When infected, leaves may turn yellow before dying and falling off. Although plants can be weakened by an infection, they don’t usually die.

Vegetable crops affected by powdery mildew include artichoke, beans, beets, carrot, cucumber, eggplant, lettuce, melon, parsnip, peas, peppers, pumpkin, radicchio, radishes, squash, tomatillo, tomatoes, and turnips.

The growing tips of fruit trees may also develop the disease. Many ornamentals—such as lilacs, zinnias, and roses—can also be infected. Succulent tissue is most susceptible to infection.

Unlike most fungi, powdery mildew spores do not need water for germination, and it is most severe in warm, dry climates. The warm days and cool nights of summer create favorable humidity for spore growth. This is also true of shady areas in the garden. The spores are spread by the wind and can overwinter on both plants and plant debris.

Powdery mildews are host specific (i.e. the mildew on roses is different from the one affecting grapes) and they cannot survive without the proper host plant. Circumstances influencing the disease’s severity include the variety of the host plant, age and condition of the plant, and weather conditions during the growing season.

Inspect plants often to catch infestation early. Once powdery mildew sets in, there is little that can be done to eradicate it.

If it does manage to get into your garden, remove the infected parts from your property. Do not compost infected plant debris. When powdery mildew is steadfast, a spray program may assist in controlling its spread.

A biological fungicide, Bacillus subtilis, is also a less toxic option. Preventative fungicides include sulfur and potassium bicarbonate.

If your plants are already infected, neem or jojoba oil may help prevent spread. As with any spray introduced to your garden, read and follow application directions. Never spray when beneficial insects are present.

Prevention is the most effective strategy for controlling powdery mildew. Plant in full sun, avoid overcrowding to provide good air flow, and limit nitrogen fertilizer to slow the production of succulent tissue.

You can also coat the upper and and lower surfaces of the leaves with a homemade spray as a preventable measure. The spray solution is made by adding 4 tablespoons of baking soda, 2 tablespoons of vegetable oil, and a few drops of dishwashing detergent to a gallon of water.

Watering at soil level—as opposed to overhead watering (i.e. via a sprinkler)—will help you avoid wetting the foliage which, in turn, could increase relative humidity.

Lastly, we recommend choosing powdery mildew-resistant plant varieties whenever possible.