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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, which 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 pathogen. Low
light intensity also favors powdery mildew development. These requirements are often
all met for several hours daily, especially on lettuce leaves near or at the soil
surface in a maturing lettuce planting. As little as 4 days are needed from infection
to production of a new crop of pathogen spores. Depending on environmental conditions
and the particular susceptibility of the lettuce variety being grown, preventative
applications of a fungicide may be needed to prevent economic loss to the crop.
The oldest leaves where initial powdery mildew infections develop will not be harvested;
however, these leaves serve as nurseries for production and release of spores, which
can infect the marketable portion of the lettuce plant. In recent field trials,
fungicides that provided excellent control of powdery mildew on lettuce included
Fontelis (penthiopyrad), Microthiol Disperss (wettable sulfur), Procure (triflumizole),
Quintec (quinoxyfen), and Rally (myclobutanil). Initiating fungicide treatments
before or at the very latest at the first sign of infection on the oldest leaves
will result in the best levels of disease control.
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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