We are approaching that time in the desert vegetable production season when downy mildew can become a concern. Development of downy mildew on crops such as lettuce, spinach, onions, broccoli, cauliflower, and cabbage is strongly regulated by the presence and duration of free moisture on plant leaves. This moisture can be supplied by rainfall, dew, and sprinkler irrigation. Optimal management of downy mildew is achieved by having a fungicide in place before disease symptoms become apparent. Less than optimal control will occur when fungicide applications are not started until downy mildew symptoms are visible on plants. This is due to the fact that there is a lag time between infection by the microscopic pathogens that cause downy mildew on these crops and the appearance of visible symptoms. This incubation period can range from 10 or more days, depending on temperature, relative humidity, and plant susceptibility to the pathogen. By the time downy mildew lesions are observed, numerous additional infection sites are developing but have not yet grown sufficiently to become visible. Fungicide evaluation trials conducted at the Yuma Agricultural Center in Arizona as well as in other states have demonstrated statistically significant reduction in disease by application of fungicides such as Actigard, Aliette, Cabrio, Curzate, Dithane, Forum, Orondis, Presidio, Manzate, 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 for pathogen resistance management. A publication entitled “Biology and Management of Downy Mildew of Lettuce” provides additional information on the biology and management of the disease on this crop and is available from the following publication Biology and Management of Downy Mildew of Lettuce.

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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.