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Rose Diseases and Their Control

Source: NDSU T.A.P.E.

Several diseases may cause problems in roses. One of the most serious is black spot. This disease is characterized by black spots on the leaves. Usually the spots are up to 1/4 inch in diameter, but sometimes they are larger. The spots have an irregular or feathery margin, and may have a gray center. Shortly after the spots develop, the affected leaf turns yellow and falls off the plant. This can lead to a premature loss of many leaves.

Control of black spot begins in the fall, when all diseased and fallen leaves should be raked up and destroyed. The disease may also attack the canes, producing purple spots on them. The disease on canes should also be pruned out and the diseased canes destroyed. About mid-summer, or whenever black spot appears, begin spraying the rose bushes with a fungicide and continue until frost. Fungicides to use for black spot control include benomyl, Daconil 2787, Funginex, Orthenex Insect and Disease Control Formula III or Orthenex Rose and Flower Spray.

Another problem frequently encountered in cool, moist springs or in shaded areas is powdery mildew. Leaves, buds and stems become covered with a powdery white coating. Young leaves curl and become distorted. Badly infected flower buds fail to open and are fringed in white. This disease is very difficult to control, but it can be accomplished by a combination of cultural practices and the use of a fungicide. Remove and destroy all diseased leaves. Spray plants with a fungicide beginning when buds first open. Use the fungicides benomyl or Funginex.

If these are not available, Flotox sulfur can be used but may be less effective.

Sometimes the leaves of rose bushes turn yellow, with the tip leaves almost colorless. The veins usually are green, and a network of very fine veins may show green on a yellow background. This is iron chlorosis, caused by a deficiency of iron in the rose plant nutrition. Our highly alkaline soils tie up the iron, making it unavailable to the plant. Iron chlorosis can be treated either by spraying iron chelate on the bushes or by applying iron chelate to the soil as recommended on the manufacturer's label. Spraying iron chelate on the leaves may help correct the immediate problem, but applying iron chelate to the soil will have a longer lasting effect.

Additional information on this topic is included in the Extension bulletin PP-469, entitled "Plant Disease Control in the Home Garden," which is available at your county Extension office.

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