July 30, 2015
Diplocarpon rosae (fungus)
This fungal disease gets its common name from the symptoms produced on the leaves. Infected leaves drop off. Defoliation is worse in hot, humid weather. Leaf petioles, twigs, and canes may also be infected.
How it’s spread
Splashing rain spreads the spores.
At least seven hours of continuous leaf wetness with temperatures above 68°F are needed for spore germination. Use trickle irrigation or water early in the day to allow roses to dry quickly. Plant the roses in sunny locations with good air circulation. Rake up and destroy infected leaves; prune out infected canes before new growth starts in spring. Choose resistant cultivars where possible. Roses vary widely in their susceptibility to black spot; many hybrid teas, particularly yellow and orange roses, are particularly susceptible to black spot.
Although black spot isn’t fatal, the disease can defoliate and weaken susceptible cultivars. Several fungicides are labeled for control.
Roses reported resistant to black spot include: Rosa wichuriana, R. roxburgii and some of the R. rugosa hybrids. Consult a current reference for a more complete listing of resistant cultivars and hybrids. Suggested reference for more detailed information on managing roses: Healthy Roses: Environmentally Friendly Ways to Manage Pests and Disorders in Your Garden and Landscape. Flint, M. L., and J. F. Karlik. 2000. Oakland: University of California ANR Publication 21589.
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