Q. My roses look absolutely terrible. The leaves are covered with black spots, turning yellow and dropping off. Help!
A. These are the classic signs of black spot, a disease caused by a fungus, Diplocarpon rosae, a serious problem in areas where high humidity or rain is common in spring and summer. The fungus spends the winter on the infected leaves and canes. The spores are spread from plant to plant by splashing water and rain. The fungus enters plant tissue, forming spots the size of a pinhead. The black spots enlarge up to a three-quarter-inch diameter as the fungus spreads; spots may join to form blotches. Plants are often killed by repeated infection.
Garden centers carry many fungicide products that will control black spot. Make sure to read the label carefully and always apply any pesticide according to label directions. You may choose to omit spraying during periods of dry, hot weather, focusing instead on sanitation. Prune off the infected canes, avoid overhead watering, and rake up and destroy the fallen leaves. In the fall, be sure to rake up and destroy all leaves, fruit, and fallen branches.
Purdue University assembled lists of black spot resistant roses in 2011. You can access the lists here.
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