Growers should keep in mind that powdery mildew can occur on fall melons in the desert. As with powdery mildew development in the spring and summer, the disease first becomes apparent as very small colonies that may be few in number. However, these spots will enlarge and produce ever larger quantities of spores, which in turn initiate more infections which increase powdery mildew coverage of melon leaves. Compared to powdery mildew in the spring and summer, the rate of disease development may be slower on fall melons due to lower temperatures as plants progress to maturity. Past fungicide evaluation trials have shown that several registered fungicides can manage powdery mildew very well, such as Microthiol Disperss (wettable sulfur) Quintec (quinoxyfen), Procure (triflumizole), and Rally (myclobutanil). In a field evaluation trial conducted last spring, three new fungicides in development provided excellent control of powdery mildew as well, indicating that additional effective compounds for disease management may be available in the future. Effectively managing powdery mildew with fungicides is best achieved by initial application before the first visible presence of the disease. Good levels of disease control also can be obtained by not starting applications until the first visible sign of powdery mildew; however, disease management success will decrease as the initial treatment is delayed and the disease becomes more established. The risk of powdery mildew within a particular melon planting will be determined by the genetic susceptibility of the melon cultivar as well as environmental factors during the growing season.

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To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.