These days the warm afternoons of late summer turn to cool nights, opening again to foggy mornings. Often plant leaves drip with moisture. These conditions make it ripe for powdery mildew to flourish, especially on vegetable crops like squash, pumpkin and cucumbers.
Powdery mildew is a white fungus that also coats the foliage of roses, apple trees and dahlias, among others. Eventually these plants will suffer by late fall. But on the coast it happens sooner rather than later.
Often it is necessary to spray with a fungicide to keep powdery mildew in check. Fortunately, organic fungicides are widely available for controlling such diseases. Control, not eradication, is the key word here. Organic fungicides are best used as preventatives rather than immediate cures. So, if squash vine leaves show symptoms of white powder, remove them and spray the new leaves on a regular basis as a preventative.
Inspecting susceptible plants weekly, and taking action at the first sign of a problem, is crucial. If you discover a plant already infected, remove all diseased portions and then begin the spray regimen. Equally importantly, pay strict attention to the rules on the label. More is not better. Some oil sprays will burn foliage if applied in heavy doses, or during the heat of the day.
Consider the following organic fungicides; they are effective preventatives:
PLANT-BASED OIL -- Jojoba beans, neem tree seeds, rosemary leaves, clove and citrus all help prevent powdery mildew. The fresh oil coating, albeit thin, forms a physical barrier between the leaf surface and the attacking organism. Regular spraying from now through autumn is effective.
SULFUR -- Sulfur is an old-time cure commonly used to prevent powdery mildew, black spot, rust and certain leaf spots. It must be used on a regular basis before a disease takes hold. Do not apply within two weeks of using an oil spray.
BACILLUS SUBTILIS -- This is a soil bacterium commonly found in the soil. It controls bacterial spot, powdery mildew, rust, gray mold, leaf blights and scab. It is most effective when used as a preventative.
An over-the-counter, ready-to-use spray called Serenade is available. Give this one a try if you are having problems with leaf spot and blight on tomatoes.
HOMEMADE SPRAY -- Mix 2 1/2 tablespoons of light, plant-based oil with 4 teaspoons of baking soda in one gallon of water. Shake and spray. This is an effective fungicide, especially for roses, dahlias, apple trees, cucumbers and squash.
Terry Kramer is a trained horticulturist and journalist. She has been writing a garden column for the Times-Standard since 1982. To get in touch with Terry, send an e-mail to firstname.lastname@example.org and put "For Terry Kramer" in the subject line, or write to Terry Kramer, c/o Times-Standard, P.O. Box 3580, Eureka, CA, 95502.