Tiny mite spreads ugly disease by feeding on roses

Roses are typically viewed as one of the most beautiful flowers, but in rare cases a tiny pest can cause gnarly looking, new growth on rose bushes.

Rose leaf-curl mites feed on roses and cause rose rosette virus, also known as RRV. The extremely small eriophyid mite feeds on plant sap from the tender stems and leaf petioles. The pest alone causes little damage while feeding, but if it is a carrier of RRV, symptoms begin to appear in the rose typically within one to three months.

There is no cure for RRV and it is not always preventable, since there are no vaccines for plant viruses.

Causes thick, succulent stems

Infected roses exhibit reddened terminal growth on infected branches, and the stems become thicker and more succulent than those on unaffected parts of the plant. These stems exhibit an abnormally high number of pliable thorns, which may be either green or red.

Infected rose bushes produce less flowers and the petals may be distorted and fewer in number. Rose leaves that develop on infected branches are smaller than normal and may be deformed similarly to herbicide injury by 2,4-D.

Lateral branches may grow excessively from main stems and create a witch’s broom symptom, much like injury from herbicide glyphosate (Roundup and other brands).

Treat nearby rose bushes

To reduce the spread of leaf-curl mites from the site of an infected rose, nearby roses can be treated with an insecticide spray containing bifenthrin or a horticultural summer oil every two weeks between April and September. This may help prevent additional plants from becoming virus infected by any sap-vectoring mites.

Symptoms of the virus generally become evident in the late spring to early summer and progress during the growing season. By late summer or fall, the plant will have a noticeable amount of abnormal, gnarly growth.

Once the rose becomes infected, RRV moves throughout the plant and the entire bush becomes infected. By the time symptoms are evident in a rose, the virus may have spread to adjacent roses by the movement of the mites.

Only affects roses

Infected plants typically die within a couple of years. The good news is RRV only affects roses, so other plants in your garden won’t get this disease unless they are closely related to the rose plant.

Since there are no treatments for plant viruses, infected roses should be immediately removed, then burned or bagged for disposal. Also remove any roots that might re-sprout later. Do not leave an uprooted, infected plant in the garden, as the mites may leave this bush for other nearby roses.

When planting new roses, space plants far enough apart so that they do not touch in order to minimize potential spread of these types of diseases.

Because RRV is systemic within the infected rose plants, grafting infected stems onto other rose plants will transmit the virus. Nursery growers may infect roses this way through poor propagation practices.

Pruning shears and other tools used on diseased roses should be disinfected with rubbing alcohol or a 10 percent diluted bleach solution before being used on healthy plants. Sap left on the pruners can contaminate other roses.

For more information on growing roses, refer to the University of Georgia Cooperative Extension publications at www.caes.uga.edu/publications/.

Rose Deformity Info: What Causes Deformed Rose Growth

If you’ve ever come across unusual rose deformities in the garden, then you are probably curious about what causes deformed rose growth. There are several things that can cause the buds, blooms and foliage to take on a strange deformed or mutated appearance in roses. Read on for more rose deformity info.

Common Causes for Deformed Rose Flowers and Leaves

Most rose deformity in the blooms, and sometimes leaves, is caused from Mother Nature herself or genetic mutations.

Proliferation – Proliferation, or vegetative center, causes deformed rose flowers. This is one of those Mother Nature’s Kitchen items. It can occur with many rose bushes, perhaps a bit more with the floribunda roses. There is some school of thought that using high nitrogen fertilizers can bring about an imbalance within the rose bush that will cause the vegetative center. The visual of this one is a mass of green growth coming from the center of the rose bloom. It can look like a knot of green growth and even new leaves coming out of the center of the bloom. The best thing to do is to prune the bloom off down to the first 5-leafset junction with the cane and let new growth and a new bloom grow out.

Genetic mutations – Another of the causes of rose deformities is really just a genetic effect, otherwise known as “an oops of nature.” These may include such things as several leaves growing together to form what appears to be one big leaf or having one bloom growing directly out of the center of a current bloom.

Most rose deformities of the foliage can be a result of fungal attacks, insect damage and viruses.

Fungal diseases – Powdery mildew will form a white powdery like cover on the rose leaves, and even when sprayed and killed, the powdery mildew leaves its mark by creating deformed rose leaves that look crinkled up.

Other fungal attacks will change the coloration of the leaves or black spots will be present all over the foliage of the rose bushes, sometimes a burnt orange looking growth will appear on the foliage. The black spots are caused by the Black Spot fungus, and the burnt orange growth is usually a fungus called Rust. It should be noted that even when the black spot fungus has been sprayed and killed by a fungicide, the black spots on the foliage that had been infected will not go away. However, the new foliage should be free of the black spots if the fungus has truly been eliminated.

Pests – Insect attacks can leave buds severely weakened to the point of them simply turning yellow and falling off of the rose bush. A common cause of this is thrips, as they like to burrow into the buds for their nutrition and cause irreparable damage to the buds. In the case of thrips, the best controlling treatment appears to be a systemic added to the soil around the bush, which is taken up by the roots. It is hard to get at the thrips and some other such insects, as they like to go deep into the buds and canes.

Other insect or caterpillar attacks will leave the foliage looking like lace. This is called skeletonizing of the foliage. Methods of treatment are a good insecticide sprayed on the roses at least twice, about 10 days apart.

I have experienced bent over heads of rose buds. They seem to form normally and then bend over to one side. This condition is called Bent Neck by some Rosarians and can be caused by rose curculios. You will usually notice tiny punctures if this is the case, as they bore in and lay eggs, then leave. They do not actually feed on the rose bush, so they are very difficult to control. The best thing to do is to prune off the bent over bud and discard it before the eggs can hatch and bring out more of the problem. The Bent Neck problem can also be caused by high nitrogen foliar fertilizers that have been used too often or not enough water uptake by the root system due to insufficient rose bush watering. The water uptake problem is seen more frequently during the hotter growing season.

Viral infections – Rose mosaic virus results in oak leaf looking yellowish markings on the leaves and Rose Rosette causes strange mutated looking, mottled (and sometimes deep reddish) growth. Rose rosette causes growth to deform in such a way that it may also have a broom like look to it. This is why some folks refer to it as Witches Broom.

Here are some rose diseases and pests for you to check out to learn more:

  • Rose Bush Diseases
  • Spider Mites on Roses
  • Leaf Cutter Bees

It helps to identify the problem prior to going at it in one particular fashion that could well miss the mark.

If you are puzzled by curling leaves on plants in your garden or landscape, you may need to do some detective work to figure out the cause. Curling leaves can be caused by many problems, including insect damage, disease, abiotic disorders, or even herbicides.

Insects

There are several insect pests that cause leaves to curl when they suck plant juices of new or young leaves that are still growing. These include aphids, thrips, and whiteflies.

Peach leaf curl

If you have peach or nectarine trees and see curled, reddish, puckered leaves, your tree likely has a disease called peach leaf curl. This plant fungus affects only peach and nectarine trees.

Abiotic damage

Leaf rolling in vegetable plants like pepper, eggplant, and tomato is very common during wet spring conditions. This isn’t caused by a disease, and no action is necessary.

Herbicides

When spraying for weeds, herbicides (weed killers) can accidently drift onto or come in contact with desirable plants, causing damage. Herbicides containing active ingredients such as glyphosate and 2,4-D can cause leaves to curl.

Determining the Cause

For further help in finding out what is causing leaf curling on your plant, use the UC IPM plant problem diagnostic tool. This easy-to-use tool contains useful photos and will help narrow down and diagnose the problem.

Leaf curling can sometimes be a difficult problem to diagnose. If you’re stumped, contact your local UC Master Gardener Program or UC Cooperative Extension Office.

Powdery Mildew

Introduction

Powdery mildew is one of the most common foliar diseases of roses. It is caused by the fungus Podosphaera pannosa. The conspicuous white growth can affect all aerial parts of the plant, but mainly new soft growth – producing microscopic spores that spread the disease. High humidity is favourable for infection, as well as plants growing in areas where air movement is poor or on plants that are grown in too much shade.

Description

A white, powdery fungal growth on the leaves and shoots. Both leaf surfaces can be affected.

There may be discolouration (yellow, reddish or purple) of the affected parts of the leaf, and heavily infected young leaves can be curled and distorted.

Mildew growth may also be found on the stems, flower stalks, calyces and petals.

Damage

Powdery mildew not only causes the foliage to curl and distort making it unsightly but the fungus also lowers photosynthetic efficiency that results in reduced plant growth and vigour.

The growing tips and flower buds may be malformed but the death of an entire plant is rare. Plants can be severely stunted if they are heavily infected early in the growing season. Rose tissue becomes more resistant to infection as it ages.

Severely infected foliage can prematurely fall off.

Environmental conditions

High relative humidity is favourable for infection. Temperatures bewteen 16-27 degrees Celsius make conditions favourable for the development and spread of the Fungus.

Plants growing in shaded areas or where air movement is poor or the soil is dry can be prone to Powdery Mildew.

Unlike many other fungal diseases, extended periods of leaf wetness are not required in order for the spores to germinate. This means that powdery mildew is often a problem during dry summers.

Life Cycle, Survival and Dispersal

All powdery mildew fungi require living plant tissue to grow. On perennial hosts such as roses, powdery mildew survives from one season to the next as vegetative strands in buds or as spherical fruiting bodies, called chasmothecia, on the bark of branches and stems.

The powdery mildew fungus overwinters as dormant mycelium in bud scales and rudimentary leaves within the dormant buds. (That is why we recommend that all the leaves are removed after winter pruning so that the leave axil does not harbor the dormant spores.)

Infected buds break open in the spring and develop into systemically infected shoots. The fungus sporulates on these shoots, producing large numbers of microscopic spores (conidia) in chains that are carried by the wind or other means to healthy rose tissue where they infect the upper and lower leaf surfaces, thus initiating a new disease cycle.

Rose powdery mildew spreads during the growing season by means of microscopic, air-borne spores produced on the powdery growth.

Treatment and Prevention

Plant roses in full sun. They should receive a full six to eight hours of sun daily.

Plants will grow more robustly and be able to resist powdery mildew better. Shade causes slower moisture evaporation thus creating a breeding zone for powdery mildew and other fungal diseases.

Plant roses in an area with good air circulation and space them well. Moisture evaporates faster. In addition the breeze will dry off the foliage.

Aerate the soil in winter. The roots of roses need an aerated soil; plants are stressed if water logging occurs and stunt new growth, thus being more susceptible to powdery mildew.

Water correctly. Plants that do not receive enough are more prone to fungal infection. Deep soakings, 3 times a week in the hot summer months will suffice.

Choose resistant varieties. Roses vary in their resistance to this disease. Use resistant varieties for low maintenance plantings.

The method of picking off diseased leaves to prevent spreading has become an old fashioned method due to the availability of new, disease tolerant roses and effective pesticides that should be used for major infestations.

Pesticides

Spot checks and preventative spraying are essential. Effective fungicides should be on the shelf in regions where this disease is prevalent. Protecting the leaves by spraying is effective.

During ideal “powdery mildew” weather conditions, spraying on a fortnightly basis is essential. The following fungicides are effective to a degree in preventing the spores to enter the leaves as well as killing spores on the leaves. The most common group contains the active ingredient Mancozeb. Of these are many fungicides registered under various trade names. Several fungicides are registered for control of powdery mildew. Because of the waxy nature of rose leaves, a spreader added to the spray will give better coverage.

We strongly recommend ‘CHRONOS’, a suspension concentrate fungicide with the active ingredient: Prochloraz zinc complex (imidazole) & Prochloraz equivalent.

The old remedy of treating powdery mildew with a baking soda spray has been shown to be ineffective.

The most common

Funginex Triforine

Rose Protector/Rosecare Propiconazole

Dithane mancozeb

Fungicides absorbed by the leaves

(These have a partial curative action as they clear the blocked capillaries)

Proplant proparmocarb

Proparmocarb proparmocarb

Chronos imidazole prochloraz zinc complex

When Using Pesticides always follow the instructions.

Resources

http://web.aces.uiuc.edu/vista/pdf_pubs/611.pdf http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=748

<http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7493.h

What’s Wrong With My Rose Bush?! Troubleshooting Common Problems

“A rose by any other name would smell as sweet,” wrote William Shakespeare. True story, unless your rose bush has mildew, is being eaten by pests, or gets a virus (yup, it happens). Not to be a total downer, but the most beautiful heirloom varieties are also the most susceptible to issues. The best way to prevent problems is to spring for a variety that has been bred to resist disease if the first place. Too late for that? Read on to learn how to play rose doctor.

Common rose diseases

1. Powdery mildew

Powdery mildew usually appears in summer, when the days are hot and dry and the nights are cool and wet. It can appear on leaves, flowers or stems and usually attacks new growth first. It causes leaves to curl and twist with white powdery spots on the top and bottom of leaves.

Creative Commons/ Scot Nelson

Powdery mildew prevention and treatment

Avoid powdery mildew by watering your roses at the ground level in the morning, allowing them to dry completely by nightfall. Powdery mildew is also brought on by overcrowding. Make sure your roses have room to breathe by planting them with enough distance apart and pruning to better facilitate air flow.

Powdery mildew can be controlled with a fungicide spray, or for a more organic solution, try mixing milk with water in a ratio of 1 part milk to 9 parts water and spraying on the foliage. This won’t completely get rid of the mildew, but it will help halt its spread.

2. Black spot

Black spot is one of the most common diseases to afflict roses. As a waterborne disease, it will appear during humid weather and spread through irrigation. Black spot appears as circular black or brown spots on the tops of leaves. The disease first shows on lower branches and moves its way upward. Leaves that are infected with black spot will eventually turn yellow and fall off the plant. This fungus attacks plants that lack good air circulation or have been sitting in wet conditions, especially overnight.

Creative Commons/ Scot Nelson

Black spot prevention and treatment

Prevent black spot by choosing rose varieties that are resistant to it. It will also help to water roses in the early morning and at ground level to avoid leaves sitting with water on them.

Fungicide sprays can help stop the spread and heal rose bushes. Begin treatment as soon as you spot this disease. Pruning affected canes and removing fallen leaves will also help stop the spread of black spot.

3. Mosaic virus

Yellow mottling on rose leaves is the first sign of mosaic virus, followed by dropped leaves and dead plants. A mild infection of mosaic may not cause any lasting problems, but severe infections can destroy a rose bush.

Creative Commons/ Malcom Manners

Mosaic virus prevention and treatment

Choose disease-resistant roses from the start. Once mosaic sets in, it can’t be cured. Affected plants will need to be removed and destroyed. (Yes, this is serious!)

4. Rust

Rust shows itself in orange spots on the underside of leaves and canes. Leaves may fall off if the plant is severely afflicted.

Creative Commons/ Malcom Manners

Rust prevention and treatment

Prevent rust by selecting disease-resistant varieties and giving plants plenty of space to breathe. Fungicides can be used to stop the spread of rust.

Common rose pests

Wondering who’s been nibbling your roses? There are a number of annoying pests just waiting to chomp down on your blooms. Frequent rose inspection will help you catch bugs before they settle in for the long haul. Be sure to look under leaves and in the soil around the base of plants as well.

1. Aphids

Creative Commons/ Olivier Bacquet

Aphids are very tiny insects that come in a range of colors from green to black. They won’t be noticed at a glance, but looking closely you can identify the little flea-like insects coating your roses.

To get rid of aphids organically, just blast them with a strong stream of water from the hose a couple of times. They’ll eventually move on. You can also use insecticidal soaps or try to encourage their natural predators, ladybugs, to make a home in your garden.

3. Sawfly/rose slug

Creative Commons/ Line Sabroe

Rose slugs are the larvae of the adult sawfly and will attack rose leaves, leaving just the skeleton of the leaf behind. These bugs can usually be found on the underside of the leaves and can make quick work of destroying your prized plants. They look just like green caterpillars and can be hand-picked off leaves or treated with insecticidal soap.

4. Leaf cutter bee

Creative Commons/Chris Worden

The leaf cutter bee gets its name from the circular holes it leaves in your bush. These bees use the foliage to build their nests. Leaf cutters don’t do much harm to rose bushes and are in fact good bugs that your garden needs for pollination. Leave them be, there’s nothing you can do and the damage is purely cosmetic. That’s a relief!

5. Spider mites

Creative Commons/ Scot Nelson

If your rose leaves are turning yellow with tiny white dots, you may just have a spider mite infestation. These teeny insects are almost invisible to the naked eye, but can do a lot of damage. Spider mites usually live on the underside of leaves and occasionally leave webbing on the stems. They can be controlled with insecticidal soap or neem oil.

6. Japanese beetles

Creative Commons/ S. Pisharam

The bane of the gardener’s existence, Japanese beetles, can cause extensive damage in a short amount of time. These insects are unmistakable with their metallic green and black bodies and come in hordes to overtake your garden. They can be driven out with the use of insecticide, hand picking and crushing, or using Japanese beetle traps far away from your roses

A fungal disease that affects peaches and nectarines, leaf curl (Taphrina deformans) is one of the most common disease problems found in backyard orchards. Symptoms appears in spring as reddish areas on developing leaves. These areas become thick and puckered causing leaves to curl and distort. When severe, leaf curl can substantially reduce fruit production.

Disease fungi overwinter as spores (conidia) underneath bark, around buds and in other protected areas. Early in the growing season, during cool, wet spring weather, the spores infect new leaves as they emerge from the buds. Later, the fungus produces great numbers of new spores which are splashed or blown from tree to tree.

Leaf curl is most active at temperatures between 50-70˚F, but can occur at relatively low temperatures. In fact, cool weather is thought to extend the infection period because new leaves are growing slowly. Wet weather is necessary for infection.

Treatment

  1. Select resistant varieties whenever possible.
  2. Leaf curl can be controlled by applying sulfur or copper-based fungicides that are labeled for use on peaches and nectarines. Spray the entire tree after 90% of the leaves have dropped in the fall and again in the early spring, just before the buds open. For best results, trees should be sprayed to the point of runoff or until they start dripping.
  3. Containing copper and pyrethrins, Bonide® Garden Dust is a safe, one-step control for many insect attacks and fungal problems. For best results, cover both the tops and undersides of leaves with a thin uniform film or dust. Depending on foliage density, 10 oz will cover 625 sq ft. Repeat applications every 7-10 days, as needed.
  4. Keep the ground beneath the trees raked up and clean, especially during winter months.
  5. Prune and destroy infected plant parts as they appear.
  6. If disease problems are severe, maintain tree health and vigor by cutting back more fruit than normal, watering regularly (avoiding wetting the leaves if possible) and apply an organic fertilizers high in nitrogen.

Rose Diseases

Roses are one of the most popular and versatile flowering shrubs grown throughout South Carolina. Most roses require a lot of care to grow and bloom properly. One of the most common causes of failure with roses is poor disease control. The three most serious diseases of roses in South Carolina are black spot, powdery mildew, and stem canker and dieback. For more information on roses see HGIC 1172, Growing Roses.

Remember that different types of roses vary greatly in their resistance to diseases and the maintenance they require. To grow roses successfully, you must select varieties that require an amount of care equal to that which you are able to provide. Shrub type roses bloom beautifully with few chemical controls needed, while the more susceptible varieties such as hybrid tea roses require an effective spray program to be in place before the growing season begins.

Black Spot

Black spot is a common and serious rose disease often reaching epidemic proportions in a season. The disease is caused by the fungus, Diplocarpon rosae. It is most severe after long wet, warm periods in the spring. Symptoms occur on rose leaves as circular, black spots surrounded by a yellow area. Infected leaves often drop from the plant. Infection continues throughout the summer months. The immature wood of first year canes develops raised, purple-red irregular blotches. Plants become stunted and produce fewer, paler flowers. By mid-summer severely infected plants may have lost all of their leaves.

Prevention & Treatment: The spread of black spot can be reduced and future infections minimized by following these cultural practices:

  • Plant Resistant Varieties: (See the following list)
  • Maintain Good Sanitation: Sanitation practices are critical in reducing future disease development. In the fall or winter remove all old leaves on the ground along with any mulch that has been contaminated with infected leaves. Replace with a fresh layer of mulch before new rose growth begins in the spring.
  • Remove & Destroy Infected Canes: Canes affected by black spot have dark or reddish areas (lesions). Severely infected plants should be pruned back in the winter or early spring to within 1 to 2 inches of the bud union, according to variety and cultivar. During the growing season, remove and dispose of infected leaves as they appear.
  • Keep Leaves Dry: It is best not to syringe plants with water, and do not use overhead irrigation, especially not in the late afternoon or early evening. Soaker hoses are an excellent way to water roses and to conserve water. Promote rapid drying of leaves by planting roses in the full sun. Space new plants far enough apart to allow for good air circulation.

Use fungicide sprays to control black spot effectively, even on resistant varieties. A rigorous fungicide program must be followed during conditions that favor disease development for susceptible cultivars. Select one of the following fungicide sprays, if disease is severe enough to warrant control: chlorothalonil, mancozeb, myclobutanil, propiconazole, or copper fungicides. See Table 1 for examples of products. Apply all chemicals according to directions on the label.

Powdery mildew is another widespread and serious disease problem of roses. It is caused by the fungus, Sphaerotheca pannosa var. rosae and produces a grayish-white powdery substance on the surfaces of young leaves, shoots and buds. Infected leaves may be distorted, and some leaf drop may occur. Flower buds may fail to open, and those that do may produce poor-quality flowers. It can occur almost anytime during the growing season when temperatures are mild (70 to 80 °F), and the relative humidity is high at night and low during the day. It is most severe in shady areas and during cooler periods.

Prevention & Treatment: Rose varieties differ in their susceptibility to powdery mildew, thus resistant varieties are the best defense against this disease. A film of water inhibits infection, so in years when rainfall is high during spring and summer, control measures may not be needed until the drier months of late summer. Remove and destroy diseased leaves and canes during the growing season. Rake up and destroy leaves under the plant in the fall.

If the disease is severe enough to warrant chemical control, select a fungicide that controls both black spot and powdery mildew. Fungicide sprays recommended for use in the home garden include: propiconazole, thiophanate-methyl, myclobutanil, sulfur, neem oil (clarified hydrophobic extract), or baking soda mixed with horticultural oil. See Table 1 for examples of products. Apply all chemicals according to directions on the label.

The following roses have some disease resistance;

Black Spot – Resistant:

  • Hybrid tea: ‘Pride N Joy’
  • Floribunda: ‘Sexy Rexy’
  • Grandiflora: ‘Prima Donna’

Black Spot & Powdery Mildew – Moderately Resistant:

  • Hybrid tea: ‘Duet,’ ‘Eiffel Tower,’ ‘Grand Slam,’ ‘Jamaica,’ ‘Matterhorn’
  • Floribunda: ‘Golden Slipper,’ ‘Saratoga’
  • Grandiflora: ‘Camelot,’ ‘John S. Armstrong,’ ‘Pink Parfait,’ ‘Queen Elizabeth’
  • Shrub roses: ‘All That Jazz,’ ‘Carefree Wonder’

Black Spot, Powdery Mildew & Cercospora Leaf Spot – Resistant

  • Rugosa roses: ‘Blanc Double de Coubert,’ ‘Fru Dagmar Hastrup’ (‘Frau Dagmar Hartopp’) ‘Rugosa Alba,’ ‘Topaz Jewel’
  • Alba rose: ‘Alba Semi-Plena’

Stem Canker & Dieback

Cankers usually appear as dead or discolored areas on rose canes and vary in color from light tan to dark purplish brown. They are caused by various species of fungi, including Botryosphaeria, Leptosphaeria, Coniothyrium and Cryptosporella. These fungi enter healthy canes through wounds caused by winter injury, improper pruning, wind, hail damage, or flower cutting. Cankers can enlarge until they entirely surround the cane, and/or reach the base (crown) of the plant spreading to other canes or killing the plant. They commonly occur on roses that have been weakened by black spot, poor nutrition or winter injury.

Prevention & Treatment: There are no fungicides specifically available to control stem canker. Keep plants healthy by controlling black spot, powdery mildew and insects. The following cultural methods can help minimize disease development.

  • Avoid Injury to the Plant During Transplanting, Cultivating, Pruning, & Flower – Cutting: Wounds are a major way the fungus enters the plant.
  • Prune Properly: To prune an outward facing bud. This will help to avoid too many branches growing into the center of the plant that may cross and rub together.
  • Remove & Destroy all Infected or Dead Portions of Canes Immediately: Make all pruning cuts well below the diseased areas, and prune about one-fourth inch above an outward-facing bud node, without cutting the nodal tissue, at a 45-degree angle. Prune live canes in the spring, not fall. Disinfect cutting tools after use on a diseased plant in a solution of 1 part household bleach to nine parts water.

Rust

Rose rust is a disease caused by the fungi Phragmidium species. It causes orange-colored spots to appear on stems and leaves. When rust is severe, an orange dust-like substance may be present on the plant surface and on the ground below the plant. Rose rust attacks all plant parts except the roots and petals. Severely diseased leaves of highly susceptible cultivars may turn yellow or brown and drop.

Prevention & Treatment: Provide good air circulation. Do not plant roses in crowded areas and prune plants to keep the centers open. Water plants before noon and avoid getting the leaves wet. Remove and destroy diseased leaves and plants. Fungicides containing myclobutanil, mancozeb or propiconazole are recommended for homeowner use. See Table 1 for examples of products. Apply all chemicals according to directions on the label.

Botrytis Blight

Rose flowers and buds are often infected with the gray-brown fuzzy growth of the gray mold fungus Botrytis cinerea. The fungus is most active when temperatures are 62 to 72 °F and conditions are moist. Infected canes have discolored sunken areas (cankers) and dieback that can extend down the stem from the flowers. Diseased flower petals have small, light-colored spots surrounded by reddish halos, which can quickly expand into large, irregular blotches. Buds fail to open and often droop. Thrips can cause similar damage to half-open buds, so inspect plants carefully.

Prevention & Treatment: Keeping the area clean is more important than anything else. Collect and discard all fading flower blossoms and leaves. Provide good air circulation, and avoid wetting the leaves when watering. Disease easily develops on canes that have been damaged, on canes that are kept too wet by the use of manure mulch, or on wet leaves. If chemical control is necessary, fungicides containing thiophanate methyl, chlorothalonil or neem oil (clarified hydrophobic extract) are available for homeowner use. Use neem oil on a trial basis, especially on open blooms and during hot weather. Neem oil is a rather weak fungicide. On dormant bushes copper fungicides can be used. See Table 1 for examples of products. Apply all chemicals according to directions on the label.

Rose Rosette Disease

Rose rosette disease is an untreatable rose disease caused by the Rose rosette virus (RRV), and is spread and introduced into the rose during feeding by the rose leaf curl mite (Phyllocoptes fructiplilus). This extremely small eriophyid mite feeds on cell sap of the tender stems and leaf petioles. The rose leaf curl mite alone causes little damage while feeding, but if it is a carrier of RRV, symptoms begin to appear in the rose typically within one to three months.

Roses exhibit reddened terminal growth on infected branches, and the stems become thicker and more succulent than those on unaffected parts of the plant. These stems exhibit an abnormally high number of pliable thorns, which may be either green or red. Rose leaves that develop on infected branches are smaller than normal and may be deformed similarly to herbicide injury by 2,4-D. Lateral branches may grow excessively from main stems and create a witch’s broom symptom quite like glyphosate (Roundup™) injury on roses. Flowering is reduced, and the petals may be distorted and fewer in number.

Rose with Rose Rosette Disease showing symptoms of reddened new growth, thicker stem, excessive thorns, and smaller leaves.
Meg Williamson, Plant and Pest Diagnostic Clinic, Clemson University

These symptoms generally become evident in the late spring to early summer and progress during the growing season. Once the rose becomes infected, RRV moves throughout the plant and the entire plant is infectious. By the time symptoms are evident in a rose, it already may have spread to adjacent plants by the movement of the eriophyid mites. Infected plants typically die within a couple of years.

Prevention & Treatment: The wild multiflora rose is very susceptible to the rose rosette disease, so any nearby wild plants should be removed and promptly disposed. Any infected, cultivated roses should be immediately removed, then burned or bagged. Also remove any roots, which might re-sprout later. Do not leave an uprooted infected plant in the garden, as the mites may leave this rose for other nearby plants. Always space rose plants so they do not touch.

Because RRV is systemic within the infected rose plants, grafting asymptomatic stems onto other rose plants will transmit the virus. Pruners used on diseased plants must be disinfested with rubbing alcohol or a dilute bleach solution before being used on uninfected plants, as sap on the pruners is contaminated with the virus.

To reduce the spread of the eriophyid mites from the site of an infected rose, nearby roses can be treated with a bifenthrin spray every two weeks between April and September. This may prevent additional plants from becoming diseased. See Table 1 for examples of brands and products containing bifenthrin. Always check product labels for the correct active ingredient. Follow label directions for use.

Rose Mosaic

The symptoms associated with Rose mosaic virus (RMV) are highly variable. Yellow wavy line patterns, ring spots and mottles in leaves will occur on some varieties of roses sometime during the growing season. In general, symptoms are most evident in the spring. Yellow net and mosaic symptoms on the leaves are also associated with RMV and detract from the overall quality of the plant. Infected plants become weakened and are more sensitive to damage caused by other stresses, such as drought or low temperatures.

Prevention & Treatment: Virus-infected plants cannot be saved. Rose mosaic spreads slowly, if at all, in established rose plantings through root grafts. Infected plants should be removed from highly prized plantings and destroyed. Buy only healthy plants from a reputable dealer; especially avoid purchasing plants showing any mosaic symptoms.

Crown Gall

This disease is caused by a soil-inhabiting bacterium, Agrobacterium tumefaciens, which infects many ornamentals in the home garden. The symptoms are rounded galls, or swellings, that occur at or just below the soil surface on stems or roots. The galls are light green or nearly white when young. As they age, the galls darken and become woody, ranging in size from small swellings to areas several inches across. The galls disrupt the flow of water and nutrients traveling up from the roots and stems, thus weakening and stunting the top of the plant.

Prevention & Treatment: To prevent crown gall, select disease-free roses. Once a plant is infected, nothing can be done since there are no chemical controls available for crown gall. Avoid injury to the roots and crown of the plant during planting and cultivating because the bacteria enter through fresh wounds. Remove infected plants as soon as galls are observed. If possible, remove and discard the soil from the area where the infected plant was located. Disinfect all cutting and pruning tools that have been used near crown gall. To disinfect tools, dip them for several minutes in a solution of 0.5 percent sodium hypochlorite (household bleach).

Table 1. Pesticides for Rose Disease & Mite Control.

Pesticide Active Ingredient Examples of Brand Names & Products
Bifenthrin Ferti-lome Broad Spectrum Insecticide Conc.; & RTS
Hi-Yield Bug Blaster Bifenthrin 2.4 Conc.; & RTS
Monterey Mite & Insect Control Concentrate
Up-Star Gold Insecticide Concentrate
Bifen I/T Concentrate
Talstar P Concentrate
Chlorothalonil Bonide Fung-onil Concentrate
GardenTech Daconil Fungicide
Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide Concentrate
Ferti-lome Broad Spectrum Landscape & Garden Fungicide Concentrate
Ortho MAX Garden Disease Control Concentrate
Tiger Brand Daconil Concentrate
Southern Ag Liquid Ornamental & Vegetable Fungicide Concentrate
Copper Fungicides Bonide Liquid Copper Concentrate (a copper soap); & RTU2
Bonide Copper Fungicide (copper sulfate); & RTU2
Camelot Fungicide/ Bactericide Concentrate (a copper soap)
Monterey Liqui-Cop Fungicide Conc. (a copper ammonium complex)
Natural Guard Copper Soap Liquid Fungicide Conc.; & RTU2
Southern Ag Liquid Copper Fungicide (a copper ammonium complex)
Horticultural oil3 Bonide All Seasons Spray Oil Concentrate; & RTU2
Ferti-lome Horticultural Oil Spray Concentrate; & RTS1
Monterey Horticultural Oil Concentrate
Southern Ag ParaFine Horticultural Oil
Summit Year Round Spray Oil Concentrate Add
Mancozeb Bonide Mancozeb Flowable with Zinc Concentrate
Southern Ag Dithane M-45
Myclobutanil Spectracide Immunox Multi-Purpose Fungicide Concentrate; & RTS1
Ferti-lome F-Stop Fungicide Concentrate
Monterey Fungi-Max
Neem oil Bonide Neem Oil Fungicide, Miticide & Insecticide Concentrate
Ferti-lome Rose, Flower & Vegetable Spray Concentrate
Monterey 70% Neem Oil Fungicide/Insecticide/Miticide
Natural Guard Neem Concentrate
Garden Safe Fungicide 3 Concentrate; & RTU2
Southern Ag Triple Action Neem Oil Concentrate
Propiconazole Banner Maxx Fungicide
Bonide Fung-onil Lawn & Garden Disease Control RTS1
Bonide Infuse Fungicide Concentrate; & RTS1
Ferti-lome Liquid Systemic Fungicide II Concentrate
Sulfur3 Bonide Sulfur Plant Fungicide (also wettable for spray)
Ferti-lome Dusting Sulfur
Hi-Yield Wettable Dusting Sulfur
Safer Brand Garden Fungicide Concentrate; & RTU2
Southern Ag Wettable or Dusting Sulfur
Thiophanate-methyl Cleary’s 3336 WP Turf & Ornamental Fungicide
Southern Ag Thiomyl Systemic Fungicide
1 RTS = Ready to Spray (hose-end applicator)
2 RTU = Ready to Use (pre-mixed spray bottle)
3 Never apply a horticultural oil spray within 2 weeks of a sulfur spray, and do not apply horticultural oils or sulfur when the temperature is above 90 °F or to drought-stressed plants.
With all pesticides, read and follow all label instructions and precautions.

Treating Common Rose Disease

The three most common rose diseases are powdery mildew, rust and black spot. Rose diseases are best prevented by providing a favorable cultural environment for the roses. Shade and moderate temperatures favor most rose diseases, so locate plants in sunny areas as much as possible and plant the bushes at least four feet apart to provide good air circulation. Avoid high Nitrogen fertilizers. A dormant spray of six tablespoons of Master Nursery® Year Round Spray Oil (Horticultural Oil) mixed with two tablespoons of Monterey Liqui-Cop® per gallon of water applied immediately after pruning will help prevent early appearance of these diseases. One tablespoon of Monterey Liqui-Cop® mixed with four tablespoons of the Master Nursery® Year Round Spray Oil in one gallon of water, applied monthly, will act as a preventive and eradicant for these diseases. Be sure to check instructions on the containers. Never apply Horticultural Oil (or any oil spray) to water stressed plants or when the temperatures will be 85˚F or greater. Likewise, do not spray plants with oil spray or with sulfur within one month of each other.

Fungicides fall into two broad categories; preventives and eradicants. Most rose fungicides are preventive and must be applied before the plant becomes infected and will prevent new infections from occurring. An eradicant can kill an existing infection.

Powdery mildew is the most common fungus disease of roses. It appears as a white to gray powder coating upper, leaf surfaces. In severe cases it will also be found on lower leaf surfaces and coating emerging flower buds. Initially, it can be rubbed off between the thumb and fore finger but soon reappears. It can be prevented if Safer® Garden Fungicide or a mixture of Monterey Liqui-Cop® (one tablespoon per gallon of water) and Master Nursery® Year Round Spray Oil (four tablespoons per gallon of water) is sprayed and covers the plant to dripping before the plant shows symptoms of the disease. Copper sprays with less than 25% Copper are of little value. A mixture of Monterey Liqui-Cop® at one tablespoon per gallon, plus Master Nursery® Year Round Spray Oil at five tablespoons per gallon H20 and Bonide® Fung-onil ™ (Chlorothalonil) are used as preventives and eradicants. The spread of powdery mildew will be reduced when the plants receive an overhead water spray. The water washes off the spores from the mildew and when spores land in water, they die. Irrigate in mid-morning so plants dry rapidly and reduce the likelihood of other fungus infections such as black spot or rust.

As part of its Pest Management Program, the UC Department of Agriculture and Natural Resources has stated, “several least-toxic fungicides are available including horticultural oils (Year Round Spray Oil, Neem Oil, Jojoba Oil) sulfur, potassium carbonate 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 protective activity”.

A black spot appears as circular black spots circled in yellow and only on upper leaf surfaces. The spots may coalesce to form large spots which do not penetrate the leaf. Black spot is spread by splashing water. It becomes less of a problem as our weather becomes warm and dry. Overhead watering is to be avoided except as mentioned above. Badly and moderately infected leaves should be removed and discarded. Do not compost.

Rust initially appears as tiny black spots on the top and bottom of a leaf. As the spots enlarge they become rust colored and finally black. The rust pustule penetrates both upper and lower leaf surfaces until the entire leaf is yellow and drops off. Rust fungus is a midseason disease and can defoliate a plant. Monterey Liqui-Cop® and Safer® Garden Fungicide are effective preventives. Pick off and discard severely infected leaves. Reapply the chemicals as new leaves appear. Master Nursery® Year Round Spray Oil or Neem Oil will also act as an eradicant. Bonide® Fung-onil™ is effective as an eradicant and preventives for mildew, black spot and rust on roses. Be sure to check instructions on the containers.

Stem cankers may appear on the canes of roses. They are indentations parallel to the stem ranging from one-half inch to three or four inches long. They are caused by a fungus and can kill the stem. There is no cure so the treatment is to cut the cane four to six inches below the canker and dispose of it. Do not compost.

Viruses may appear on roses, usually on bushes sold ten or more years ago. Most commonly they appear on new spring foliage, stressed plants or at the end of the growing season. At least two different kinds of viruses could infect roses. One form leaves zig-zag lines that resemble lightning bolts. The other produces yellow circles on the green leaves; these virus symptoms may temporarily disappear when the plant is actively growing. Both viruses may produce a slight stunting of growth but will do no other harm. They are spread by grafting a healthy cutting to a diseased rootstock or a diseased cutting to a healthy rootstock. The virus becomes systemic in the plant and remains there for the life of the plant. There is no cure or remedy.

For further information, see UC ANR Publication 7493: Pest Notes “Powdery Mildew on Ornamentals”.

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