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

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

Problem
The young leaves, young twigs, and flower buds are covered with a thin layer of grayish white powdery material. Infected leaves may be distorted and curled and many may turn yellow or purplish and drop off. New growth is often stunted, and young canes may be killed. Badly infected flower buds do not open properly. In late summer, tiny black dots (spore-producing bodies) may be scattered over the powdery covering like ground pepper.

Analysis
This common plant disease is caused by a fungus (Sphaerotheca pannosa var. rosae). It is one of the most widespread and serious diseases of roses. The powdery covering consists of fungal strands and spores. The spores are spread by the wind to healthy plants. The fungus saps the plant nutrients, causing distortion, discoloring, and often death of the leaves and canes. Powdery mildew may occur on roses any time during the growing season when rainfall is low or absent, temperatures are between 70° and 80°F, nighttime relative humidity is high, and daytime relative humidity is low. In areas where there is high rainfall in spring and summer, control may not be needed until the drier months of late summer. Rose varieties differ in their susceptibility to powdery mildew.

Solution
Apply a fungicide containing triforine at the first sign of mildew. Repeat the spray at intervals of 7 to 10 days if mildew reappears. Rake up and destroy leaves in the fall. For a list of roses tolerant of and susceptible to powdery mildew, see Roses Resistant to Black Spot, Powdery Mildew or Rust.

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