A recent newspaper article cited the high incidence of mildew on lettuce during the current production season. But lettuce can be infected with downy mildew as well as powdery mildew. Which mildew was it? The article was probably referring to downy mildew, which has been prominent this season. However, this is the time of year in Yuma County when both downy and powdery mildew can be present in lettuce fields. There are some straightforward clues that will help identify which mildew disease is present. Symptoms. Spots or lesions caused by downy mildew can first appear as one or more yellowish areas on the upper leaf surface usually bordered by leaf veins, giving the affected portions of the leaf a somewhat angular appearance. In time, these lesions will increase in number and older ones will turn brown and necrotic. On the other hand, first evidence of powdery mildew is the appearance of very small circular colonies of the pathogen growing on the upper or lower lettuce leaf surfaces. Over time, these colonies enlarge, produce spores that start new colonies, and eventually the entire leaf surface can become covered with the powdery mildew pathogen. Spores. Spores of the downy mildew pathogen (Bremia lactucae) usually arise from the underside of leaves, are egg-shaped, and are produced singly on the ends of branching mycelium, much like fruit are borne on many types of fruit trees. Large numbers of this spore-bearing mycelium and attached spores produce the downy white growth that we see, again usually on the bottom surface of leaves. Keep in mind that on head lettuce, the underside of the leaf can appear to be the upper surface due to the curving of leaves to form the lettuce head. However, for the powdery mildew pathogen (Golovinomyces cichoracearum), spores are produced in chains on pathogen mycelium growing on either side of lettuce leaves, giving the appearance of powder on the leaf. Required environment. Development of downy mildew requires free moisture on leaves, which can be supplied by rainfall, dew, or sprinkler irrigation. In contrast, powdery mildew development requires moderate to high relative humidity, but not free moisture. In fact, free water will actually kill spores of the powdery mildew pathogen. Due to changing environmental conditions during growth and maturation of a lettuce crop, it is possible to have symptoms of both mildew diseases on lettuce at the same time.

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.