Downy mildew has been appearing since early December in several areas within the southwestern Arizona lettuce production region. At first, one might consider this unusual as we have not received much rainfall during this time. On the other hand, dew on plants has been common and temperatures were above normal for a good portion of December; thus, temperature and moisture requirements for downy mildew development were in place. Downy mildew is best managed by having a fungicide in place before disease symptoms become apparent. Good levels of disease suppression can also be obtained by initiation of 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, 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. The last few days of low relative humidity will help arrest downy mildew development; however, constant vigilance is needed, as future occurrences of dew and rainfall will favor further disease expansion.

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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.