The moist weather pattern now affecting Arizona in general and Yuma County in particular is providing environmental conditions favorable for development of downy mildew on crops susceptible to this plant disease. Rainfall in the vegetable production regions of Yuma County on Jan 4th and 5th ranged from about 0.11 to 0.17 inches and has been followed by mornings with high humidity and extended leaf wetness periods. 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 not already initiated, now is the time to start a preventative fungicide application program for downy mildew on susceptible crops. Waiting until 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 not yet visible due to lack of sufficient development. Fungicide evaluation trials conducted at the Yuma Agricultural Center as well as in other states have demonstrated significant reduction in disease by application of fungicides such as Actigard, Aliette, Cabrio, Curzate, Dithane, Forum, 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.