Powdery Mildew

Powdery mildew is the name given to diseases resulting from infection by fungi that produce a white to gray, powdery growth on the surfaces of leaves and sometimes other plant parts. Leaves may yellow, then brown and die. Infected tissues may be distorted and misshapen.

There are many kinds of powdery mildew fungi, and most are highly specialized. For example, the powdery mildew that infects squash plants will infect some other plants in the cucurbit family but will not infect roses, and the powdery mildew from roses will not attack zinnias (and vice versa), although the fungus that infects zinnias also infects many other members of the composite family. Powdery mildew fungi are obligate parasites; that is, they can grow only on living plant tissue. When the mildew-infected plant part dies, so does the mildew unless chasmothecia (spherical, resting stages of the fungus) are formed.

Most powdery mildew fungi grow over the surface of the leaf, sending short food-absorbing projections (haustoria) into the epidermal cells. The fungi produce masses of spores (conidia), which become airborne and spread to other plants. Powdery mildew spores require no external moisture for germination, while most other fungi require free water in the form of dew, guttation, rain, or water from overhead irrigation for germination and infection. Conidia of powdery mildew (except those that infect grasses) die in water. Spores may be dispersed, however, by splashing water.

The fungus survives in the absence of susceptible host tissues by forming a sexual stage (chasmothecia) resistant to drying and other adverse environmental conditions. With many perennial plants, such as rose, the fungus survives as mycelium in dormant buds or actively on plant tissues. Powdery mildews are particularly severe in semiarid regions, such as most of California, and are less troublesome in high rainfall areas.

Powdery mildews are favored by warm days and cool nights and moderate temperatures (68° to 86°F). At leaf temperatures above 90°F, some mildew spores and colonies are killed. Shade or low light intensities as well as high relative humidity (greater than 95%) favor powdery mildew fungi. Greenhouse conditions are often ideal for development of the disease.

The best control is through the use of resistant cultivars. However, little attention has been paid to the development of resistant cultivars in flower and nursery crops. Increased air movement around the plants in the greenhouse tends to reduce infection potential of mildews. Plants that have been treated with anti-transpirants are less likely to develop powdery mildew infections

In general, there are two types of fungicidal control: eradication of existing infections and protection of healthy tissues. In practice, some products provide both protection and eradication, especially when good wetting of the plant is achieved. To achieve good wetting, some of these products may require the addition of surfactants.

The fungus has developed resistance to some of these fungicides. Rotate the different fungicides to help slow down the development of fungicide resistance within fungal strains.