Identifying Powdery Mildew

When powdery mildew finds your garden, it won't be difficult to spot. Each of the numerous powdery-mildew fungal strains attacks a specific group of hosts, and all leave the same calling card: plants looking as if they had been coated with powered sugar. Other signs of infection vary from stage to stage, but the signature white-to-grayish leaf blotches are unmistakable. Identifying the disease is the easy part.

Because powdery-mildew fungi are host-specific, identifying the disease on one variety of plant doesn't mean it's about to spread throughout your garden. The common Golovinomyces cichoracearum strain, for example, limits itself to annual zinnias (Zinnia spp.) and other Daisy-family (Compositae, Asteraceae) plants. Sphaerotheca fuliginea and Erysiphe cichoracearum, on the other hand, target only cucurbits such as zucchini (Cucurbita pepo) and cucumbers (Cucumis sativus).

Even before the telltale blotches appear, infected leaves may twist, curl or become discolored with reddish areas or scorching. Flower buds sometimes refuse to open.

In most cases, however, powdery mildew surfaces as white spots gradually merging to cover the stems and both sides of the leaves. Less frequently, they appear on fruit or flowers. The blotches never penetrate deeper than the uppermost layer of cells, or epidermis. That's deep enough, however, for them to siphon off a plant's growth-sustaining nutrients.

An exception to the both-sides rule is the Leveillula taurica powdery-mildew strain attacking tomatoes (Lycopersicon spp.) and other Nightshade (Solanaceae) family plants. Its blotches affect only the backs of the leaves.

Unless treated, powdery-mildew infection leads to yellowing, dying leaves that drop prematurely. On fruiting plants and vegetables, leaf loss may lead to sunburn. Continued nutrient loss slows growth and leads to loss of vigor.

Severe powdery mildew may also cause reduced cold resistance. For deciduous trees that drop their leaves in fall, this means delayed spring growth.

Powdery mildew infection begins when wind carries fungal spores from previously infected plants or fallen leaves to new hosts. Given the right conditions, the spores germinate into white spots in as little as a week, and the white spots begin producing spores of their own.

Combine dry plants, dry weather and temperatures in the 60- to 80-degree Fahrenheit range with shade, and you have ideal powder-mildewy conditions. Direct sun, wet surfaces and temperatures above 95 degrees kill the spores.

Take action as soon as you identify a powdery-mildew infection. Prune infested plant parts and dispose of them in sealed plastic bags. To keep the spores from spreading, disinfect your pruning tools in rubbing alcohol between cuts and after you finish.

To minimize symptoms, consider spraying with a homemade solution of 1 part skim or raw milk to 9 parts water.

Raw or nonfat milk

Measuring cups

Clean, 1-quart (32-ounce) spray bottle

Measure and pour 1/3 cup of raw or nonfat milk into a clean spray bottle.

Measure and pour 3 cups of water into the bottle.

Spray the infected plants thoroughly, being sure to hit both sides of the leaves.

Repeat weekly while conditions favor powdery-mildew infection.

The National Gardening Association estimates that this treatment reduces symptoms up to 90 percent.

Practice these preventive measures to reduce the likelihood of infection:

Promptly clean up and dispose of plant debris during the growing season, and remove all dead annuals and vegetable plants in the fall. Don't compost the material.

When possible, cut back overhead branches so more sun reaches the plants. 

Water from above to wash off spores before they germinate and kill the ones landing before the water dries.

Allow enough space between your plants for good air circulation.

Replace chronically infected plants with mildew-resistant varieties. 

Return to watering your plants from beneath after the risk of mildew subsides. Watering from above exposes them to other types of fungal infections.