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This is the time during the lettuce production season in Yuma County that
downy and powdery mildew can both be present in the field. 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 these 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 bottom 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 the development of a lettuce crop, it
is possible to have symptoms of both mildew diseases on lettuce at the same time.
Disease Triangle
Click picture to listen to Mike's update
To contact Mike Matheron go to: matheron@ag.arizona.edu.
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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
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