Lettuce Downy Mildew (January 22, 2014)

The last rainfall received in the Yuma area greater than 0.01 inch occurred November 21 to 23, according to the AZMET weather station at the U of A Yuma Agricultural Center. Since downy mildew development depends on the presence of free moisture on plant leaves, why can downy mildew appear in some lettuce fields in spite of our current lack of rainfall? The answer lies in the development of dew on leaves. Irrigation supplies ample moisture to plants, and the microenvironment within developing lettuce crops can result in development of dew. The number of days and hours per day that dew is present can affect downy mildew development and severity. Also, the relatively warm temperatures we have had recently favor disease development when leaf wetness requirements are met. Optimum management of downy mildew 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, 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 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. 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 development and expansion of downy mildew activity.

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