   Powdery Mildew - November 24, 1999 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Powdery mildew is a fungal plant disease. It appears as a growth on the surface of susceptible plants from the middle of summer until late fall. What we see upon quick inspection is a white dust on the leaf surface. No big deal, right? Not! What is really going on is an insidious attack pillaging the cells of their hard earned carbohydrates. Several species of powdery mildews exist. Interestingly enough, these fungi are more abundant in semiarid areas, than in areas of high rainfall where other diseases thrive. Unlike other fungi, powdery mildew spores do not require standing water for germination. High humidity, shady locations, and crowded planting, and/or poor air circulation do promote spore germination. The spores are easily transported by the wind and, as with most fungal spores, they are everywhere in the natural environment. To start the infection, the spore lands on a leaf and forms a tangle of fungal threads, called hyphae, on the surface. Special organs, called haustoria, penetrate the epidermal cells in search of food. Starting out slender, haustoria expand once they are inside the cell. They then take a round or branched form to increase their surface area to allow more efficient uptake of the host plant's carbohydrates. Powdery mildew occurs on so many plant species that an all-inclusive list is not possible in this short article. In this installment we will look at a few of the most common host plants: roses, lilacs, and cucurbits (squash, cucumber, melons, and gourds). Life histories and treatment strategies of powdery mildews vary slightly from species to species. The key is leaning to recognize the disease. Rose and lilac (and other ornamental plant) mildews are serious diseases in Arizona. In roses, some varieties are more susceptible than others. The first sign is a slight curling of the leaves. The mycelia may be barely noticeable. It becomes progressively more conspicuous on the surface until it covers buds and decreases and distorts blooms. Late in the growing season, mildew tends to be concentrated on the stems. Conventional treatments include sulfur, copper hydroxide, copper sulfate, chlorothalonil, or thiophanate-methyl. Always read the product label carefully and follow instructions. Culturally, make sure roses are well pruned and planted in a sunny area. Prune lilacs after bloom otherwise you will prune off the buds containing blooms. Over-fertilization with nitrogen can also increase powdery mildew susceptibility. The species of powdery mildew that affects cucurbits was first reported in North America in 1890. Besides cucurbits, this species attacks lettuce, chrysanthemums, clematis, yarrow, many composites (daisies and sunflowers), campanula, mallow, phlox, sage, and a long list of others. Cucurbits always seem to become infected about midsummer. Conventional treatments are similar to that for roses and lilacs. However, make sure to double check the application rate and follow directions on the product label. Sulfur can cause plant injury if the temperature is above 75 degrees F. Biocontrols are also becoming available. One is product is called AQ10. It is a fungus (Amplomyces quisqualis) that attacks powdery mildew fungus. The spores germinate and grow into powdery mildew mycelia and the AQ10 parasitizes powdery mildew. Neem oil, while usually used as a botanical insecticide, is also effective at controlling powdery mildew. The oil is extracted from the depulped seeds of the neem tree: a native to the Indian subcontinent. Another product, M-pede, is used widely by organic producers to control powdery mildew. M-pede is made from potassium salts of fatty acids and functions both as an insecticide and fungicide. Remember that cultural practices can decrease the incidence of many plant pests and diseases. Healthy plants can often resist pest attacks by themselves. Finally, by understanding the biology of the pest organism, you will gain insight and reduce losses due to plant pests. The University of Arizona Cooperative Extension has publications and information on other plant diseases and their treatment. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |