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Powdery Mildew

Powdery mildew. Photo: Brain Bushe

Photo: Brian Bushe, CTAHR

Powdery mildew on roses. Photo: Dr. W. Nishijima

Photo: Dr. Wayne Nishijima

Problem

There are white, powdery spots on the leaves and flowers of your vegetables.

Description  

Powdery mildew is a common disease on a wide variety of vegetables and ornamentals. It is caused by different fungi and is species specific, though the symptoms and method of control will be similar.  Symptoms of powdery mildew include white, powdery spots on both surfaces of the leaves which may spread to stems and flowers, and occasionally, to fruit.  On tomatoes, onions, peppers and artichokes, you may see yellow patches rather than powdery growth. Leaves may turn yellow, die, and fall off of the plant. There may also be some distortion of leaves. Plants in the cucurbit family are especially susceptible (cucumber, melons, squash, and pumpkins.)

Information

Control

Plant disease-resistant varieties. Plant in full sun, with good air circulation, and use slow release fertilizer so that you don’t over fertilize your vegetables. Fungicides will help, but must be used at the first sign of infection.  See an extension agent about the use of fungicide or consult the UC Davis IPM publication listed above.

You can also use neem oil, jojoba oil, wetable sulfur or horticultural oil to treat powdery mildew. Do not apply oils when temperatures are above 90 deg.F or within two weeks of spraying plants with wetable sulfur. Sulfur products work best when applied before symptoms appear or at the first signs of disease. Read and follow product labels carefully. Reapply every 7-10 days until all symptoms are gone.

Kendal Lyon, Hawaii Island Master Gardeners