Diseases of lettuce (
Powdery mildew
Powdery mildew is a foliar disease caused by the obligately parasitic fungus Erysiphe cichoracearum. Disease primarily affects plants approaching maturity. Unlike most foliar fungal diseases, it can occur in dry weather and develops in the absence of free water on leaf surfaces. Initial signs are small tufts of fungal growth on upper or lower leaf surfaces. As disease develops, much of the leaf may become covered by the fungal hyphae, giving it a powdery or dusty appearance (photo 1). Masses of spores which are easily windborne are produced in chains from the hyphae. Low light intensity and high humidity (but not presence of free water) favor disease development.
Powdery mildew can be managed if tolerant varieties of lettuce are available and by the preventive application of Azoxystrobin or sulfur before disease begins.
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February 20, 2013