Cooperative Extension Service
The University of Georgia College of Agricultural and Environmental Sciences
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Mila Pearce
Extension Plant Pathologist
Blackspot
Botrytis Blight
Crown Gall
Downy Mildew
Powdery Mildew
Cankers and Dieback
Rose Mosaic Virus (RMV)
References
Evidence suggests roses were cultivated in North Africa 5,000 years ago. Today, roses have a wholesale value of more than $116 million and they are the most popular garden plants in the world. Here, we will discuss the most common diseases on roses found in Georgia.
The disease usually begins on leaves at the base of the cane. Leaves and stems exhibit brown to black circular spots approximately ¼ to ½ inch in diameter and may have a yellow margin or halo (Fig 1). Small black structures resembling BBs may be visible within the lesions. Spots may coalesce and leaves eventually fall from the plant. Severe defoliation reduces flower production and overall plant health. The canes exhibit irregular purplish blotches on the first-year canes. Flower petals may have reddish flecks and distorted growth. Disease is seldom lethal unless compounded by stressful environmental factors.
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| Figure 1. Circular black spots typical of blackspot. |
Diplocarpon survives winter on diseased canes and fallen leaves. Wind and rain carry spores to new growth the next spring. Leaves are most susceptible when they are 6-14 days old. Leaves must be wet for at least 7 hours and temperatures must be 71-77 degrees F for germination. Spots usually occur 10 -14 days after infection. Frequent rains spread more and more disease as spores are constantly produced. If disease is severe, rose may be completely defoliated by June.
Resistant varieties of roses are available. In the fall, collect fallen leaves and discard them. Prune infected canes and ensure roses get plenty of sunlight and good air circulation. Irrigate only as needed and early in the morning. Chemical controls are also available, but they must be used shortly after bud break and continue as long as weather conditions are conducive for disease development. Contact your local county extention agent for a list of these chemicals.
Buds and flowers are most often the victims of Botrytis, but canes can also become infected. Symptoms on flowers are small tan spots surrounded by a maroon halo. Spots quickly enlarge into irregular brown blotches engulfing much of the petal. Buds will not open and appear to droop from the stem. Petals and buds soon become covered with a gray fuzzy growth (Fig 2). If disease is severe, infection can extend into the canes and cause cankers and dieback.
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| Figure 2. The beginning of gray fuzzy growth characteristic of Botrytis sporulation. |
Botrytis enjoys periods of cool, humid weather with the optimal temperature at 59-70 degrees F. Spores are usually dispersed by air and invade plant tissue through a wound or injury. This most often occurs on older, senescent flowers.
Control of Botrytis can be accomplished by removing old flowers and fallen leaves. Space plants appropriately so sunlight and air can easily pass. Fungicides are also effective when used properly. Contact your local county extension agent for a list of these chemicals.
Tissue overgrowths or galls can be found at or just below the soil surface on the crown and on the roots of plants (Fig 3). Galls are irregularly rounded and rough in texture. Galls vary in size from ½ inch to several inches in diameter. Young galls are light green or white, and the tissue is soft. Advanced galls are hard and brown to black in color. Roses severely infected become stunted, producing only a few blooms.
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| Figure 3. Tissue growth on the crown. |
The crown gall bacterium enters plants through wounds made by transplanting, cultivation, grafting, and pruning. Other wounds caused by insects, animals or people can serve as avenues for infection to occur. Affected plants may be stunted, produce small chlorotic leaves and may become more sensitive to environmental stresses such as winter injury. Severely infected plants decline and eventually die. Agrobacterium can survive in the soil for 2 to 3 years.
To control crown gall, avoid wounding susceptible plants at or near the soil line. Prune infected branches and roots well back into healthy tissue. To prevent possible spread of the disease, sterilize pruning tools between cuts with a 10 percent household bleach solution. Plant removal may be necessary when galls are extensive. Chemical treatment may be practical for valuable landscape plants. Contact your local county agent for a list of chemicals.
Symptoms include lesions on the leaves, stems and flowers. Leaf lesions are the most obvious and appear angular, purplish to brown, and are often accompanied by yellowing of sur-rounding tissues (Fig 4). Severe defoliation may occur as a result of infection. The symptoms are suggestive of chemical injury or possibly nutrient stress. The downy mildew fungus also forms a downy mass of spores on the underside of leaf lesions, but sometimes this is difficult to see.
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| Figure 4. Angular, purple lesions indicative of downy mildew. |
Downy mildew over winters in the canes of infected plants. When weather conditions are cool (50-75 degrees F) and humid, the disease begins to sporulate and infect new growth. The optimal temperature for rose downy mildew is 64 degrees F. Overhead irrigation and rainfall not only splash downy mildew spores from leaf to leaf and from plant to plant, but also provide the wet leaf surface required for downy mildew spore germination. Once conditions turn warm (~85 degrees F) and dry, the disease is kept in check until the next cool, humid period.
Downy mildew can be reduced by increasing air circulation through pruning and avoiding frequent overhead irrigation. Control with fungicides is very difficult; environmental management is much more likely to be effective.
Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder-like growth (Fig 5). Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group. The disease is most commonly observed on the upper sides of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open.
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| Figure 5. Gray talcum-powder-like growth associated with powdery mildew. |
Most powdery mildew fungi produce airborne spores and infect plants when temperatures are moderate (60 to 80 degrees F) with high humidity. Unlike most other fungi that infect plants, powdery mildew fungi do not require free water on the plant surface in order to germinate and infect. Overcrowding and shading will keep plants cool and promote higher humidity. These conditions are highly conducive to powdery mildew development. Mildew can spread rapidly since the disease cycle can be completed in as little as 72 hours. It commonly takes 7 to 10 days from the time of infection to the development of symptoms and secondary spore production.
Selecting a powdery mildew resistant rose is the most important control measure if planting a rose garden. Do not crowd plants and ensure roses are in full sunlight. Prune infected canes and rake any fallen leaves. Fungicides are also effective when used properly. Contact your local county extension agent for a list of these chemicals.
The first symptoms are small, circular lesions on the canes; the spots are pale yellow, reddish, or bluish purple. They gradually enlarge, turn brown or grayish white (often with a darker margin), and may partially or completely girdle the cane (Fig 6). Complete girdling results in dieback or poor growth of the plant parts above the affected areas. Cankered areas may be sprinkled with black, speck-sized, fungus-fruiting bodies. When left unchecked, infections may spread downward into the crown, causing entire rose plants to wilt, wither and die.
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| Figure 6. Stem partially girdled by a canker. |
The fungi over winter in infected canes and plant debris. When conditions are conducive, spores are spread by wind and splashing water. Infection usually occurs through wounds when temperatures are warm and wet, humid conditions. Disease spread may also occur by contaminated pruners.
Prune diseased canes in the spring prior to budbreak. Maintain plant vigor with proper fertilizaton and water. It is helpful to avoid injuries and to provide adequate winter protection. Disinfect pruning tools with each cut. Fungicides are not effective for control of canker diseases.
Symptoms may be caused by one or a combination of the viruses mentioned above. RMV has a wide range of symptoms including ring-spots, wavy lines, chlorotic vein banding, an oakleaf pattern, a mosaic pattern, and a yellow net pattern (Fig 7). Symptoms appear in late spring and are present throughout the growing season. Symptom expression varies with the cultivar and the time of year. Some plants infected with rose mosaic virus remain symptomless.
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| Figure 7. Mosaic pattern common for RMV. |
Transmission of the virus is through vegetative propagation of infected plant material. Infected grafting material can include buds, scions or root stocks. It is also suspected the causal virus can be spread through infected pollen, although this route of infection is not considered an important avenue of transmission. Plant-to-plant spread via aphids (or other insects) is not thought to occur.
Complete removal of the plant is recommended to prevent spread to neighboring plants. Temporary control may be achieved by pruning away stems where infected leaves appear. The virus is not spread via mechanical means such as pruning. Because the disease is systemic, it can only be spread by grafting infected plant tissue onto uninfected tissue.
American Phytopathological Society http://www.apsnet.org/online/image.asp
Clemson University http://hgic.clemson.edu/factsheets/HGIC2106.htm
Hagan, A., and Mullen, J. Diseases of Roses and Their Control. 2000. Alabama Cooperative Extension System. ANR-505
Horst, K.R. Compendium of Rose Diseases. 1996. APS Press, St. Paul, Minn.
Jones, R.K., and Benson, D.M. Rose Diseases and Their Control in the Home Garden. North Carolina State University. Ornamental Disease Information Note 2.
University of Alicante http://www.ua.es/
University of Maryland http://www.agnr.umd.edu; www.agnr.umd.edu/ .../large/canker
West Virginia University Extension Service http://www.wvu.edu/~agexten/ipm/disease/rosecank.htm
Bulletin 1280/June, 2005
The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service, the University of Georgia College of Agricultural and Environmental Sciences offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, gender or disability.
An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating.
Josef M. Broder, Interim Dean and Director