Lettuce Downy Mildew (November 11, 2015)

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 control of downy mildew will result when fungicide applications are not started until downy mildew symptoms are seen on plants. This is due to the fact that there is a lag time between infection by the pathogen (Bremia lactucae) and appearance of visible symptoms. This incubation period can range from 10 or more days, 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 yet 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, Orondis (registration pending), 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. A new publication entitled “Biology and Management of Downy Mildew of Lettuce” provides additional information on the biology and management of this disease and is available.