Powdery Mildew on Vegetables - July 14, 2010
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County


Powdery mildew is a common fungal plant disease. There are many different species of powdery mildew fungi and each species only attacks specific plants. In our area, it is especially common on peas and cucurbits (squash, melons, pumpkins, and cucumbers). However, several other vegetable crops are affected by powdery mildews, including artichoke, beans, beets, carrot, eggplant, lettuce, parsnips, peppers, radicchio, radishes, tomatillo, tomatoes, and turnips.

Interestingly enough, these fungi are more abundant in semiarid areas, than in areas of high rainfall where other diseases thrive. Unlike other fungi, powdery mildew spores do not require standing water for germination. However, high humidity, shady locations, and crowded planting, and/or poor air circulation do promote spore germination. The spores are easily transported by the wind and, as with most fungal spores, they are everywhere in the environment.

All powdery mildew fungi require living plant tissue to grow. To start an infection, the spore lands on a leaf and forms a tangle of fungal threads, called hyphae, on the surface. Special organs, called haustoria, penetrate the epidermal cells in search of food. Starting out slender, haustoria expand once they are inside the cell. They then take a round or branched form to increase their surface area to allow more efficient uptake of the host plant's carbohydrates. Once the infection is easily visible to the naked eye, a thin layer of the fungus has spread over the leaf surface which is covered with reproductive spores. Special resting spores are also produced late in the growing season allowing the fungus to survive cold winters.

The best method of control is prevention. Planting powdery mildew resistant vegetable varieties when available (or avoiding the most susceptible varieties), planting in the full sun, and following good cultural practices will adequately control powdery mildew in many cases. However, very susceptible vegetables such as cucurbits may require fungicide treatment. Several least-toxic fungicides are available. These products work best as a preventative treatment but must be applied no later than the first sign of disease. Some of these products include horticultural oils, sulfur, potassium bicarbonate, and baking soda.

Biocontrols are also becoming available. One is product is called AQ10. It is a fungus (Amplomyces quisqualis) that attacks powdery mildew fungus. The spores germinate and grow into powdery mildew mycelia and the AQ10 parasitizes powdery mildew. Another powdery mildew biocontrol products contain the bacteria Bacillus subtilis (Serenade Garden). Neem oil, while usually used as a botanical insecticide, is also effective at controlling powdery mildew. The oil is extracted from the depulped seeds of the neem tree: a native to the Indian subcontinent.

A home remedy can be made by combining 2-1/2 tablespoons of horticultural oil (Sunspray Ultra-Fine, Saf-T-Side, etc.) in a gallon of water and adding 4 teaspoons baking soda. This solution is sprayed on plants to prevent powdery mildew infections. Sprays of baking soda can injure the plant, so use these materials with caution. Also, baking soda sprays can have deleterious effects on soil structure and should be used sparingly.

There are several effective fungicides available for different gardening situations and plant species. Be certain the product you purchase is labeled for the intended use(s) and follow label directions.

Cultural practices and sanitation can also decrease the incidence of many plant pests and diseases. Healthy plants can often resist pest attacks by themselves. In addition to planting in sunny areas as much as possible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow-release or organic fertilizer. Overhead sprinkling may help reduce powdery mildew because spores are washed off the plant. However, overhead sprinklers are not as efficient as drip or furrow irrigation and could contribute to other pest problems. Removing infected plants and keeping them out of the compost pile is a sanitation practice.

Naming of companies or products is neither meant to imply endorsement by the author nor criticism of similar companies or products not mentioned.

Follow the Backyard Gardener on Twitter use the link on the BYG website (see link below). If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: July 8, 2010
Content Questions/Comments: jschalau@ag.arizona.edu

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