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.


Get local stories you won't find anywhere else right to your inbox.
Sign up here

Just Posted

A man was sentenced to more than a year in jail for a late-night robbery in Courtenay last April. Photo by Mike Chouinard
Late-night Courtenay robbery results in 500-plus days in jail

Heatley’s sentence also includes probation, DNA order, firearms ban

A 3.0-magnitude earthquake occurred off Ucluelet just after 12:30 a.m. on April 10 and was reportedly felt as far south as Oregon. (Map via United States Geological Survey)
Quake off Ucluelet reportedly felt as far south as Oregon

Magnitude 1.5 earthquake also reported off Vancouver Island’s west coast hours earlier

The CSRHD board has diverging views on its relationship with Island Health. File photo
Comox-Strathcona board ponders relationship with Island Health

“We have to stand together … to get our share of the health budget.”

The CVCDA Accessibility Project is creating safer ramps at the Courtenay facility. Photo supplied
Accessibility project underway at Comox Valley Child Development Association

Over the past month, you may have noticed some construction underway behind… Continue reading

B.C. Health Minister Adrian Dix and Premier John Horgan describe vaccine rollout at the legislature, March 29, 2021. (B.C. government)
1,262 more COVID-19 infections in B.C. Friday, 9,574 active cases

Province’s mass vaccination reaches one million people

Cannabis bought in British Columbia (Ashley Wadhwani/Black Press Media)
Is it time to start thinking about greener ways to package cannabis?

Packaging suppliers are still figuring eco-friendly and affordable packaging options that fit the mandates of Cannabis Regulations

Two-year-old Ivy McLeod, seen here on April 9, 2021 with four-year-old sister Elena and mom Vanessa, was born with limb differences. The family, including husband/dad Sean McLeod, is looking for a family puppy that also has a limb difference. (Jenna Hauck/ Chilliwack Progress)
B.C. family looking for puppy with limb difference, just like 2-year-old Ivy

Ivy McLeod born as bilateral amputee, now her family wants to find ‘companion’ puppy for her

A vehicle that was driven through the wall of a parkade at Uptown Shopping Centre and into the nearby Walmart on April 9 was removed through another hole in the wall later that night. (Photo via Saanich Police Department and Ayush Kakkar)
Vehicle launched into B.C. Walmart removed following rescue of trapped workers

Crews cut new hole in parkade wall to remove vehicle safely

Four members with Divers for Cleaner Lakes and Oceans were out at Cultus Lake on March 28 and 29 hauling trash out of the waters. (Henry Wang)
PHOTOS: Out-of-town divers remove 100s of pounds of trash from Cultus Lake

Members of Divers for Cleaner Lakes and Oceans hauled out 470 pounds of trash over two days

As of Saturday, April 10, people born in 1961 are the latest to be eligible for a COVID-19 vaccine. (Black Press files)
B.C. residents age 60+ can now register to get their COVID-19 vaccine

Vaccine registration is now open to people born in 1961 or earlier

A new saline gargle test, made in B.C., will soon be replacing COVID-19 nasal swab tests for kids. (PHSA screenshot)
Take-home COVID-19 tests available for some B.C. students who fall ill at school

BC Children’s Hospital plans to provide 1,200 kits to Vancouver district schools this April

Ruming Jiang and his dog Chiu Chiu are doing fine following a brush with hypothermia that saw several people work together to get them out of the Fraser River near Langley’s Derby Reach Park on March 25, 2021 (Special to the Advance Times)
Man finds men who rescued him from drowning in B.C.’s Fraser River

A grateful Ruming Jiang says he will thank them again, this time in person when the pandemic ends

The 10-part Netflix series Maid, which is being exclusively shot in Greater Victoria, was filming near Prospect Lake in Saanich last month. (Photo courtesy Fred Haynes)
Province announces $150,000 towards South Island film studio, fulfilling B.C. NDP promise

Investment to fund movie studio feasibility study at Camosun College

Most Read