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February is a month when powdery mildew can make its initial appearance in maturing
lettuce plantings. The disease, caused by the fungus Golovinomyces cichoracearum,
is first observed as very small spots of white fungal growth on upper and lower
leaf surfaces of the oldest leaves. From these initial infection sites, the fungus
continues to grow on the leaf surface and release vast quantities of spores which
are carried in the air, when upon landing on lettuce leaves initiate additional
infections under favorable temperature and moisture conditions. The most favorable
temperature range for spore germination is 65 to 77 °F. Relative humidity at or
above 85% is required for infection, growth and sporulation by the
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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