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Powdery Mildew of Vegetables1
Ken Pernezny and W.M. Stall2Powdery 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 occasionally found on strawberries and watermelons in the state. Powdery mildew of okra, squash, cucumbers, muskmelons, honeydews, and pumpkins is caused by the fungus Podosphaera xanthii (Sphaerotheca fulginea). 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 spots on leaves ( Figure 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 ( Figure 2 ) or other plant parts ( Figure 3 ). 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 become yellow, then become dry and brown ( Figure 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 ( Figure 5 ).
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5. The fungus causing the disease 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.
CONTROL
Crop rotation and many other cultural practices seem to have little effect on powdery mildew incidence and development. However, healthy, vigorous leaves and stems are less prone to infection. Plants under nutritional stress in most cases will develop powdery mildew much sooner than plants the same age grown under a good nutritional program.Cultivars of many crops have degrees of resistance or susceptibility to powdery mildew. For example, cucumber cultivars Poinsett and Gemini (slicing types) and Carolina, Galaxie, Explorer, Premier, and Calypso (pickling types) have various levels of resistance to powdery mildew. Bean cultivars have also been noted to contain varying degrees of tolerance to leaf and/or pod infection. Growing cultivars with resistance or tolerance to the disease is recommended during periods of high powdery mildew incidence.
Even with the above practices, chemical control often is needed. Under low disease pressure, some materials applied for downy mildew control may give satisfactory control of powdery mildew. However, under moderate to heavy mildew disease pressure, specific fungicides are recommended. Check with your county agent for updated registrations, recommendations, and harvest limits.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the assistance of Gary W. Elstrom in preparation of this fact sheet.
Footnotes
1. This document is PP-14, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date April, 1994. Revised May, 2005. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.2. Ken Pernezny, professor, Plant Pathology Department, Everglades Research and Education Center, Belle Glade FL; W.M. Stall, professor, Horticultural Sciences Department; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.
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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
Copyright Information
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