST2808)

4 h

Yes

Sporatec

(clove oil, rosemary oil, thyme oil)

Yes

Cueva

(copper occanoate)

M1

4 h

Yes

Kumulus DF Fungicide-Acaricide, Micro Sulf, Microfine Sulfur, Microthiol Disperss, Sulfur 90W, Wettable Sulfur, Yellow Jacket Dusting Sulfur, Yellow Jacket Wettable Sulfur (sulfur)

M2

24 h

Yes—for Kumulus, Microfine Sulfur, MicroSulf, Microthiol Disperss

Do not apply during periods of warm weather to avoid phytotoxicity

Top Cop with Sulfur (basic copper sulfate + sulfur)

M1 & M2

24 h

No

Fontelis,

Vertisan (penthiopyrad)

12 h

No

Headline,

Headline SC (pyraclostrobin)

12 h

No

Priaxor

(fluxapyroxad + pyraclostrobin)

7 & 11

12 h

No

*REI: Restricted Entry Interval

**OMRI: Organic Materials Review Institute. OMRI-approved products can be used for organic production.

Footnotes

This document is PP311, one of a series of the Department of Plant Pathology, UF/IFAS Extension. Original publication date May 2014. Revised June 2017. Visit the EDIS website at http://edis.ifas.ufl.edu.

Qingren Wang, commercial vegetable Extension agent and pesticide trainer, UF/IFAS Extension Miami-Dade County; Shouan Zhang, associate professor, Department of Plant Pathology, UF/IFAS Tropical Research and Education Center; and Teresa Olczyk, director, UF/IFAS Extension Miami-Dade County; UF/IFAS Extension, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office.
U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.

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