Powdery Mildew of Vegetables1
Ken Pernezny, Don Maynard, and W.M. Stall2
Introduction
Powdery mildew is a serious disease of beans, southern peas, okra, squash, cucumbers, muskmelons, honeydews, and pumpkins in Florida. The disease occurs also on English peas and is found on strawberries and watermelons in the state. Powdery mildew of okra, squash, cucumbers, muskmelons, honeydews, pumpkins, and watermelons is caused by the fungus Podosphaera xanthii (=Sphaerotheca fulginea) or, occasionally, Erysiphe cichoracearum. The fungus Erysiphe polygoni causes powdery mildew of beans, southern peas, and English peas. Powdery mildew of strawberries is caused by the fungus Sphaerotheca macularis.
The disease is found mainly on the older leaves and stems of plants. Yields of many of the infected vegetables are reduced due to premature foliage loss. In honeydew and muskmelon severe leaf infection usually results in lower fruit sugar content and subsequent reduction of fruit quality. In a few crops direct damage to the marketable produce occurs.
Symptoms and Disease Development
The fungus is usually first noted as subtle, small, round, whitish or yellow spots on leaves (Fig. 1) and sometimes stems. The spots enlarge and coalesce rapidly and a white mass resembling talcum powder becomes evident on the upper surface of older leaves (Fig.2) or other plant parts (. Young leaves are almost immune. A large part of the talc-like powder on the leaf surface is composed of spores. These spores are easily blown by winds to nearby susceptible plants.
Heavily infected leaves yellow, then become dry and brown (Fig. 4). Extensive premature defoliation of the older leaves can ensue if the disease is not controlled. Yield reduction from defoliation is proportional to the severity and length of time plants are infected. Severe economic losses can occur in beans when pods are infected. The pods develop purplish spots and become distorted. (Fig. 5).
Powdery mildew fungi can reproduce under relatively dry conditions. Increased humidity can increase the severity of the disease, and infection is enhanced during periods of heavy dew. Unlike downy mildew, powdery mildew can and does become severe during periods of low rainfall in the winter and spring months in Florida.
It is not known for certain how the fungus survives between crop seasons. The fungus is thought to survive on wild cucurbit and other weeds year round
Copyright 2009 University of Florida
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