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What is it?

Rose rust is a plant disease caused by any of several species of the fungi known as phragmidium. This fungus infests only rose plants.

What does it look like?

Roses will develop brown or yellowish spots on the topsides of the leaves up to ¼ inch in diameter. These spots will appear in spring or late fall with the lower leaves being affected first. The undersides of the leaves will have blotches or spots, which contain a black, orange or red powdery material, which can easily be scraped off. Infected leaves may become dry and twisted and in severe cases may drop off the plant. Occasionally young twigs may also be infected. Rose plants with severe infection will lack vigor.

How does it manifest?

Wind spreads the orange fungus to rose leaves. Moisture such as rain, dew and fog assist the spores in entering the tissue on the underside of the leaves. The ideal temperature range for this to occur is 55° to 75° F. Spores survive winter on dead leaves, and when wet in the spring the fungi produce new spores, which cause new infections.

What can you do about it?

When you notice the first signs of rust, carefully pick off and destroy all infected leaves. Spray the plant with a fungicide containing lime-sulfur or triforine spray. Continue to repeat the treatment at intervals of 7 to 14 days for as long as temperature and moisture remain favorable for infection. In the fall, rake and destroy all infected leaves. Be sure to prune and destroy any twigs that show infection. Apply an additional lime-sulfur spray during the dormant season. Alternatives to this method of disease maintenance are to select resistant varieties when planting.