The moist weather pattern affecting Arizona in general and Yuma County in particular for the past 3 weeks is providing the environmental conditions needed for rapid development of downy mildew on susceptible crops. From Nov 20 through Dec 8, rainfall recorded at the Yuma Valley, Yuma South, N. Gila, and Roll AZMET stations was 1.34, 0.87, 1.11, and 1.59 inches, respectively. Evenings with relative humidity high enough for dew formation occurred virtually every day during the same time period, setting up perfect conditions for disease development. Downy mildew severity is a function of the duration of plant wetness, with more nights and following mornings with wet plants resulting in increasing disease. If you are not already dealing with downy mildew, now is the time to consider initiation of a preventative fungicide application program for the disease on susceptible crops. Waiting until the symptoms of downy mildew are visible before beginning fungicide applications can result in a less than satisfactory level of disease management. By the time downy mildew lesions are observed, many more are likely to be present but have not developed to a sufficient extent to be visible. Fungicide evaluation trials conducted at the Yuma Agricultural Center as well as in other states have demonstrated statistically significant reduction in disease by application of fungicides such as Actigard, Aliette, Cabrio, Curzate, Forum, Manzate, Orondis, Presidio, Previcur Flex, Prophyt, Ranman, Revus, and Tanos. Check current registration status of any of these fungicides prior to use. Several different modes of action are represented by these compounds, thus facilitating alternation among different chemistries for effective disease management as well as pathogen resistance management. Periods of low relative humidity and little or no dew on leaves will help suppress but not totally stop downy mildew development. Constant vigilance is needed throughout the remainder of the crop growing season, as future occurrences of dew and rainfall will favor further development and expansion of downy mildew activity.

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.