Problem Identifier Unit Screen//v.w/ RR Template

Problem Identifier Unit Screen//v.w/ RR Template Black Spot

Problem
Circular black spots with fringed margins appear on the upper surfaces of the leaves in the spring. The tissue around the spots or the entire leaf may turn yellow, and the infected leaves may drop prematurely. Severely infected plants may lose all of their leaves by midsummer. Flower production is often reduced and quality is poor.

Analysis
Black spot is caused by a fungus (Diplocarpon rosae) that is a severe problem in areas where high humidity or rain is common in spring and summer. The fungus spends the winter on infected leaves and canes. The spores are spread from plant to plant by splashing water and rain. The fungus enters the tissue, forming spots the size of a pinhead. The black spots enlarge, up to ¾ inch in diameter, as the fungus spreads; spots may join to form blotches. Twigs may also be infected. Plants are often killed by repeated infection.

Solution
Spray with a fungicide containing triforine or chlorothalonil. Repeat the treatment at intervals of 7 to 10 days for as long as the weather remains wet. Spraying may be omitted during hot, dry spells in summer. Prune off infected canes. Avoid overhead watering. In the fall, rake up and destroy the fallen leaves. After pruning plants during the dormant season, spray with a lime-sulfur spray. The following spring, when new growth starts, begin the spray program again. Plant resistant varieties (for a list, see Roses Resistant to Black Spot, Powdery Mildew or Rust).

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