No cure for powdery mildew — only prevention and a modicum of control

Powdery mildew is found throughout North America and is easily recognizable by its white to greyish, talcum powder-like circles that appear on leaves, flowers and fruits of various vegetables, fruits, perennials and shrubs.

Drat! First the tomatoes get blossom end rot, now there is powdery mildew on a few cucumber leaves in the greenhouse.

Luckily … because it is the end of the season … there should not be too much damage to the fruits.

Powdery mildew is found throughout North America and is easily recognizable by its white to greyish, talcum powder-like circles that appear 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, cukes, beans, peas, melons, apples, pears, strawberries, gooseberries and grapes.

Leaves covered by powdery mildew cannot manufacture enough food, which can seriously impact on plant growth and fruit development, depending on the rate of infection. But rarely does the mildew kill the plant. It just looks horribly unsightly.

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.

Throughout the growing season, the fungi produce mycelium and spores on the surface of affected foliage. The spores are then carried by even the gentlest of wind currents to other plants.

Strangely enough, it is the very wind that 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.

But this late in the season, we are almost hooped in protecting our plants and crops from powdery mildew. Right now, the cooler nights (we had four degrees Celsius the other night here in Black Creek) and our gorgeous sunny days are exactly the right conditions these fungi prefer.

Once a plant has been infected, the mycelium will continue to spread on the leaf surface regardless of the moisture conditions.

Best line of defence is to remove the affected leaves as soon as you spot them and bag them for the garbage. Do not put them in the compost unless you have a very hot pile.

A friend reminded me that a mixture of one part cow’s milk to nine parts water, mixed in a sprayer, is an effective treatment for powdery mildew. Indeed, research studies on infected wheat and zucchinis have shown it to be a relatively successful treatment. (You can use skim, one per cent, two per cent or homogenized … just remember to rinse out the sprayer thoroughly after use.)

Another good treatment is one teaspoon (15 ml) baking soda dissolved into one quart (roughly one litre) of water. Carolyn Herriot in her book, A Year Down the Garden Path – A 52 Week Organic Gardening Guide, has cited the addition of one teaspoon (15 ml) vegetable oil and a few drops of insecticidal soap to emulsify the oil.

Spraying this mixture onto affected plants raises the pH into a more alkaline range … producing a more inhospitable environment for the spores and thereby restricting germination.

We are reaching the time of the season when you should notice tiny black circles about the size of a pinhead on some of the infected leaves. OK … a magnifying glass or microscope would probably help.

These black circles are called cleistothecia and are the sexual reproductive stage by which the powdery mildew fungus … whichever one it is … is able to overwinter. They remain on the infected leaves or drop onto the soil, where they patiently wait for the temperature to warm come the following spring and start reproducing into new infections.

Alas, there is no known cure for powdery mildew … only prevention and a modicum of control once it appears. To reduce the risk of it being a problem next year, make sure you clean up any infected plant debris.

And look for those vegetable, fruit or perennial varieties that have been specifically bred to resist the powdery mildew fungi when you start shopping for next year’s seeds.

Happy harvesting!

Leslie Cox co-owns Growing Concern Cottage Garden in Black Creek. Her column appears every second Friday.

 

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