Here are steps you can take to prevent powdery mildew from blemishing your garden.
A gray, talcum powder-like coating that covers the leaves, flowers, and even fruit of some of your vegetables, perennials, and shrubs.
Where it's a problem
Powedery mildew is found througout North America.
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.
Plants it attacks
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.
Why it's a problem
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.
Organic damage control
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.
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.
Spraying leaves with baking soda (1 teaspoon in 1 quart water) raises the pH, creating an inhospitable environment for powdery mildew.