We’ve been getting a lot of much-needed rain in the past couple of weeks, followed by cool, humid nights and breezy days. This is the perfect breeding ground for the spread of powdery mildew in the garden, and a couple of weeks ago, as I was pruning my bee balm (monarda), I found the yellowing lower leaves and powdery white coating typical of this fungal disease.
If powdery mildew isn’t treated, it can result in stunted growth, twisted and distorted stems and leaves and the death of the affected plant and all plants exposed to the disease.
Powdery mildew starts out as a few spores but can quickly spread through crowding of plants and the wind. Bee balm is particularly susceptible to this disease. Other plants that need to be watched include black-eyed Susan (rudbeckia), hydrangeas, lilac (syringa), phlox, photinia, privet (ligustrum vulgare), roses and zinnias, as well as vegetables such as cucumbers, summer/winter squash, pumpkins, melons, okra, peas and beans.
The first thing a gardener must do to get rid of powdery mildew is to remove the yellowing or powder-coated leaves of an affected plant, taking care not to touch healthy plant material. Throw this diseased material in your garbage. Do not compost it or attempt to recycle it as yard debris.
Treatment for the disease should be done as soon as you see any signs of yellowing leaves or gray or white powdery coating. In the past, recommendations included treating the leaves with horticultural oils like Neem oil that would smother the powdery mildew fungus. Homemade remedies made with a mixture of baking soda and soap have also been popular.
A new treatment involving the use of a product found in almost every home was discovered by scientists working in Brazil in 1999. They found that spraying plant material weekly with a dilution of one part milk to nine parts water during periods when powdery mildew might occur was as effective as the chemical fungicides currently on the market. Subsequent experiments in Australia and the United States have confirmed the efficacy of this treatment.
It seems that milk contains a natural germicide, as well as naturally occurring salts and amino acids that are taken up by the sprayed plants. Its secondary effect as a foliar fertilizer can help boost the plants’ immunity to fungal diseases. Skim milk and powdered milk work as just as well as whole milk.
So, grab a pint of milk from your fridge, and mix it with water (in a 1:9 ratio as explained above) in your sprayer. Make sure it is a sunny day and apply early so that the milk has a chance to dry on the leaves of the affected plants.
More milk is not better — make sure that the milk doesn’t exceed the recommended percentage, as some studies have shown that more milk can result in culturing a different mold. This solution works best before your garden is in flower.
This toxin-free application has worked for me — so far, the powdery mildew I found last week has not spread or infected other plants — and my garden is full of susceptible plants. So go out and give your garden a milk shake. You and your plants will both enjoy the treat.
Eleni Silverman is a Master Gardener, Vice President of the Belle Haven Garden Club, Chair of the Landscape Committee at Thomas Jefferson High School for Science and Technology and author of the garden blog "Belle Haven Garden Maven." She admits to a fascination with all things gardening, believes even compost is engaging, and will eagerly discuss the relative merits of leaf mold versus hardwood mulch.