Excerpt from the Popular Garden Series magabook Orcharding with permission from its publisher, BowTie magazines, a division of BowTie Inc. Purchase Orcharding here.
When talking about disease, it’s important to start by recognizing that a disease is not the same as a pathogen. Pathogens (fungi, bacteria and viruses) cause disease, but in order to do so pathogens require a host and a hospitable environment. The reason this is so important in IPM is that if you disrupt any one of those elements, the disease will not develop. You can change the host by planting resistant varieties, you can modify the environment or you can attack the pathogen.
Powdery mildew affects apples, peaches, apricots, cherries, plums and pecans. It causes a powdery, white growth on leaves and can cause leaves to be stiff and distorted if the leaves are infected while young. On apples, powdery mildew causes net-like russeting and overall reduction in yield. On stone fruit, powdery mildew causes white patches on young fruit that can become green or reddish at the margins. The fruits, except for cherries, become more resistant to infection as they mature, getting net-like, corky patches instead. Older leaves are also more resistant to infection.
There are many types of powdery mildew, and each type overwinters in slightly different forms. Roses are thought to be an important alternate host for the powdery mildew that infects apricots and plums. Spores are released in the spring and spread through the air to susceptible hosts. Management of powdery mildew emphasizes preventing inoculation. Start with resistant varieties if powdery mildew is a problem in your area. Good air circulation and sunny conditions can help prevent powdery mildew. Removing nearby roses can help apricots and plums avoid infection. Once infection occurs, pruning out infected shoots, especially on apples, is important.
Spray programs for powdery mildew are usually protective, although some products can serve as eradicants. Chemical treatments include sulpher (never apply sulpher with, or within two weeks of, oil), some horticultural oils, potassium bicarbonate, sodium bicarbonate and some biological fungicides. A wide range of conventional fungicides is also available