An outbreak of powdery mildew is messing with personal crops

Marijuana growing equipment suppliers in Toronto have been getting a steady stream of phone calls from people inquiring about the “white stuff” on their plants

Macro of top bud of green and red cannabis female plant with lot of trichomes.


Global warming has extended the weed growing season, but this year it has also led to a flare-up of powdery mildew, or PM, in outdoor gardens and many indoor grow ops as well.

“This was the summer for powdery mildew,” says Thomas Henry, owner of Growers World Superstore on the Danforth in Toronto.

If left untreated, PM will make your weed crop totally unusable. Henry speculates that increased humidity and cooler summer temperatures made the conditions ripe this year for PM.

PM is a parasitic fungus. It starts on the leaves, moves to the stems and then takes over your buds. The mildew originates from spores that are spread by wind or physical contact from neighbouring plants. That’s right, this fungal disease is a problem for other plant species as well.

PM has a dusty matte-white look, sort of like flour or fake snow. The inexperienced cannabis grower may mistake it for trichomes (the part of the plant that gets you stoned), which are also white. But trichomes have a sparkly look, sort of like the shimmer of real snow or cocaine.

Trevor Wilkinson owner of Grow It All on Geary in Toronto says he has never seen a PM outbreak as bad as  this year’s. He has been getting a steady stream of phone calls from people inquiring about the “white stuff” on their plants.

“Can I smoke it?” is a question Wilkinson is asked often these days. Common sense would dictate that smoking is something you might want to avoid. But if caught in time, PM is relatively easy to control.

A friend of mine, who lives in the Queen West Village, had a mild case on a plant she grew on her balcony. She took the advice of a “friendly guy” on YouTube.

She added 250 millilitres of hydrogen peroxide (3 per cent solution) to four gallons of water and then soaked the infected branches in the solution, agitating gently to separate the PM from the plant. She then thoroughly rinsed the plant under a shower.

“I wondered if it would wash off some of the good stuff,” she says. “I suspect it did. My weed is quite weak, as well as being a very body stone, but I actually love it that way. Totally worth it to salvage the crop.”

Another grower in the east end of the city was not so lucky. The PM covered practically the whole plant. She has been growing pot off and on for at least 15 years and had never experienced PM. She tried a spray solution of 10 millilitres of mineral oil to a litre of water. She applied the spray over three days. She says it cleared up most of the PM. The mineral oil works by suffocating the mildew. At harvest time she washed the mineral oil away as best she could with a hose.

“It was very effective. It was amazing,” she says. “Some remained, but I washed it all away under a warm water spray.”

PM is almost impossible to stop in late flowering, so early detection is essential.

Plants under stress are typically attacked first. Infection usually starts in the lower canopy and spreads to the top of the plant. Maintaining healthy plants means taking preventative measures. You have to be a sleuth, inspecting the corners, edges and lower portions of your garden frequently.

There are things you can do to prevent the start of  PM. The first is to remove infected leaves and plants. The second is not to water your plants at night. Also, keep your plants apart so that there is good air circulation between them. As your plants mature it is helpful to remove the lowest leaves to increase airflow in and around the lower part of the canopy. You may want to decide to harvest early if you think mildew is going to be a problem – or just don’t want to deal with it.

For indoor grows, avoid prolonged periods of high humidity – 50 to 60 per cent is ideal. Levels of over 80 per cent will almost guarantee infection. Good ventilation is essential. Constant air movement inhibits mildew and lowers humidity. A dehumidifier can be quite effective in keeping mildew away.

Also, spraying plants regularly with alkaline water at 8.0-plus pH is a good idea. But you should stop spraying when buds begin to mature, as alkaline water does appear to affect the growth of the plant. The effect, however, is temporary.

You can also try spraying your plants with a mixture of an ounce of potassium bicarbonate in a gallon of water together with 1-1/2 cups of milk.

But not all is lost if your plants have been infected. There’s a type of hash you can make, called bubble hash. The process of making this hash, which involves using ice and water and very fine screens, takes the PM right out. Bubble hash kits start at $100.

If you’re a newbie to growing weed and are in doubt, you can always take your suspicious buds to your local marijuana growing equipment supplier. But to avoid problems in the future, make sure to keep on top of the white stuff.


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