Humid moist weather favors the development of two common rose diseases, powdery mildew and blackspot. Powdery mildew prefers cool humid weather while blackspot develops under warm wet conditions.

One of the most serious rose diseases is blackspot. As its name indicates the symptoms are irregular black spots on the leaves up to a 1/4 inch in size and occasionally larger. Shortly after the spots develop the affected leaf turns yellow and falls off. This can cause a premature loss of many leaves. Sanitation is the first step in control. Rake up and destroy all diseases and fallen leaves in the fall. If the disease has produced purple spots on the canes, they should be pruned out and destroyed. If and when blackspot symptoms appear begin spraying with a fungicide and continue every 7 to 14 days as needed. Fungicides are available at local garden centers.

Powdery mildew is another disease frequently encountered during cool moist weather or in shady areas. The leaves, buds and stems become covered with a powdery white coating. The young leaves curl and become distorted and badly infected flower buds fail to open. Again sanitation is important. Remove and destroy all infected leaves and flowers. You should spray your roses with a fungicide, beginning when the buds first open and repeat every 10-14 days as long as necessary. Fungicides are available locally.

Another problem, but not a disease of roses, is iron chlorosis. Symptoms to look for are: leaves which turn yellow with veins remaining green in color. This condition is caused by a deficiency of iron in the rose plants nutrition. Our soil which is very alkaline ties up the iron in the soil making it unavailable to the plant. Iron chlorosis can be treated by spraying iron chelate on the roses or by applying it to the soil. Spraying iron on the leaves is a very short term solution.

This page was last updated May 2003

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