Rose diseases

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Growing Roses (p.11)

ANR-157, Reprinted October 1996. Recommended for Extension use by Dave Williams, Extension Horticulturist, Associate Professor, Horticulture, Auburn University. Former authors and revisers include Bridget Behe, former Extension Horticulturist; Patricia Cobb, Extension Entomologist; Austin Hagan, Extension Plant Pathologist; Kim Sheffer, former Horticulturist; Ron Shumack, Extension Horticulturist; George Stritikus, Extension Agent, and Dave Williams, Extension Horticulturist; all at Auburn University.

Rose Diseases

Because of the long growing season, high annual rainfall, heavy dews, and relatively mild winters in Alabama, roses are subjected to prolonged attack by several plant diseases. Fortunately, control measures are available for most of these diseases.

Six diseases that commonly occur in Alabama are listed below, with a brief description of each disease and the recommended control practices.

Black Spot (Diplocarpon rosae)--Black spot is probably the most damaging disease of roses in Alabama. Symptoms consist of circular black spots with jagged margins appearing on the leaves. Frequently, the spots are surrounded by a yellow halo. Infected leaves turn yellow and fall prematurely. When the attack is severe and allowed to continue, repeated defoliation will occur, resulting in a weakened plant. The fungal spores are spread primarily by splashing rain or water. Germination of the spores and infection occur when free water remains on the leaf surface for a period of 6 hours or longer. Leaf spots develop within 5 to 10 days.

Powdery Mildew (Sphaerotheca pannose)--This fungal disease occurs in Alabama during the spring and fall. It is rarely a problem during the mid-summer months when black spot is at its worst. Frequent rains and temperatures above 80 degrees F inhibit powdery mildew. The presence of the powdery mildew fungus is confirmed by the white powdery masses of spores on young leaves, shoots, and buds. Symptoms consist of distorted foliage and stunted shoots.

Fungal spores of powdery mildew are easily wind-borne and are responsible for its spread. Under conditions of relatively high humidity these spores germinate and infection occurs. Unlike the black spot fungus, powdery mildew does not require free water for spore germination.

Control (Black Spot and Powdery Mildew):Sanitation is the first step in controlling both black spot and powdery mildew. Leaves (either attached or on the ground) that appear to be infected should be removed and disposed of properly. This practice should be continued throughout the flowering season. Since the fungus overwinters on diseased leaves and canes, this practice should be intensified near the end of the growing season. Sometimes it is necessary to prune canes severely in the spring prior to new growth. In addition, remove and replace mulch beneath diseased plants. A dormant spray of liquid lime-sulfur (1:15) is also recommended at the time of leaf emergence. The best results will be obtained if the plants and ground are sprayed after the old mulch is removed and before fresh mulch is applied.

Good control of black spot and powdery mildew can be realized with a season-long fungicide spray program. Important points to consider are regularity of spraying and thorough coverage. Fungicides act as a protective shield. Once infection occurs, however, they are of little value to already infected plant tissue.

Crown Gall (Agrobacterium tumefaciens)--This disease causes a gradual decline of the rose bush. The bacterium that causes this disease is usually introduced into an area through contaminated nursery stock. Once present, it can survive for several years, even in the absence of a host plant. The bacteria enter the plant through wounds made during planting, cultivating, and grafting, as well as those made from chewing insects and rodents. Galls are spherical, woody growths with rough surfaces, and they may exceed 6 inches in diameter. Crown gall reduces plant vitality, causes stunting, and inhibits blossom development. Aboveground symptoms are similar to those caused by nematodes.

Control (Crown Gall): There are effective controls for existing crown gall infections. Diseased plants should be removed and destroyed. Infested areas may be replanted after fumigation or solarization. Transplants should be free of galls and handled carefully to avoid root injury. Use root dips when planting susceptible plants.

Stem Canker and Cane Blight (Botrytis blight)--Stem canker and cane blight may be caused by one of several fungi. These fungi, however, usually attack weakened or damaged plants. These fungi, therefore, may indicate the presence of a more serious problems such as crown gall or low soil fertility. Symptoms may consist of dark brown to black, sunken lesions on canes. Terminal stems die when canes are completely girdled.

Control (Stem Canker and Cane Blight): Follow sanitation practices suggested for black spot and powdery mildew. Fungicides that are effective in controlling black spot will also control most stem and cane diseases. A regular spray program throughout the growing season is advisable.

Viruses--Several kinds of viruses attack roses in Alabama. Most are confined to minor leaf discolorations and are of little importance. Since most viruses can be transmitted through grafting, roses suspected of having virus infections should not be used in propagation. Most viruses may also be transmitted by insects (primarily aphids). Growers may therefore wish to remove infected plants from rose gardens to prevent the possible spread to healthy plants.

Varietal Resistance--Roses vary considerably in their reaction to black spot and powdery mildew as well as other diseases. If a low-maintenance rose bed is desired, choose a variety with multiple disease resistance. Remember, many of the prize-winning roses are quite susceptible to one or more diseases and may require a rigid disease control program.

See Extension Publications ANR-401, "Disease and Insect Control for Roses" (entomology) and ANR-505, "Diseases of Roses and Their Control" (plant pathology) for more information on control procedures for rose diseases.

Use pesticides only according to the directions on the label. Follow all directions, precautions, and restrictions that are listed. Do not use pesticides on plants that are not listed on the label.

The pesticide rates in this publication are recommended only if they are registered with the Environmental Protection Agency and the Alabama Department of Agriculture and Industries. If a registration is changed or canceled, the rate listed here is no longer recommended. Before you apply any pesticide, check with your county Extension agent for the latest information.

Trade names are used only to give specific information. The Alabama Cooperative Extension System does not endorse or guarantee any product and does not recommend one product instead of another that might be similar.

For more information, call your county Extension office. Look in your telephone directory under your county's name to find the number.

Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, religion, sex, age, veteran status, or disability.

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