Downy mildew (Plasmopara viticola) affects many plants and appears as yellow to white patches on the upper surfaces of older leaves. On the undersides, these areas are covered with white to grayish, cotton-like fungi. These “downy” masses are most often noticed after rain or heavy dew and disappear soon after sunny weather resumes. As the disease progresses leaves may eventually turn crisp and brown and fall off even though the plant has ample water.

Downy mildew occurs in cool, moist weather usually in early spring or late fall. Spore production is favored by temperatures cooler than 65˚F. and by relative humidities approaching 100%. This disease overwinters on plant debris and in the soil. Fungal spores can be carried by insects, wind, rain or garden tools.

Contents

Treatment

The best way to prevent downy mildew is to avoid the environmental conditions that favor the disease.

  1. Prune or stake plants and remove any weeds to improve air circulation.
  2. Water in the early morning hours, or use a soaker hose, to give the plants time to dry out during the day.
  3. Keep the ground under infected plants clean during the fall and winter to prevent the disease from spreading.
  4. Remove and destroy any plants with serious infection (see Fall Garden Cleanup).
  5. Choose resistant varieties whenever possible.
  6. Downy mildew is comparatively easy to control on most plants when the foliage and fruit are kept protected by a copper spray. Begin treatments two weeks before disease normally appears or when weather forecasts predict a long period of wet weather. Alternatively, begin treatment when disease first appears, and repeat at 7-10 day intervals for as long as needed.
  7. The systemic action of Organocide® Plant Doctor moves throughout the entire plant to treat common disease problems. Mix 3/4 tsp per gallon of water and apply to foliage. Spray to run-off, as required for disease control.

Downy mildew and powdery mildew are often confused. If you’re looking for how to get rid of powdery mildew, check here. But downy mildew is a completely separate problem, even if it might look similar. Both are common issues in the garden.

Today, we’ll go over everything there is to know about downy mildew. I’ll tell you how to treat it, but more importantly, I’ll help you determine how to prevent it so you don’t get it again!

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Helpful Products For Downy Mildew Control:

  • Neem Oil
  • Bonide Copper Fungicide
  • Organocide Plant Doctor
  • Bonide Fung-Onil

Downy Mildew Overview

Common Name(s) Downy mildew, downy mold
Scientific Name(s) Peronospora, Bremia, Plasmopara, and Basidiophora genuses
Family Peronosporaceae
Origin Worldwide depending on species
Plants Affected Extremely wide range of plants, including ornamentals and food crops.
Common Remedies Neem oil as a preventative or light-infection treatment. Copper fungicides or phosphorus acid fungicides can treat larger scale infections. Prevention is the best cure.

What Is Downy Mildew?

Downy mildew spotting on the upper surface of cucurbit leaves. Source: NYSIPM Image Gallery

Anyone who lives in a humid climate is likely experienced with treating downy mildew. With a heavy preference for wet surroundings, this fungal mold lives in the soil and later colonizes the leaves of plants, causing severe damage.

This downy mold is a member of the Peronosporaceae family. There are multiple variations of Peronosporaceae that make up the water mold family, each with slightly different plant preferences.

Peronospora and Plasmopara species are particularly dangerous for most gardeners. Bremia and Basidiophora can also become problematic, especially on ornamental plants.

Technically, Peronosporaceae are not a true fungus. Research has shown that downy mold has similar traits to fungal development, but they are oomycete microbes, not fungi. For gardening purposes, we often treat it as if it were a fungal disease because of how they spread.

Downy mildew grows on and into the leaves of their preferred plants, living off the plant’s water supply. While small amounts of downy mildew aren’t likely to do major harm to your garden, it’s a symptom of a larger water-related issue.

Unfortunately, many food crops and some flowers and shrubs are susceptible to this fungal infection.

While it rarely causes damage to more than 25% of a given crop field in agricultural surroundings, in the home garden it can become quite destructive. It’s important to take it out before it can spread too widely!

Life Cycle of Downy Mildew

A closeup of downy mildew on the upper part of a cucurbit leaf. Source: NYSIPM Image Gallery

In most gardens, downy mildew begins as a fungal spore in the soil. These spores are windblown, so can appear almost anywhere.

Any moisture which splashes soil onto the leaves of plants can help move the spore onto your leaves. Once there, it will latch into the underside of the leaf. It penetrates the leaf’s surface with its mycelia and begins to grow.

As the downy mildew grows, it causes spotting on the upper side of leaves. Underneath the leaf, a mat of sporangia forms, ranging from white through grey and even purplish in color. These release more spores into your yard.

Spores which land on leaves will begin to colonize the leaves if the conditions are right. However, they are only viable for a short period of time.

Others will be dispersed into the soil via plant debris rotting away, just waiting for the chance to get into a juicy plant. Spores can overwinter in the soil and reappear the moment infected soil gets onto plant leaves. Soil may be infected for up to five years.

The mildew’s mycelia, or fungus-like root system, can spread throughout your plant’s stems. New growth may already be infected by downy mildew if the plant’s had issues with it before.

Plants which are fully infected by Peronospora’s mycelia may produce damaged fruit or have visible problems with newly-formed growth.

In hops, the cones themselves can become colonized, essentially destroying them before they can fully form. Cucumbers can have a similar problem with young fruit, especially if they aren’t put on a trellis.

Needless to say, it’s essential to remain vigilant and watch for damage, especially in places that had issues with downy mildew in the past!

Evolution of Downy Mildew

The underside of a basil leaf with downy mildew. Source: Starr Environmental

Unfortunately, like so many other plant diseases, this fungus-like growth can adapt and change over time.

In 2007, basil downy mildew (Peronospora belbahrii) was first identified in the U.S. By 2012, it was reported throughout the United States and into Canada.

It’s believed that this form was an evolution of an old strain which was only reported in Uganda in 1933, and that it evolved into a new and more aggressive variation.

As there are many other causes for yellowing of basil leaves, it often isn’t properly treated, which can spell doom for the basil plant. However, the slightly purplish-grey spores on the underside of leaves are an indication of this problem.

Worse yet, it was discovered that it can be transmitted through infected seed, which is likely how it managed to make its way around the world.

Hot water sterilization of the spores isn’t viable. Some seed companies are sterilizing the exterior of basil seeds via steam treatments now with good effect.

While this version impacts basil, there are other new variations of downy mildew.

Peronospora lamii was discovered on coleus in 2005 in New York, and by late 2006 had spread throughout the rest of the country.

Downy mildew of impatiens, Plasmopara obduscens, has developed rapidly since 2011 on the eastern coast as well. While primarily centered in the area around Massachusetts, it has started to spread to other states via infected plant sales.

Difficult as it is to deal with, it’s good to be aware that this plant disease does in fact regularly evolve to impact other forms. This helps us in research to work towards an eventual complete cure.

Symptoms of Downy Mildew

I’ve roughly discussed the symptoms that appear from downy mildew, but let’s get into more detail.

Early Downy Mildew Detection

Downy mildew on sorghum. Source: CIMMYT

A colony of downy mildew initially forms on the underside of the leaf. This can give the leaf’s underside a whitish, purplish, bluish or bluish-gray speckling.

Once the spore has locked into the leaf structure, the top of the leaf begins to yellow in spots to mirror the location of the sporangia underneath the leaf. The sporangia itself is white to bluish-white in color as it begins to create new spores.

The downy mildew may further colonize the leaf it’s on and spread to form a fuzzy-looking coating of sporangia.

Later Downy Mildew Progression

This squash has downy mildew and powdery mildew at the same time. Notice the yellowed leaf, plus the whitish dusting on the upper surface. Both diseases spread in wet, cool conditions. Source: lis chere

As leaf spots die out, the mildew sporangia turns grey and powdery, and spores release to fall on other leaves. Branches and leaves may become distorted or die off entirely.

From the time of infection to the time that new spores form usually takes 7-10 days, but can happen in as little as 4 days if the conditions are right. 85% or higher humidity at the soil’s surface tends to speed the progression.

Left untreated or constantly reinfected, the plant may die. However, it’s more common that a plant will be colonized by the mycelia.

This makes the plant a possible danger to others in the garden, as an outbreak could possibly recur. However, effective deterrent measures will reduce that likelihood. Control measures may wipe out the infection.

Controlling Downy Mildew

Downy mildew on coleus. Notice the deformed center growth. It’s also got a pest problem! Source: bill barber

It’s tricky to control this particular plant disease, and in part that’s because we’re still learning a lot about it. While commercial growers make use of heavy chemical sprays to eliminate it,

Downy Mildew Treatment

Once downy mildew has begun to colonize a plant, it’s essential to strike quickly to try to lessen its ability to do severe damage to the rest of your garden.

Our old friend, neem oil, should be our first line of defense. Applied early on, it can stop downy mildew from colonizing a plant. However, it won’t kill any of the disease that’s already gotten into the plant’s stems.

I prefer to use a copper fungicide like Bonide Copper Fungicide to battle downy mildew. It’s an organic treatment which can be applied every 7-10 days or until the plant has visibly recovered from its current outbreak.

For more severe issues, Organocide Plant Doctor should be used. This broad-spectrum systemic fungicide can be used as a soil drench around the plant as well as being sprayed onto plant surfaces. It works its way through the plant’s tissues, dealing with a variety of possible diseases.

It’s important to note that in neither case are these fungicides guaranteed to eliminate downy mildew. Both are considered to have moderate to good effect on this disease, but there are still strains which persist.

However, copper based fungicides are widely used throughout the Pacific northwest as both a preventative and a fungicide, and phosphorous acid treatments like Organocide are beginning to become popular.

While there are chemical treatments which are proving effective against downy mildew, they tend to be hard for the average home gardener to get. These are generally used in large-scale agriculture and can be dangerous to apply without proper safety measures.

Chlorothalonil fungicides such as Bonide Fung-Onil are typically the only chemical variations that home gardeners have ready access to. This can work, so if neither of the organic options work, it may be an option for you.

Overall, the best treatment for downy mildew is prevention.

Preventing Downy Mildew

A closeup of the underside of a basil leaf with downy mildew. Source: Starr Environmental

When preventing downy mildew, the first place to look is at your watering habits.

Only water in the early morning hours. This ensures that your plants have time to dry out during the day.

Keep them pruned to allow for good air circulation, as this also helps any wet foliage to dry. Remove visibly-damaged leaves once you see them to try to prevent further spread into the plant’s system.

Stake plants or secure them in a fashion where air can still get around to all surfaces of the plant. If using a cage, thin out some branches to ensure you have full air circulation. This is especially important with plants like tomatoes that can form dense growth.

Using a soaker hose instead of a standard spray nozzle will reduce the amount of splashing of soil onto leaves. This can protect your plant’s foliage from damaging spores.

If growing in a greenhouse, reduce the ambient humidity by opening vents and adding fans to provide airflow. Doing so will also help prevent problems with powdery mildew and some pests, like whiteflies.

Many varieties of plant are available now which are resistant to downy mildew. When at all possible, choose those varieties to avoid the problem entirely.

Ensure the area beneath your plants is kept clean and free of debris. You can mulch if necessary, but avoid buildup of fallen leaves, especially those which may be harboring spores.

Control weeds around your plants to reduce airflow issues at the base of your plants. Removing weeds as they appear is always a good idea, as it reduces the appearance of weeds in subsequent years.

Finally, severely diseased plants should be removed and destroyed for the safety of the rest of your garden.

While nobody wants to lose a prized plant, the long-term effects of downy mildew in the garden can be catastrophic to the rest of your plants. It’s better to remove very diseased plants just to ensure others don’t get it.

The upshot of all of this is that if you practice good garden maintenance regularly, you likely won’t have severe problems from downy mildew.

Once the weather warms up, it’s far less likely to appear, which may give you the break you need to recover from the cooler months of the year!

Frequently Asked Questions

Downy mildew on maize tends to cause yellow streaking down the fibrous leaves. Source: CIMMYT

Q: What plants get downy mildew?

A: It depends on the type of downy mildew, and there are a lot.

Whether you grow ornamental plants or fruits and vegetables, there are forms of downy mildew which might develop in your garden. I’m currently battling downy mildew on some roses, for instance. And last year I had problems with downy mildew on cucumbers, which is a completely different species.

A number of varieties are regular colonizers of greenhouse-grown plants, both edible and ornamental. Downy mildew can appear indoors as well, so keep a watchful eye on your houseplants!

Researchers are still learning a lot about these oomycetes and how they strike. And as they can and have evolved in the past, there’s likely to be new strains that appear. It’s best to act to prevent these problems before they can take hold.

On the bright side, there’s likely to be advances in science which lead us towards better treatments in time. Until then, keep a watchful eye out for the symptoms of downy mildew, and practice preventative measures!

Hopefully you now understand downy mildew a lot better than you did before. Have you ever had this common infection in your garden? What did you do to treat it? Let us know in the comment section!

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Downy Mildew

Downy mildew, caused by a fungal organism, and is most destructive to cucumber and cantaloupe, though all cucurbits are susceptible. Symptoms first appear as pale green areas on the upper leaf surfaces. These change to yellow angular spots. A fine white-to-grayish downy growth soon appears on the lower leaf surface. Infected leaves generally die but may remain erect while the edges of the leaf blades curl inward. Usually, the leaves near the center of a hill or row are infected first. The infected area spreads outward, causing defoliation, stunted growth, and poor fruit development. The entire plant may eventually be killed.

The fungus is easily carried by wind currents, rain splash, farm implements, or the hands and clothes of farm workers. It is favored by cool to moderately warm temperatures, but tolerates hot days, although long periods of dry hot weather can stifle the spread of the disease. Unlike powdery mildew, it requires humidity to flourish. Therefore, downy mildew is most aggressive when heavy dews, fog, and frequent rains occur.

Downy mildew does not overwinter beyond Mexico and the southernmost tier of U.S. states, where it survives on cultivated and wild cucurbit plants. Spores are blown northward each season as favorable seasonal conditions advance. As a result, the disease is most common on late summer plantings and is infrequently seen on spring cucurbits.

Monitor Plants For Downy Mildew

Keeping abreast of when, and how severely, downy mildew is occurring in your area can help you determine the proper time to treat it. The North American Plant Disease Forecast Center is an online forecasting network that tracks outbreaks of downy mildew from March through the end of the growing season.

Data is posted twice weekly. Growers can use the website to identify areas where an outbreak is reported, as well as spore movement in that area. The site also offers information on control measures, photos, and more. Because the website relies on growers and others to report the outbreaks, it isn’t comprehensive or foolproof. It is, however, a useful monitoring tool.

One of the principal means of managing downy mildew in cantaloupe and cucumber is the use of genetically resistant cultivars. Resistance has not been developed in other cucurbits, though some squash varieties like Super Select and Zucchini Select are considered to be tolerant, as are cucumber varieties like ‘Poinsett’ and ‘Galaxie’. The Virginia Extension publication Downy Mildew of Cucurbits identifies other resistant cucumber cultivars. Gardeners are advised to contact Cooperative Extension and local seed suppliers for assistance in selecting resistant varieties that also perform well in their location.

Cultural Controls

Because this disease is carried to most fields on light winds, cultural practices like crop rotation and sanitation have a limited effect on the incidence of downy mildew. Still, there are several things that growers can do to suppress the disease. Growing vigorous plants, capable of withstanding or repelling disease onslaughts, is the first step. This involves careful irrigation and soil fertility management.

Good soil fertility management can often be backed up with foliar fertilization, which some growers believe can assist in pest resistance.

Further cultural considerations include selecting growing sites with good air drainage, full sunlight, and low humidity. Using drip irrigation, or scheduling overhead irrigation to avoid excessive leaf wetness, will also reduce disease incidence. When detected early, disease spread might be slowed somewhat by removing and destroying infected plants, and by taking care not to transport the disease by hand or on infected tools and equipment.

Fungicides

Neem oil is a botanical fungicide; it is a multi-purpose insecticide, miticide, and fungicide labeled for control of both downy and powdery mildews on cucurbits.

Neem products, once considered largely benign to beneficial insects, have demonstrated some negative impacts. Washington State research has found neem to be toxic to lady beetles, especially in their early larval stages. Being an oil formulation, neem can also harm bees and should be applied when they are not active in the field. Therefore, while neem oil is suitable for organic production, it should not be used without clear need and plenty of caution.

Peroxides Organic growers and others in alternative agriculture have often mentioned hydrogen peroxide (H2O2) as a disease preventive for crops. While documentation on the use of food- and/or pharmacy-grade peroxide in managing plant diseases is sketchy, BioSafe Systems has recently released a peroxigen formulation under the name of OxiDate™, which is labeled as a broad-spectrum bactericide and fungicide. Downy and powdery mildews of cucurbits are among the diseases it is said to control. Among the listed benefits are biodegradability, little to no phytotoxicity, and the ability to kill fungal spores on contact.

Although the Organic Materials Review Institute (OMRI) had previously approved OxiDate for organic production, it removed the product from its listing in spring 2002 because of non-compliance with federal regulations. If reformulated, it may be approved again in the future.

Downy mildew of cucumber, melon and squash

What causes downy mildew

Downy mildew does not create viable oospores (thick-walled resting structures) that would allow it to survive Minnesota’s harsh winter. It cannot survive on plant debris and only grows on living plant tissue. For downy mildew to occur in Minnesota, air currents must blow the spores.

In eastern states with similar conditions, downy mildew does not arrive until the end of the growing season, often in August. However, downy mildew has arrived in the Midwest earlier in the season. This early infection is possibly due to a change in the pathogen’s biology or due to greenhouse production of cucurbits that allows the pathogen to overwinter on living plants.

Downy mildew can start an infection in a wide range of temperatures (41-86° F) but is most severe from 59-68° F. The pathogen needs moisture on the leaf surface in order to germinate and start a new infection.

Under humid conditions, downy mildew rapidly reproduces and spreads, resulting in severe crop damage. The pathogen can move on air currents, splashing water and on the tools and hands of workers.

There are several strains of downy mildew that attack different crops. It is possible to see a healthy pumpkin field alongside a severely diseased cucumber field for this reason.

Cucurbits, Downy Mildew

Cultural Controls & Prevention:

The main means of control are fungicide applications, the use of resistant cultivars, and cultural practices. Maximum control can be achieved only with a combination of these measures.

  • Monitor disease occurrence and weather forecasts at http://cdm.ipmpipe.org/.
  • Maximize the distance between cucurbit fields to limit potential inoculum sources.
  • Many commercial cultivars of cucumber have good levels of resistance to Downy Mildew. Watermelon and melon cultivars are available with low levels of resistance. Squash and pumpkin cultivars are resistant to some pathotypes but are very susceptible to compatible pathotypes. See variety tables posted at http://vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm.
  • Use plant spacings which reduce the density of the plant canopy. Avoid overhead irrigation. Both these practices are aimed at minimizing the length of leaf wetness periods.
  • Choose planting sites with good air movement and without shading. Avoid overhead irrigation in early morning when leaves are wet from dew or late in the day when leaves will not have an opportunity to dry before dew forms.
  • Apply broad-spectrum protective fungicides before detection and systemic narrow-spectrum fungicides when downy mildew occurs early in crop production.

Chemical Controls & Pesticides:

For Current information on disease recommendations ins specific crops including information on chemical control & pesticide management, please visit the New England Vegetable Management Guide website.

Crops that are affected by this disease:

  • Cucumber, Muskmelon, and Watermelon
  • Pumpkin, Squash, and Gourds

Powdery Mildew vs Downy Mildew

When dealing with disease in the garden, it is always best to know what you’re up against before you start treating. Keeping symptoms straight can be confusing, and keeping names straight adds another layer of deduction. Downy mildew or powdery mildew—while both are called “mildew,” there are differences in how they look on your plants and how you treat the disease.

So how do you tell the difference at first inspection?

Powdery Mildew vs Downy Mildew

Powdery Mildew Downy Mildew
Fungal spots have a circular white appearance Fungal spots have an angular and gray appearance
Fungus can be anywhere on the leaf surface Fungus is limited by leaf veins
Leaves yellow after fungus has been present for a while Leaves may yellow before the presence of fungus is even evident

Powdery Mildew

Powdery mildew is a serious fungal disease that attacks a wide range of plants. While nonresistant crapemyrtle trees are especially susceptible, this disease can also attack flowers like zinnias, roses, and gerbera daisy, as well as edibles like squashes, strawberries, and tomatoes.

One of the most noticeable symptoms of powdery mildew is the spots or patches of white powder that can be found usually on the leaves. These spots enlarge and become a dusty white or gray coating. Symptoms of powdery mildew usually appear late in the growing season, especially when nights are cooler and there is little rain but humidity is high.

Powdery mildew thrives in moist conditions—including humid weather—shady areas, and areas where plants are crowded. This fungus robs your plant of nutrients and will result in stunting of leaves, buds, and fruits.

The symptoms of downy mildew can be a bit misleading, because at first glance, they’re similar to the symptoms of nutritional deficiencies or even virus diseases. Young plants and new leaf growth are most vulnerable to this fungal disease. In cases of downy mildew infection, leaves become yellowish or speckled, leaf edges may curl downwards, and faint gray fuzz may appear on the undersides of leaves.

As the disease progresses, white-ish to gray, downy-looking growth will be visible along the undersides of leaves, sometimes flecked with tiny black spots. On the upper surface, yellow leaf spots will appear angular in shape and bound by leaf veins, sometimes creating a “quilted” look. Eventually leaves and flowers drop, leaving you with a plant of mostly stems.

Basils, impatiens, melons, and viburnum are particularly susceptible to downy mildew. There are certain conditions that are more ideal for downy mildew to flourish: high humidity, cool temperatures, and plant crowding.

Treatments

You can treat powdery or downy mildew by spraying the infected plants with a fungicide labeled specifically for the type of mildew you are dealing with, be it powdery or downy. As always, it is important to follow all label instructions.

A Pound of Prevention

While treatments exist, prevention is always the best way to deal with plant diseases. Preventing powdery or downy mildew can be accomplished by following common recommendations for managing fungal diseases. Susceptible plants should be planted in sunny locations when possible, and regardless of whether they’re planted in the sun or shade, air should be able to move freely around plants (i.e., no crowding). Additionally, if you are providing your plants with supplemental water you should water only the roots—not the foliage—in the morning. Watering only the roots is important to avoid getting the foliage wet, and doing so in the morning allows any foliage that does get splashed an opportunity to dry out.

UF/IFAS Publications

  • Downy Mildew of Basil in South Florida
  • Downy Mildew on Impatiens
  • Downy Mildew on Viburnum
  • Management of Cucurbit Downy Mildew in Florida
  • Rose Pests and Diseases in Florida

Also on Gardening Solutions

  • Powdery Mildew

Downy mildew

Downy mildew, disease of plants, especially in cool humid regions, caused by several funguslike organisms of the phylum Oomycota. White, gray, bluish, or violet downy patches of mildew form mostly on the undersides of leaves in damp weather. Pale green to yellow or brown areas usually develop on the upper leaf surface opposite the downy growth. Affected leaves often wilt, wither, and die early. Stems, flowers, and fruits are sometimes infected. Seedlings may wilt and collapse. Garden plants, bush fruits, vegetables, and certain trees, shrubs, field crops, and weeds are susceptible. Downy mildew is commonly caused by members of the oomycete genus Sclerospora, but other pathogens include species of Bremia, Peronospora, Phytophthora, Plasmopara, and Pseudoperonospora.

downy mildewPearl millet (Pennisetum glaucum) affected by downy mildew caused by the fungus Sclerospora graminicola.© Erick Boy

Downy mildew can be avoided by rotating annual flowers and vegetables and by avoiding overwatering, overcrowding, and poorly drained soil. Other avoidance measures are growing resistant varieties, sowing disease-free seed, removing diseased parts and crop refuse, eliminating weeds, and maintaining balanced soil fertility. The application of copper or either of the fungicides maneb or zineb is effective against many downy mildews, but the amount of residue on vegetables must be considered.

Downy Mildews of Ornamental Plants

Downy mildew diseases can cause serious losses in many floriculture and greenhouse crops. In recent years, new downy mildew diseases have been discovered on coleus, basil, and garden impatiens. Downy mildews present a challenge to growers both because they can be present but not obvious, and because they are difficult to control with fungicides once established. The pathogens are very different from powdery mildews- they attack different plants under very different environmental conditions, and are controlled by different classes of fungicides. Downy mildew diseases are caused by oomycetes, a group of fungus-like organisms that also includes Pythium and Phytophthora species.

Most of the downy mildew fungi are very host specific and infect only one plant family. Pathogens include species of Peronospora, Pseudoperonospora, Bremia, Plasmopara, and Basidiophora. Downy mildews infect almost all ornamental plants as well as some indoor plants. Greenhouse crops reported to have downy mildew diseases include snapdragon, Salvia, alyssum, pansy, sunflower, rosemary, Primula, Osteospermum, Impatiens walleriana, coleus, statice, Verbena, ornamental cabbage, basil, and Cineraria. Perennial hosts include Aster, Buddleia, Coreopsis, Geranium (not Pelargonium), Geum, Gerbera, Lamium, Delphinium, Veronica, and Viola.

For more information about downy mildews of basil, coleus, and impatiens, please see individual fact sheets.

Signs and Symptoms

Downy mildews begin as yellow to light green, irregular leaf lesions that may become purple to dark brown and are delimited by leaf veins. Under humid conditions, the fungus sporulates on leaf undersides, producing a downy growth which may be white, tan, gray, or purple.

Downy mildews vary in aggressiveness, and symptoms will vary among host species. As the disease progresses, lower leaves often wither and fall off. The fungus grows both locally and systemically and it can escape notice until conditions are right for sporulation. Systemic symptoms can include stunting, leaf distortion, epinasty, shortened internodes, and a decrease in the quantity and quality of flowers.

Disease Cycle

Downy mildews reproduce through special structures called sporangia that develop on leaf undersides. Air currents and splashing water dislodge these sporangia and spread the pathogen to nearby healthy plants. The period between infection and symptom development can vary from days to weeks and is dependent largely upon environmental conditions. Some downy mildews produce long lasting survival spores called oospores that can persist in the soil indefinitely. Others may survive in weed hosts near greenhouses. Downy mildews may also be seed-borne.

Downy mildews thrive under the cool, wet conditions of spring and fall. The pathogens need wet leaves and high relative humidity (greater than 85 %) to cause disease; under these conditions, disease can occur rapidly and is difficult to control.

Cultural Management

Cultural management practices for downy mildew diseases consist of excluding the disease from the greenhouse, managing relative humidity, and strict sanitation.

  • Plants should only be purchased from reputable and trusted sources and should be thoroughly inspected before their introduction into production areas.
  • Keep relative humidity below 85 % to inhibit sporulation on infected plants and stop germination of spores on healthy plants. This can be done by venting and heating, which fills the greenhouse with warm, drier air. The use of fans in greenhouses improves horizontal air flow and prevents cold spots where condensation develops from occurring. The density of the plant canopy should be reduced by spacing plants to allow for maximum air circulation and sunlight availability.
  • Scout regularly for early symptoms of the disease. Diseased plants, infected debris, and soil should be promptly removed and destroyed. Asymptomatic plants immediately surrounding diseased plants should also be removed and destroyed.
  • Control weeds inside and outside of greenhouses, as they may also serve as hosts for downy mildews.
  • Avoid using downy mildew- prone plants in ornamental beds near greenhouses.
  • Growing downy mildew resistant cultivars is advisable; however, very few of these are currently available.

Chemical Management

Fungicides will be ineffective unless used in conjunction with good cultural management practices. Fungicides should be applied preventively for maximum efficacy. The highest levels of control are obtained when fungicides are applied preventively. Many products which give excellent control preventively provide little to no control when used as curatives.

The use of fungicides to manage downy mildew diseases has become more difficult because of the development of fungicide resistant strains. Practices for the prevention of fungicide resistance development include:

  • beginning a regular spray program with a protectant fungicide
  • limiting the number of applications of particular fungicides applied per season
  • rotating fungicide applications among FRAC groups (modes of action)
  • applying systemic chemicals in combination with a protectant

A full list of fungicides registered for use on downy mildew of greenhouse ornamentals can be found in the New England Greenhouse Floriculture Guide: https://ag.umass.edu/greenhouse-floriculture/publications-resources/new-england-greenhouse-floriculture-guide

Plant Health February 2017

Downy Mildew: Conditions and Controls
By Kathie E. Kalmowitz

6. What plant species are recognized as susceptible to downy mildew?

In annual plant production, Impatiens walleriana species and coleus have been studied and documented most frequently in recent years. The downy mildew species infecting these plants are different and the environmental conditions for optimal infection differ as well. Other susceptible annuals include pansy and snapdragon. Susceptible perennials include aster, coreopsis, geum, lupine, rudbeckia and veronica. Downy mildew can be a major production disease in woody plant production, particularly in Viburnum spp. Downy mildew is host specific with roses, and occurs from stems to all plant parts. Preventative protection is required.

As winter progresses and temperatures and humidity fluctuate in your operation, be aware that conditions favoring downy mildew can be present.

Always read and follow label directions.

Note: Stature, Pageant and Intrinsic are registered trademarks, and Orkestra is a trademark of BASF Corporation. Adorn is a registered trademark of Valent Corporation. Strike Plus is a registered trademark of OHP. Segovis is a registered trademark of Syngenta Crop Protection.

Kathie E. Kalmowitz

Kathie E. Kalmowitz is a technical specialist for BASF Turf and Ornamentals. For more information about BASF and downy mildew control, go to www.basf.com.

Features

Crop Protection
Inputs
Downy Mildew vs. Powdery Mildew

Downy mildew and powdery mildew are diseases that may look similar at first glance, but are actually very different.

Both usually affect only the leaves, but downy mildew can be identified from the fungal layer on the underside of leaf, that develops in moist weather and is accompanied by leaf spots on the top of the leaf. Powdery mildew causes white, powdery, fungal growth in the absence of any leaf spotting.

Downy Mildew
Under humid, cool conditions, downy mildew spores appear in large numbers on the lower surfaces of leaves, growing in tree-like formation on branched fruiting structures. In the presence of water from recent irrigation, rain or heavy fog, the spores will germinate within four hours. In fact, sporulation on leaf surfaces may occur in three days under ideal conditions of 18°C temperatures. Below 5°C the spores won’t germinate, and they’re killed by exposure to 27°C temperatures for 24 hours; dry, warm, clear days inhibit spore production. Roses, for example, are unaffected by downy mildew when humidity is less than 85% and unlike powdery mildew spores, which are spread by air movement; downy mildew is spread by splashing water.

Downy Mildew on Snapdragon

Symptoms of Downy Mildew
• During moist weather, the undersides of the spots are covered with a layer of fungus that can be white to purplish to almost black.
• The spot is brighter on the upper leaf surface than on the lower leaf surface.
• The spots may turn brown or may remain yellow.
• The entire leaf dies quickly.
• Usually older leaves are affected first, then younger leaves. Petioles remain green after the leaf blade dies.

Prevention of Downy Mildew
• Promote good air circulation and leaf drying; use wide spacing between plants.
• Avoid overhead irrigation.
• Water early in the day to allow leaves to dry thoroughly.

Control of Downy Mildew
NEW! Acrobat 50WP from BASF which contains the active ingredient dimethomorph, is now registered for the control of downy mildew on greenhouse ornamentals. It is not registered for use on cut flowers.

Powdery Mildew
Powdery mildew is caused by the fungi Erysiphe cichoracearum and Sphaerotheca fuliginea. It is favored by moderate temperatures. These fungi are unusual in that they do not require the leaves to be wet for them to infect the leaf, but rather the humidity must be high. They produce chains of spores when the humidity is low.

Powdery Mildew on Poinsettia

Symptoms of Powdery Mildew
• A whitish, powdery, fungal growth is present on the upper surface of plant parts.
• Fungal growth often starts on the shaded undersurface of the crown leaves.
• Severely affected leaves can become dry and brittle, or can wither and die.

Prevention of Powdery Mildew
• Employ good air circulation and low humidity.
• Separate new plantings from old plantings.
• Control weeds that can host the fungi.

Control of Powdery Mildew
NEW! Pristine WG Fungicide from BASF which contains the two active ingredients boscalid and pyraclostrobin, has received a full registration for the control of powdery mildew in greenhouse tomatoes and greenhouse cucumbers.
Registered Control Products
(products in bold are marketed and distributed by Engage Agro)

Downy Mildew
• Acrobat 50WP – systemic

Powdery Mildew
• Actinovate
• Banner Maxx – Outdoor only
• Compass O
• Daconil Ultrex – contact
• Folpan
• Funginex – systemic
• Meltatox
• Milstop
• Nova
• Phyton 27 – contact and systemic
• Pristine WG – (greenhouse vegetable only) – systemic
• Rhapsody
• Senator 70 WP – locally systemic

For more general information about THE LOUPE, contact [email protected]

Downy mildew of grapevines

Symptoms

Downy mildew attacks all green parts of the grapevine.

Leaves

The symptoms vary with leaf age. On young leaves (in spring), the disease will appear on the upper surface as small yellow spots referred to as oil spots. They are about 10mm diameter often with a chocolate halo (Figures 1 and 2). These spots tend to grow to about 50mm diameter as they mature and the halo fades. As they enlarge they may appear to cover most of the leaf, especially if there is more than one spot on the leaf (Figure 3). On red varieties the oil spot can appear red (Figure 4).

Figure 1 A single oil spot on a grapevine leaf Figure 2 Downy mildew oilspots on a leaf with a chocolate covered halo that fades as it ages Figure 3 Multiple infections can lead to several oil spots on a single leaf which can merge Figure 4 A red oil spot. On occasion these appear on red varieties

After warm humid nights, a dense, raised, white cottony growth develops on the underside of the yellow oil spots (Figure 5). This is commonly referred to as ‘white down’. As the spots age naturally, or after a sporulation event (see later) or hot weather, their centres dry out and become a reddish brown leaving an outer ring of yellow (Figure 6).

The fungus in this yellow ring remains active and given favourable conditions at night, can produce a ring of ‘white down’ on this outer active edge (Figure 7).

Figure 5 White down (spores) develop beneath oil spots after warm, humid nights Figure 6 An old oil spot that has died in the centre but remained active with an outer ring of yellowFigure 7 After favourable conditions the active yellow ring surrounding the dead tissue is still capable of producing new spores

Later in the growing season (late summer and autumn) on mature leaves, leaf infections will appear as small, angular, yellow spots that are limited in growth by veins (Figure 8). These form a tapestry-like (mosaic) pattern that soon turn reddish brown. Defoliation can occur in severely affected vines.

Figure 8 Infection on mature leaves will be yellow-brown and remain small, confined by the leaf veins forming a mosaic pattern

Shoots

Infection on young shoots, stems and tendrils is seen as oily brown areas (Figure 9). These oily patches may spread into leaf stalks, which turn brown and may die. After warm humid nights these oily patches may also sporulate and be covered with white down.

Figure 9 Downy mildew leaf infection has spread to the shoot causing the oily black appearance

Inflorescences, bunches and berries

Infection on inflorescences, young berries and bunches are seen as oily brown areas. After suitable warm humid nights they may be covered with white down (Figure 10). Infected inflorescences and young bunches rapidly turn brown and wither (Figure 11). Infected young berries stop growing, harden and may later develop a purple hue. They turn dark brown, shrivel and fall from bunches.

Figure 10 Downy mildew infection (white spores) of a grapevine inflorescence Figure 11 Infection near flowering can result in the death of the bunches which then turn brown

Berries become resistant to infection when they are pea size (5-6mm diameter). However, they may still be killed if the berry or bunch stems become infected. They may also sunburn and fail to ripen if defoliation occurs from leaf infection (Figure 12).

Figure 12 Downy mildew infection of a grape bunch showing infected berries with white down and resistant healthy green berries

Looks like

Young downy mildew infections may be confused with powdery mildew, caused by Erysiphe necator. Downy mildew spots are oily with a chocolate halo and develop ‘white down’ on the underside of the leaf.

Powdery mildew spots are often smaller and yellow-green which then develop a thin layer of ash-grey powdery spores that may eventually cover both sides of the leaf (see Powdery mildew of grapevines in Western Australia).

Yellow spots on leaves may also be due to spray drift damage from herbicides such as paraquat (e.g. Spray.Seed®) or sucking insect damage. Paraquat damage is distinguishable from downy mildew as it does not grow and tends to develop a small brown spot in the centre of the yellow area (Figure 13).

Figure 13 Herbicide damage can be confused with downy mildew

Often downy mildew infection occurs at the bottom of leaf tips or at the junction of the petiole, where water has collected, and when the infection dies or is killed through management techniques this can be confused with botrytis leaf infection (Figure 14).

Frost damage to leaves can cause mosaic symptoms similar to those seen in mature leaf infection of downy mildew (Figure 15). Confirmation that it is downy mildew and not another form of damage requires a bag test where the downy mildew will produce white down on the underside of the oilspot whilst the others will not (see ‘Monitoring’).

Figure 14: Downy mildew infection at the end of a leaf tip that has died and looks similar to a botrytis leaf infection. Figure 15 Frost damage to a vine leaf can be confused with downy mildew infection

White growth on the underside of a leaf may also be due to grape leaf blister mite damage. This white growth is distinguishable from downy mildew as it forms within blister-like green galls that bulge on the upper side of the leaf.

White fungal growth on a leaf may also be due to other fungi such as Penicillium, Aspergillus or Rhizopus. These infections will eventually change to green, blue, black or brown. Many can also cause grapevine bunch rots.

Plasmopara viticola is specific to grapevines (for example, Vitis vinifera), although not all Vitis spp. are susceptible. The American rootstock species and hybrids are less susceptible or resistant (refer to ‘Varietal susceptibility’).

Other species of downy mildew, such as those found on cucurbits and roses, do not attack grapevines. Although, weather conditions that favour the development and spread of grapevine downy mildew also may encourage the development and spread of cucurbit and rose downy mildews.

Damage and loss

Severe infection will cause leaves to fall prematurely, reducing yield and berry sugar content and will expose remaining bunches to sunburn. Total crop loss may occur if severe infection is not managed, especially near flowering. If partial bunch infection occurs this can cause significant aesthetic issues for table grape growers. Severe leaf fall also can cause yield loss in the following season due to the inability of the vine to store reserves.

Varietal susceptibility

All varieties of Vitis vinifera are highly susceptible to downy mildew infection. The strain of the disease in Western Australia does not infect or show symptoms on V. rupestris, V. cordifolia and V.rotundifolia and V. riparia vines. These are often used as rootstocks in WA.

Disease cycle

Downy mildew is an obligate parasite (meaning it requires a living host) and therefore it grows on all green parts of the vine. However, there is one overwintering stage of downy mildew development that is not found on green tissue.

Overwintering

Oospores (resting bodies) are formed in late summer or autumn from the mycelium within leaves, shoots or berries (Figure 16). These resting bodies fall to the ground when leaves and bunch parts fall in autumn. There they overwinter in infected leaves and litter in the soil for 3-5 years (possibly up to 10 years).

Oospores are the sexual structures of downy mildew. An oospore has a thick wall that makes it less susceptible to fungicides and adverse weather conditions, such as exposure to the sun, than zoospores or sporangia (see below) (Figure 17).

Figure 16 Downy mildew oospores develop within grape leaves in Autumn. The leaf veins highlight the size of the oospores. Figure 17 Close up of downy mildew oospores showing the double wall, making oospores resistant to fungicides and the weather.

Primary infection (soil to vine)

The 10:10:24 ‘rule of thumb’ refers to the conditions required for primary (first) infection to occur. At least 10mm rainfall (and irrigation) is required while the temperature is 10°C or more over a 24-hour period. Not all 10:10:24 conditions are suitable for a primary infection but this ‘rule of thumb’ provides a guide to monitor for favourable primary infection conditions when no other options are available.

More specifically the conditions required for oospores to germinate are:

  • soil needs to be wet for at least 16 hours
  • usually achieved by 3-5mm rainfall (and/or irrigation).
  • temperature also needs to remain above 10°C.

The germinated oospores then release zoospores (that swim in free water) which then need to be splashed by rain or irrigation to the vine canopy before the end of the 24 hour period. This process usually requires another 3-5mm of rain (and/or irrigation) to ensure sufficient splash and leaf wetness for infection on the underside of the leaves. For this, the foliage must remain wet for at least 2-3 hours at 20°C (or 4-5 hours at 10°C) for the spores to infect the leaf and complete the primary infection cycle.

Soil moisture levels are important for the germination of oospores. A combination of rainfall and irrigation together can be sufficient for germintion to occur.

Oil spots

The zoospores released during primary infection that establish on the underside of the leaf begin to grow hyphae. These hyphae grow inside the leaves to form oilspots that appear 5-17 days (but more often 5-10 days) after infection has occurred. The development of oilspots is quickest in warm weather (18-27°C). At warmer or cooler temperatures the incubation period is longer.

Primary infection levels are usually low with only 1-3 oilspots developing per 50m of vine row per germination event. Hence, primary infections are very difficult to find if only a few germination events have occurred at the beginning of the season.

Secondary infection (leaf to: leaf, shoot, inflorescence, berries, stalk)

Active oilspots need to be present before secondary infections can occur. These oilspots (and surfaces of other diseased tissue) produce sporangia (seen as white down) on suitable warm wet nights. Oilspots are known to survive the high summer temperatures in WA. During these periods the oil spot may remain inactive, by not producing sporangia, but when conditions become suitable again sporulation will resume.

Sporulation requires at least four hours of darkness to develop, during which time the temperature is 13°C or more and humidity is 98% or more.

To then cause infection the foliage must also be wet for at least 2-3 hours once sporulation has occurred. The wet foliage can be the result of rainfall, overhead irrigation or occasionally from heavy dew.

Secondary infections can occur any time during the growing season whenever oil spots are present and conditions are favourable.

Sporangia are the asexual structures of downy mildew. Sporangia are produced on tiny treelike structures known as sporangiophores. A single sporangium can in turn produce between 1-10 zoospores. Zoospores are able to move (swim) through water but are spread mainly by wind and rainsplash.

Spread

Primary infections begin the disease cycle by providing a source of oil spots. Overseas experience has shown that primary infection events occur throughout the growing season and are important for disease development and spread.

Secondary infections produce spores that can be spread by wind and rain to establish new infection sites within the local vicinity of the primary oil spot. Secondary infections can drive the disease to epidemic levels rapidly if conditions are favourable.

Monitoring

Good management of downy mildew is dependent on good monitoring of favourable weather conditions for primary and secondary infection events and of disease progress in the vineyard.

Automatic weather stations can be used for monitoring and predicting weather events. Weather stations collect information on temperature, rainfall, leaf wetness and humidity (suitable for downy mildew) and process the data for the likelihood of a primary or secondary infection event. Alternatively a maximum/minimum thermometer and a rain gauge can be used as a guide.

Monitoring in the vineyard is important to confirm possible infections from weather events. Physical examination can confirm whether a pre-infection or post-infection fungicide is required and will also indicate if spray coverage was effective after an event.

Monitoring in the vineyard should occur every 7-14 days when weather conditions are favourable from 3-4 weeks after budburst (that is, when shoots are approximately 10cm long).

Some general guidelines on vineyard monitoring include:

  • Vines to be monitored should be representative of the block being assessed.
  • Monitor for possible source areas of downy mildew infection such as wetter more sheltered parts of the vineyard (for example, near windbreaks and sheds), vines with dense canopies or areas that have previously been infected.
  • Inspect both sides of 200 vines by scanning the foliage between mid-morning and mid-afternoon.
  • Spend about 30 seconds per vine. Less time will be required early in the season when vine canopies are small.
  • Focus on the canopy near the ground in lower lying areas where the soil may remain wet for extended periods.
    • The disease is difficult to detect in dense shaded canopies. The foliage may need to be parted to scan the inner leaves.
    • Use a hand lens to check suspect spots on leaves for evidence of the fungus.
    • Take samples of suspect leaves and bunches and conduct a bag test (The bag test for downy mildew of grapes).
    • Tag suspect sample sites so they may become the reference points if the sample is confirmed positive.

Monitoring for downy mildew can be combined with monitoring for other pests (for example, diseases and insects).

Note: Other fungi may infect leaves and bunches. Downy mildew spores are white and raised. Powdery mildew spores are ash-grey and are a uniform thin layer. Botrytis spores are cream to grey-brown. Penicillium, Aspergillus or Rhizopus are white and raised but change to green, blue, black or brown. If there is any uncertainty suspect samples may be submitted to a pathology laboratory for confirmation DDLS – Plant pathology services.

Oilspots that have already developed ‘white down’ on their underside (whether this fungal growth is fresh or old) may not develop any more ‘white down’ even after a bag test is conducted.

Management options

Cultural

Oospores may spread from property to property and region to region by the movement of infected leaves and litter in the soil or on vines in late summer and autumn. Avoid distribution of infected soil and plant matter by equipment and machinery (for example, mechanical harvesters, leaf pluckers, trimmers and utilities), by soil still adhered to rootlings, or by potted vines from nurseries.

Sporangia may spread across property boundaries by wind. It is very difficult to prevent the spread of these spores. It is believed that sporangia rarely spread more than 200m by wind.

Canopy management practices that encourage air movement will help to dry out leaves and improve sunlight and spray penetration. This will help to prevent infection. Such practices include:

  • lower planting density
  • trellising and pruning to open the canopy
  • shoot training to open the canopy
  • vine trimming and hedging
  • lateral shoot thinning
  • leaf plucking.

Vegetative growth may also be managed by the selection of appropriate rootstocks prior to planting and by careful application of fertilisers (for example, nitrogen). Excessive growth leads to dense shaded canopies that may encourage the development of downy mildew.

Chemical

All chemicals registered for use against downy mildew of grapes in WA are listed in the department’s Viticulture Spray Guide. Consultation with the Australian Wine Research Institute (AWRI) ‘Dog Book’ should occur prior to use of chemicals for any further application requirements.

Pre-infection fungicides (applied as close as possible but prior to an infection event)

Pre-infection (protectant) fungicides help to prevent downy mildew zoospores from entering the green vine tissue. Spray coverage needs to be excellent to adequately protect all of this green tissue. In particular they need to be applied to the underside of leaves and the back of bunches. It is important to time their application as close as possible but prior to the possible infection event (for example, when possible primary or secondary weather events are forecast).

A pre-infection spray program tends to be used where downy mildew is well established in a region or vineyard and occurs frequently. Growers who are unable to conduct careful monitoring tend to use a pre-infection spray program. Large vineyards and table grape growers tend also to use pre-infection spray programs or those with soils which make access limited after a rain event, delaying post-infection spray application.

Pre-infection fungicides have limited movement from the areas where they are deposited and any new growth after the spray has been applied will not be protected. Rain and overhead irrigation will dilute or may wash the protectant sprays off the vine. Hence, further applications will be required before the next possible infection event.

A pre-infection spray program often requires application on a 7-14 day schedule. This may be expanded to a 21 day program later in the season as shoot growth slows and possible infection events are less. As flowering is the critical period to prevent crop loss, the spray program may need to be tightened to every 5-7 days to coincide with possible infection events.

(Note: In table grapes pre-infection fungicides should only be used provided it is prior to 10mm berry size as spray residues on berries may occur).

Post-infection fungicides (applied as soon as possible after an infection event)

Post-infection (eradicant) fungicides are systemic and penetrate the vine tissue killing the downy mildew fungus from within the vine tissue. Use of these fungicides involves withholding sprays until an infection event has occurred. Relying on post-infection fungicides requires careful monitoring and has a greater risk of downy mildew becoming established. However, if downy mildew is not established in the region or vineyard and few possible infection events occur it has the advantage of using fewer sprays that have greater effectiveness.

Post-infection fungicides work best when applied as soon as possible after an infection event – within five days of infection and before oilspots appear. No additional spraying should be required until weather conditions favour another possible infection event. In this situation, pre-infection fungicides may be used once again.

Once the fungus is visible it is difficult to kill. A single post-infection spray is usually not effective, although it may reduce the number of spores and limit spread of the disease. Hence, follow up sprays of post-infection fungicides may be required after the initial post-infection fungicide spray.

(Note: In table grapes not all post-infection fungicides can be used after 10mm berry size for the control of downy mildew. Contact your nearest Department of Primary Industries and Regional Development office or consultant for further information).

Did the post-infection fungicide work? Use The bag test for downy mildew of grapes to check whether the post-infection fungicide was effective. Conduct the tests 24 hours and then, if required, another three days after spraying with samples of healthy bunches, limp and browning bunches and leaves with oilspots. Keep each sample in a separate moist bag.

Consider immediate repeat spraying with a post-infection fungicide if fresh white down is evident on the underside of the leaf or on bunches of these samples when inspected the next morning.

Post-infection products should be used after an infection event if there is any concern that the pre-infection fungicides applied were not adequate. Browning bunches may be too badly infected for the post-infection fungicide to prevent crop loss and only normal looking bunches will benefit from the spray.

Fungicide resistance

Due to the ability of Plasmopara viticola to sexually reproduce within Australian vineyards and the use of eradicant fungicides for control it is considered high risk for the development of resistance. Fungicide resistance has been detected in WA for this disease. The continual use of one fungicide or one group of fungicides increases the risk of resistance developing to that fungicide or that group of fungicides. To reduce the risk of resistance developing within your vineyard always read the chemical label and regularly consult the resistance management guidelines for grapevine downy mildew produced by Croplife Australia. Always read the chemical label thoroughly prior to use.

Tips For The Control Of Downy Mildew

A common but under diagnosed problem in the spring garden is a disease called downy mildew. This disease can damage or stunt plants and is difficult to diagnose. But, if you are familiar with the different ways this disease presents itself and with the conditions it which is can grow in, you will be better able to take steps to control downy mildew in your garden.

What is Downy Mildew?

Often times, when gardeners hear the name downy mildew, they think this disease is related to another common garden disease called powdery mildew. While the two have very similar names, they are two very different diseases.

Downy mildew is caused mostly by organisms that belong to either the Peronospora or Plasmopara genus. While powdery mildew is cause by a true fungus, downy mildew is cause by parasitic organisms that are more closely related to algae.

Because it is closely related to algae, downy mildew needs water to survive and spread. It also needs cooler temperatures. You are most likely to see downy mildew in your plants in the spring, where rainfall is frequent and temperatures stay cool.

One of the tricky things about downy mildew is that it can appear different ways, depending on what kinds of plants it is infecting.

Most often, an infection of downy mildew will also include a fuzzy, soft looking growth that can be white, grey, brown or purple. This growth is most commonly seen on the lower leaves of the plant. This growth is where this disease gets its name from, due to its downy appearance.

Other common symptoms for downy mildew include mottling or spots on the leaves. The spotting will be yellow, light green, brown, black or purple. In some cases, the mottling may look like chlorosis.

Plants that are affected by downy mildew, may be stunted or have leaf loss.

The best control of downy mildew is to make sure that your plants do not get it in the first place. Because downy mildew needs water to survive, the very best thing you can do to prevent downy mildew is to water your plants from below. Water that sits on the leaves of the plant gives the downy mildew a way to infect and spread on the plant. The spore of downy mildews spreads by literally swimming through water until they come across live plant material to infect. If there is no water on your plant leaves, the downy mildew cannot travel to or infect your plants.

Good garden hygiene is also crucial to stopping downy mildew from developing in your garden. This disease overwinters on dead plant material, so removing dead plant material from your garden in the fall will help prevent the disease in the following spring.

If your plants become infected with downy mildew, the organic control of downy mildew is your best bet. The reason is that once a plant is infected with downy mildew, there are no effective chemical controls, though if you have a reoccurring problem with downy mildew, there are some preventative chemicals you can use. Downy mildew is not a fungus, so fungicides will not work on it.

Once your plants have downy mildew, the best thing you can do is to try to eliminate moisture and humidity around the plants. As mentioned, make sure your are watering from below. If possible, try to improve air circulation through selective pruning. In enclosed environments, like in the house or in a greenhouse, reducing the humidity will help as well.

Regardless of what you do, downy mildew normally clears itself up in the outdoor garden once the weather warms up, as this disease does not survive well in warm temperatures. If your plants only have a mild case of downy mildew, your best option may be to simply wait for warmer weather.

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