Wednesday, September 2, 2009

Powdery mildew on vegetables

The telltale talcum powder look of powdery mildew infections is common on squash and pumpkin vines now. This disease can cause yellow leaf patches on some plants, and distorted buds, stems and leaves on others. Leaves may drop prematurely and flower buds fail to open.

Powdery mildew damages plants by decreasing photosynthesis and removing nutrients from the host plant. Infections weaken plants and leave them vulnerable to other pests. Mildews are host specific and the ones seen on the vine crops will not affect onions or fruit trees.

It is generally thought that plants in areas with poor air circulation are prone to infection. Mildews are different than many other plant diseases and don’t require wet leaves for infection. Warmth and adequate humidity are sufficient. Dense plantings hold humidity on calm days and are often enough to set off disease development..

Do plant vegetables in full sun as shade contributes to weaker plants and also longer moisture retention. Prune or thin plants to increase light and air circulation. Direct water on the soil and don’t wet plant leaves. Avoid excessive fertilizer that promotes succulent leaf tissue that is more easily penetrated by disease organisms. Look for powdery mildew resistant varieties if available.

Most fungicides are preventives meant to be applied before disease appears or at least in the early stages. Examples are sulfur and potassium bicarbonate (sold as Remedy). Read label directions before application. With sulfur, be mindful that it can damage some melon and squash varieties. Do not apply sulfur when temperatures are at of above 90 degrees F or plant injury can result. In the photo right an eradicant stopped the disease and weather conditions were unfavorable for development so new growth is unaffected.

A few fungicides can kill existing mildew infections (eradicants) but are still best applied at the earliest sign of disease. The natural Neem plant-based fungicides (Greenlight Powdery Mildew Killer) and horticultural oils are eradicants. Don’t apply oil to drought-stressed plants or within two weeks of a sulfur spray application. Don’t spray oils at temperatures at or above 90 degrees F.

Thanks to Mary Small for information used in this post

Photo credit 4 powdery mildew photos, Carl Wilson

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