Mildew LeafHere's what the dreaded powdery mildew looks like!© Copyright, Steve Davis, 2017

Complete Guide to Stopping Powdery Mildew On Your Marijuana Plants

There’s something in the air that marijuana growers don’t like—powdery mildew.

This hardworking spore floats around in the air, and also lives on leaves, other surfaces and materials, and in soil.

If you’re unlucky, powdery mildew spores find your cannabis leaves and infects them.

The infection looks like clumped, sticky sand poured onto your marijuana leaves.

Take a look at the photo accompanying this article and you’ll see this plague taking over a leaf.

If your cannabis grow room conditions become favorable for powdery mildew, and especially if you’re growing a marijuana strain prone to it, this mildew can ruin your plants during bloom phase.

Bloom phase is the prime time when powdery mildew comes on, although you sometimes see it coming in on hydroponics clones or otherwise during veg phase.

Powdery mildew likes the following conditions:

• Relative humidity between 60-80%. Temperatures between 61-71 degrees Fahrenheit. • Marijuana plants fed too much nitrogen. • Poor fan aeration and air movement. Poor grow room air exchange. • Dense cannabis plants. Lack of light penetration. • Damaged leaves. • Marijuana strains susceptible to powdery mildew.

Preventing infection is very possible, but totally stopping it after it’s started is difficult.

Powdery mildew is said to “live inside and outside your marijuana plant.”

This means that it isn’t just living on your leaves.

Powdery mildew inserts itself into your marijuana plants, so it becomes a systemic infection, not just a surface infection.

But powdery mildew cannot be stopped using a systemic anti-mildew feed, so only partial control after infection is possible.

Stopping Powdery Mildew

The good news is you can still harvest an optimum amount of bud even if a powdery mildew infection starts—but only if you almost totally control the infection so it doesn’t take over your entire set of buds.

You prevent or control powdery mildew infection using the following methods:

  • Don’t allow marijuana grow room conditions to favor powdery mildew. Choose strains not prone to powdery mildew.
  • If powdery mildew hits in grow phase, or in bloom phase before resin glands have developed, you can use: a sulfur burner; Rhino Skin foliar spray at 6.8 pH and 160 ppm; potassium bicarbonate, Neem, or baking soda as a foliar spray (one half tablespoon of baking soda per gallon at 6.8 pH).
  • Reduce amount of nitrogen you’re feeding your marijuana plants.
  • Increase the amount of light your cannabis plants receive, particularly UV-B and UV-C wavelengths.
  • Spray Physan 20 on your marijuana leaves, but no later than 3 weeks into bloom phase. • Treat plants with beneficial microbe foliar spray containing Voodoo Juice, Tarantula, Piranha at 6.9 pH.
  • Decrease watering. Decrease ambient humidity to 51% night and day.
  • Maintain grow room temperature at 75F day and 72F night.
  • Root feed using plant inoculant Bud Factor X and cannabis plant stress reducer B-52.
  • Sterilize fans, vents, ducts, exhaust fans. Increase fan and venting aeration and air movement. Increase plant spacing. Increase grow room air exchange rate and volume.
  • Monitor your leaves every day of your marijuana plants’ lives and immediately cut all infected marijuana leaves/stalks/stems and dispose far away from your marijuana grow room.
  • Disinfect all clothes, tools, and other materials that could have spores on them.
  • After you harvest the infected marijuana crop, remove all plants and totally disinfect room, ducts, fans, tanks, reservoirs using Physan 20, and after that, bleach.

I just grew a cannabis crop of Ghost Train Haze that got powdery mildew on it starting week 4 of bloom phase.

Too late for sulfur burning or any kind of foliar spraying other than beneficial microbes and Rhino Skin.

But because I caught it early and used the non-spray methods outlined above, it only infected my large leaves, and never got onto my buds or sugar leaves.

I was lucky to get the same quality and quantity of harvest from my cannabis plants I’d have gotten if my marijuana plants hadn’t been infected.

This was a rare victory.

In most cases when powdery mildew hits your crops after weeks 3-4 of bloom phase, it can’t be stopped.

The spores cover your buds, and your crop is wrecked.

For another take on dealing with powdery mildew in your marijuana garden, check this article out, and remember, in some cases you can stop powdery mildew from infecting and/or ruining your cannabis plants.

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