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Controlling powdery mildew in the garden

May 04, 2002|By Thomas Turini, Imperial County-University of California Cooperative Extension

A white powdery substance on leaf surfaces of many types of plants is a familiar sight for many gardeners.

This white substance is made up of many fungal spores. Spores are to fungi what seeds are to plants. Like seeds, spores will germinate under favorable conditions. When these spores are on a susceptible plant, they can cause disease.

These fungi cause the disease powdery mildew. There are many kinds of powdery mildews. Different powdery mildew fungi cause disease on different plants.

Generally, powdery mildews do not require moist conditions and normally do well when the weather is warm. Therefore, the powdery mildews thrive under Imperial Valley conditions.


>> Identification

This disease can be recognized easily on most plants by the white powdery growth that forms on shoots and both sides of leaves. Occasionally it can be seen on flowers and fruit. However, the type of powdery mildew that attacks tomatoes, eggplants, peppers and some ornamentals produces yellow patches on leaves but often no powdery growth.

On vegetables, powdery mildew appears first on the upper leaf surface of older leaves. Then symptoms spread to the undersides of leaves and stems. Affected leaves may turn completely yellow, die, fall off and exposed fruit beneath may become sunburned. On some plants, powdery mildew may cause the leaves to twist or distort. Powdery mildew does not directly damage most vegetables, although pea pods may develop brownish spots. The sugar content of fruit is often reduced, and the flavor of melons and squash may be affected. As compared to a healthy plant, a severely infected plant may produce fewer fruit.

On perennial plants such as grapes, fruit trees and roses, this disease can be severe. On these plants, the disease attacks new growth including buds, shoots, and flowers as well as leaves. Young tissues are stunted, distorted and covered with a white, powdery growth. On apples and grapes, young fruit develop rough, reddish scars.

>> Biology

All powdery mildew fungi require living plant tissue to grow. On some plants, the fungi can survive on leaves that remain on the plants through winter. Year-round availability of crop or weed hosts is important for the survival of the powdery mildew fungi that infect tomatoes, peppers, and eggplants. Special resting spores are produced that allow survival of the species that cause the disease in lettuce and peas and certain other crops including several perennials.

Most powdery mildew fungi grow as thin strands on the surface of the affected plant. Spores, which are capable of infecting susceptible plants, make up most of the powdery growth. Chains of these spores can be seen with a hand lens. Powdery mildew spores are carried by wind to new hosts.

All powdery mildew species can germinate and infect in the absence of water. In fact, spores of some powdery mildew fungi are killed and germination and fungal growth are prevented by water on plant surfaces. Moderate temperatures and shady conditions are generally the most favorable for powdery mildew development.

>> Control

In most cases, planting resistant vegetable varieties or avoiding the most susceptible varieties and following good cultural practices will adequately control powdery mildew.

However, under some conditions, fruit and ornamentals require protection with fungicide sprays. Fungicide applications are most often needed on susceptible varieties of apples, grapes, cucurbits, roses, and crape myrtle.

>> Resistant varieties

Cantaloupe and honeydew, cucumber, pea, rose, crape myrtle and euonymus varieties resistant to powdery mildew are available. However, few fruit or grape varieties have mildew resistance. Be aware of control needs when planting more susceptible varieties.

>> Cultural practices

If possible, avoid planting in a shaded area. Provide adequate but not excessive amounts of water and avoid excess fertilizer. Keep grapes and other perennials carefully pruned and trained to allow exposure of fruit to sunlight and to insure that air moves through the canopy. As new shoots begin to develop on perennial plants, watch closely for the appearance of powdery mildew.

>> Fungicides

In some situations, especially in the production of apples, grapes, susceptible melons, squash and roses, fungicides may be needed. Sulfur is available at nurseries and in the home garden section of department stores. Sulfur in a dust form as well as formulations of sulfur that can be sprayed on the plants are effective against powdery mildew provided that plant coverage with these materials is adequate. However, sulfur can be damaging to some squash and melon varieties. To prevent plant injury to any plant, do not apply sulfur when temperature is near or over 95°F.

In general, fungicide applications are most likely to be successful when applied at the earliest signs of the disease. Once the disease is severe, it is usually too late for effective control with fungicides.

Closely follow label directions when working with pesticides.

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