How To Protect Your Summer Plants Against Powdery Mildew

Here's what you need to know to deal with this garden destroyer.

July 21, 2017
powdery mildew
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Powdery mildew is a gray, talcum powder-like coating that covers the leaves, flowers, and even fruit of some of your favorite summer vegetables, perennials, and shrubs. Powdery mildew can affect plants in all regions of North America.

Powdery mildew fungal spores are spread by wind and overwinter on plants, and in plant debris. Unlike mildews that appear in bathrooms or basements, powdery mildew does not need direct contact with water in order to grow. The warm days and cool nights of late summer create an ideal climate for spore growth and dispersal.

(Whether you're starting your first garden or switching to organic, Rodale’s Basic Organic Gardening has all the answers and advice you need—get your copy today!)

mildew on leaves
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How Plants Are Affected

Powdery mildew is the blanket name for a few different species of fungi that infect many ornamentals, such as beebalm (Monarda), lilacs (Syringa), zinnias, roses, and garden phlox (P. paniculata). It also affects vegetables, including beans, cucumbers, grapes, melons, and squash.

Damage To Plants

Powdery mildew is unattractive and it can affect the flavor and reduce yields of some fruits and vegetables. Although plants are unsightly and can be weakened by an infection, they do not usually die. Powdery mildew on ornamentals is an aesthetic issue, and not usually worth treating. Prevention and control is more important for vegetables.

spraying for powdery mildew
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How To Organically Control Powdery Mildew

Powdery mildew can be prevented, and it can be controlled once it appears, but it can't be cured. The key to preventing it is planting mildew-resistant or mildew- tolerant varieties. Resistant varieties get less mildew than susceptible varieties; tolerant varieties may get some mildew, but it shouldn't affect the performance of the plant. Prevention also includes siting plants where they will have good air circulation, and exposing as much leaf surface as possible to direct sunlight, which inhibits spore germination.

To control minor infestations, pick off affected plant parts and either compost them in a hot compost pile or bag them tightly and put them in the trash.

Related: 7 Easy DIY Recipes For Deterring Garden Pests, Diseases, And Unwanted Wildlife

Homemade Sprays

Research studies in 1999 and 2003 on infected zucchini and winter wheat (respectively) indicated that spraying cow's milk slowed the spread of the disease.

To try this at home, mix 1 part milk with 9 parts water and spray the stems and tops of leaves with the solution. Reapply after rain.

Also, spraying leaves with baking soda (1 teaspoon in 1 quart water) raises the pH, creating an inhospitable environment for powdery mildew.

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