A fungal disease which is found throughout North America, powdery mildew is easily recognizable by its white or greyish, talcum powder-like circles appearing on leaves, flowers and fruits of various vegetables, fruits, perennials and shrubs. The list includes roses, lilacs, dahlias, begonias, delphiniums, phlox, monarda (bee balm), euphorbias (spurge), catalpa (bean tree), zinnias…as well as squash, cucumbers, beans, peas, melons, apples, pears, strawberries, gooseberries, and grapes.
Zucchini ‘Dark Star’ with powdery mildew
Leaves covered by powdery mildew cannot manufacture enough food which can impact on plant growth and fruit development, depending on the rate of infection. But rarely does the mildew kill the plant. It just looks horrible.
There are a number of different fungi species responsible for powdery mildew. Some are species specific, others will attack a wider range of plant varieties. In regions of high humidity and moderate temperatures, the fungi produce mycelium and spores on the surface of affected foliage. The spores are then carried by wind currents to other plants.
Strangely enough, it is the wind which will reduce the risk of fungal infection. Providing adequate spacing between plants will increase air circulation and decrease the moisture retention on the leaves. Opening up shaded areas to more sunlight will also help.
Autumn is another trying time of year for protecting our plants and crops from powdery mildew. A lot of fungi spores repose in the soil and come fall, the rains splashing up onto the plants will often carry a few spores with the droplets.
No matter the time of year, once a plant has been infected, the mycelium will continue to spread on the leaf surface regardless of the moisture conditions.
And there is no known cure for powdery mildew…only prevention and a modicum of control once it appears. Best line of defence is to remove the affected leaves as soon as you spot them. Always put the infected plant parts in the garbage. Never put them in the compost unless you have a very hot pile.
To prevent powdery mildew on susceptible plants, mix up either of the following recipes. Both are reasonably effective.
One part cow’s milk
Nine parts water
Combine ingredients in a sprayer, if you have a lot of plants to treat…or a spray bottle if treating just one or two. Will have to be repeated after each rainfall.
1 gal water
1 tbsp baking soda
1 tbsp vegetable oil
1 tbsp dish soap
Combine all ingredients, in order as listed, in a sprayer or spray bottle. Preferably apply this mixture on cloudy days with no threat of rain. Applying on sunny days risks sunburn on the leaves.
Will protect the plant longer because of the addition of vegetable oil and soap in this recipe. However, it must be reapplied after a few rainfalls, or an especially hard downpour.
Submitted by Leslie Cox
CVHS member and writer as the “Duchess of Dirt”