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

An RCMP cruiser looks on as a military search and rescue helicopter winds down near Bridesville, B.C. Tuesday, Dec. 1. Photo courtesy of RCMP Cpl. Jesse O’Donaghey
Boundary Mountie and suspect airlifted from ravine after foot chase

Military aircraft were dispatched from Comox, B.C., say RCMP

The process of integrating Union Bay services into the regional district can now begin. Record file photo
Union Bay transfer to region targeted for July 2021

Three services will be rolled into Comox Valley Regional District

Mike Aldersey, the Port McNeill base manager for West Coast Helicopters has been awarded the prestigious Agar/Stringer Award by the Helicopter Association of Canada. (Submitted photo)
Vancouver Island pilot receives coveted helicopter industry award

Port McNeill based Mike Aldersey is the recipient of the 2o2o Agar/Stringer Award given out to select few Canadians

12-year-old Ella Smiley captured some video of orcas on a sea lion hunt on Nov. 28 at Kitty Coleman Park, just north of Courtenay. Photo by Ella Smiley
VIDEO: Orcas hunt sea lion near Kitty Coleman Park

Twelve-year-old Ella Smiley, of Comox Valley Wildlife Sightings, caught up with a… Continue reading

The School District 71 DPAC hosted an online forum for candidates hoping to fill a vacant board of education position. Screenshot, SD71 DPAC Facebook page
Six make their pitch to fill empty school board seat in Comox Valley

District’s parents advisory council hosted the online forum for Area C candidates

A tongue-in-cheek message about wearing a face mask to curb the spread of COVID-19 on a sign outside a church near Royal Columbia Hospital, in New Westminster, B.C., on Sunday, Nov. 29, 2020. THE CANADIAN PRESS/Darryl Dyck
B.C.’s COVID-19 infection count climbs back up to 656

20 more people in hospital, active cases still rising

Letter to the editor.
LETTER – Horgan’s election promise of COVID relief cash is money foolishly spent

Dear editor, Would you dip into your child’s registered education fund to… Continue reading

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

A logo for Netflix on a remote control is seen in Portland, Ore.,Aug. 13, 2020. Experts in taxation and media say a plan announced Monday by the government will ultimately add to the cost of digital services and goods sold by foreign companies. THE CANADIAN PRESS/AP-Jenny Kane
‘Netflix tax’ for digital media likely to raise prices for consumers, experts say

The government says Canadian companies already collect those taxes when they make digital sales

BIG SALMON ranch in Washington State. (Center for Whale Research handout)
Non-profit buys Chinook ranch in hopes of increasing feed for southern resident killer whales

The ranch, which borders both sides of Washington State’s Elwha River, is a hotspot for chinook salmon

Gaming content was big on YouTube in 2020. (Black Press Media files)
What did Canadians watch on Youtube during isolation? Workouts, bird feeders

Whether it was getting fit or ‘speaking moistly,’ Canadians had time to spare this year

Fraudsters projected to use pet scams to gouge over $3M from customers: BBB

The pandemic heavily contributed to the number of puppy scams

A teacher places the finishing touches on the welcome sign at Hunter’s Glen Junior Public School which is part of the Toronto District School Board (TDSB) during the COVID-19 pandemic in Scarborough, Ont., on Sept. 14, 2020. THE CANADIAN PRESS/Nathan Denette
Hindsight 2020: How do you preserve a year many Canadians would rather forget?

Figuring out how to preserve the story of the pandemic poses a series of challenges

Haley Callison. (Facebook photo)
Former B.C. pro hockey player frustrated with COVID-deniers after horrific bout with virus

Haleigh Callison hopes people will follow precautions and tone down the rhetoric

Most Read