   Watch for Powdery Mildew - July 4, 2001 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Powdery mildew often appears on vegetable, ornamentals, and fruit trees during the summer months. Most commonly, it appears on the leaves of cucurbits (squash, cucumbers, and melons) and roses during the summer months. Powdery mildew is caused by a fungus that forms white powdery patches on leaves, stems, or fruits. The powdery material is actually a mass of fungal strands (called mycelium), which may also contain spores (reproductive structures). Once this disease appears, it is very difficult to control. There are several species of fungi that are called powdery mildew and each species has a set of closely related host plants that they infect. For example, powdery mildew that infects roses will not infect cucurbits and vice versa. Some plant species tend to be infected by powdery mildew when they are not well suited to the environment in which they are planted. For example, a Euonymous that is planted in the shade may be more susceptible to powdery mildew than one planted in full sun. Conversely, cucurbits seem to get powdery mildew toward the end of the growing season regardless of planting site and cultural practices. Powdery mildew infects plant hosts first by wind depositing a spore on the plant surface. The spore germinates and develops a mycelium that spreads over the plant. On leaves, it most often grows on the upper surfaces. The fungus nourishes itself by penetrating into plant cells with root-like structures called haustoria, which remove mineral nutrients, sugars, and other compounds. On annuals and shrubs, the disease infects older leaves first. During heavy infections, these leaves often turn brown and die. On tree fruits, grapes, and berries, new shoots are often infected and killed. Powdery mildew is most likely to infect plants during periods of moderate to high humidity and temperatures. Good air circulation is beneficial but does not prevent infection. Spores germinate on dry plant surfaces. Water on leaf surfaces actually inhibits spore germination. Powdery mildew resistant varieties of some plants are now available. Read seed packets and plant descriptions to identify these varieties. Cultural practices can also be used to minimize powdery mildew damage. Some of these are planting on full sun, avoiding areas with poor air movement, avoiding overcrowding of plantings, and using overhead irrigation to prevent spore germination. Several compounds/chemicals can be applied to minimize damage from powdery mildew. All of these are most effective if applied when the very first signs of infection appear. Sulfur has been used for centuries to control powdery mildew. However, be aware that sulfur can damage some crops, especially if temperatures are above 90 degrees F. Another least toxic method of powdery mildew control is using baking soda. One formulation that I have seen is 1 tsp baking soda, one quart of water, and a few drops of liquid soap. Commercially produced, synthetic fungicides that control powdery mildew are also available. As with any pesticide, read the label carefully and follow directions explicitly. These products will list crops that are approved for application, mixing instructions, personal protection needed, and other important information that allow you to safely use these products. Some of these are Bayleton (triadimefon), Rally (mycobutanil), Rubigan (fenarimol), and Funginex (triforine). Watch for powdery mildew with the onset of the monsoon season and be prepared to deal with it. Look cultural control opportunities first: reducing shade or increasing air circulation. Try sulfur or baking soda next. If it is causing significant damage to a high value plant or crop, then explore some of the commercial fungicides as a final option. The University of Arizona Cooperative Extension has publications and information on gardening and diseases. 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. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |