Lettuce downy mildew (November 26, 2014)

We are approaching that time in the desert lettuce production season when downy mildew can become a concern. Development of downy mildew on lettuce 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 but good levels of downy mildew control can also be obtained by starting fungicide applications at the very first sign of the disease; however, remember that 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.