Zucchini and other members of the squash family are vigorous growers, and within a few weeks can cover remarkably large areas of a garden. If given enough water from irrigation or rainfall, their large leaves spread out to shade the soil, which helps to retain soil moisture. While this is generally good for the plants, a moist environment (along with warmth and shade) encourages fungal diseases, particularly powdery mildew.
Powdery mildew appears as a slightly fuzzy white, grayish-white or gray growth on leaf surfaces. Because it often begins on the leaf stem and veins, it is sometimes mistaken for an accidental dusting by wind-blown powder or dust. It spreads rapidly to cover most or all of the leaf, both upper and lower surfaces. This prevents photosynthesis by the plant, resulting in plant death.
Because powdery mildew is such a widespread problem for home gardeners and commercial growers, seed breeders have developed varieties that exhibit resistance. This is indicated by the letters PMR, for powdery mildew resistance, after the plant name. If the resistance is strong, the letters PMRR are used. New varieties with resistance are introduced every year, and to grow them is surely the best way to avoid problems with this disease.
The earlier an infection is spotted, the greater the chance of successful treatment. When there are relatively few fungal spores on and around the plant, even a simple remedy using baking soda is very effective. Mix 1 tsp. of baking soda in a quart of water. Add 1 tsp. of a horticultural oil, such as SunSpray UFP®, which helps the mixture to spread and stick on leaves. Mix thoroughly, then spray the entire plant, particularly the tops and undersides of all leaves. Do not spray this mixture on a hot, sunny day; choose a cloudy morning.
There are many species of powdery mildew. The species that attacks zucchini and related members of the cucurbit family is called Podosphaera fusca. Fortunately, mildew is very particular about where it settles, so powdery mildew on roses, say, will not spread to your zucchini.
Since fungal spores are carried on the wind and by insects, protecting your plants with floating row cover is an easy way to greatly reduce the risk of infection and prevent damage from insect pests. Floating row cover is a super-light synthetic fabric that allows water and air to reach your plants, but denies access to insects and wind-borne spores. It is available at garden centers and by mail order.