One or more wet rainy days followed by mornings with persistent dew provide the ideal environment for development of downy mildew on susceptible crops. Up to now, rainfall has been scarce this vegetable growing season, with only a few hundredths of an inch falling last Nov 7 and 8 in Yuma County. Does this mean that Pest Control Advisors do not need to worry about downy mildew? Not necessarily, as moisture can develop on leaves in the absence of actual rainfall, when calm conditions exist in moist fields leading to high relative humidity within the crop canopy. Also, sprinkler irrigation can supply moisture needed for 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. Dew remaining on crop leaves past mid-morning for several days is a warning sign that downy mildew could develop. Disease management outcome is best if a preventative fungicide application program is initiated when these moist conditions are observed in the field. Waiting until the symptoms of downy mildew are visible before beginning fungicide applications can result in a less than optimal level of disease management. By the time downy mildew lesions are observed, many more are probably 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 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.