Lettuce Downy Mildew (January 7, 2015)

Recorded rainfall during December, 2014 ranged from 0.28 to 0.38 inches at the four Yuma County AZMET weather stations (Yuma Valley, Yuma South, Yuma North Gila, and Roll). Since downy mildew development depends on the presence of free moisture on plant leaves, one may expect, based on rainfall alone, that disease incidence and severity would be relatively equal in the regions where these AZMET weather stations are located. However, rainfall is not the only source of free moisture in lettuce fields. The presence of dew on leaves is also very important. Another look at December weather data reveals that 100% relative humidity was recorded 1, 6, 8, and 12 days, respectively, at the Yuma Valley, Yuma North Gila, Roll, and Yuma South AZMET stations. The risk of downy mildew development and its severity is a function not just of rainfall but also includes the number of days and hours per day that dew is present, which in turn is dependent on the microclimate within a particular field. Clearly, the risk of downy mildew development in Yuma County lettuce fields is dependent on field location. Optimum disease management is achieved by having a fungicide in place before disease symptoms become apparent. Good levels of disease suppression can also be obtained by initiating fungicide applications at the very first sign of the disease; however, it is important to remember that there is a time lag 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 the susceptibility of the particular lettuce variety to the pathogen. By the time lettuce downy mildew lesions are first observed, many more are likely to be 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. 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.

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