Question: Why did rose leaves develop black spots, turn yellow, and drop all summer long?
Answer: Black spot, caused by the fungus Diplocarpon rosae is the most common problem of roses grown in Indiana gardens. This disease is favored by wet summer weather. Infected leaves develop black spots and the leaves respond by turning yellow and falling from the plant leaving the rose with less energy to produce flowers. Some popular roses are so susceptible that in order to produce acceptable flowers they require spraying every week. There are many home garden fungicide products available for rose disease management. However, black spot resistant roses are also available. Keep rose foliage dry by avoiding sprinkler irrigation. See Botany and Plant Pathology Extension Publication BP-26-W for more information on growing roses and on rose black spot disease management.
--Dr. John Hartman, Extension Plant Pathologist, University of Kentucky, Lexington
Edited for Indiana by Gail Ruhl, Purdue University