The moist weather pattern recently affecting Arizona in general and Yuma County in particular is providing environmental conditions favorable for downy mildew development on crops susceptible to this disease. Rainfall in vegetable production regions of Yuma County from Dec 21 to Jan 1 have ranged from 0.8 to 1.5 inches. These periods of rainfall plus the associated high relative humidity and extended periods of leaf wetness provide ideal conditions for rapid development of downy mildew. Disease severity is a function of the daily duration of plant wetness, with increasing number of nights and following mornings with wet plants resulting in increasing disease. If not already initiated, now is the time to start a preventative fungicide application program for downy mildew 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 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, Reason, Revus, and Tanos. 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 arrest downy mildew development. However, constant vigilance is needed, as future occurrences.

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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.