Lettuce Downy Mildew (November 27, 2013)

The storm that passed through Arizona last week deposited a significant amount of rainfall in the region, ranging from 2/3 of an inch at the Yuma Valley Agricultural Center to about 2.5 inches in the North Gila Valley. This moisture plus dew on lettuce in the days following the rain increases chances for development of downy mildew. This disease is best managed by having a fungicide in place before disease symptoms become apparent. Good levels of downy mildew control can also be obtained by starting fungicide applications at the very first sign of the disease; however, there is a lag time between infection by the pathogen (Bremia lactucae) and appearance of visible symptoms. This incubation period can range from 3 days to longer than a week, depending on temperature, relative humidity, and lettuce variety susceptibility to the pathogen. By the time lettuce downy mildew lesions are observed, many more are likely present but have not matured to a sufficient extent to be 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, 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 pathogen resistance management.

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To contact Mike Matheron go to: matheron@ag.arizona.edu.