White powder on the leaves: is it powdery mildew, and what do I do first?

GROWER ANSWER

If it looks like a dusting of flour that wipes off with a finger and comes back a few days later, it's most likely powdery mildew. It lives on the leaf surface, it likes a crowded canopy, still air and humid nights, and it travels on the air. You don't scrub it away. You stop it spreading, then fix the room that invited it.

Stop the spread, then fix the room.

Is it mildew?

• Powdery mildew: white or gray powdery spots that grow into patches, usually on the tops of the leaves first. It wipes off, then comes back. Left alone, it moves leaf to leaf.

• Dust or dried spray residue: wipes off and stays off.

• Tiny pale specks that move, or fine webbing: that's mites, not mildew. See the bugs page below.

• Silvery streaks with tiny black dots: thrips, also on the bugs page.

• Gray fuzz or brown mush inside a flower: that's gray mold, a different problem, and it needs action today. See the bud rot guide.

Not sure? Bring us a leaf in a sealed bag, or send a close photo of both sides of it. Naming it first costs nothing.

Why it showed up

Mildew doesn't need water sitting on the leaf the way gray mold does. It needs a pocket of damp, still air and a leaf to land on. The usual setup: a dense canopy with leaves touching, a dead corner the fan never reaches, and nights where the humidity jumps 15 to 20 points when the lights go off and stays there. It also rides in on new plants and on clothes, so a fresh cutting from a friend's tent is worth a close look before it joins yours.

What to do first

1. Take off the worst leaves. Bag them inside the tent before you carry them out, so you don't shake spores over the rest. Keep it targeted: about 5 to 10% of the leaves in one pass, 15% at most on a thriving plant. In the first 2 to 3 weeks after the switch to flower, light cleanup only.

2. Open the canopy and move the air. Gentle air through every part of the plant: leaves moving, not whipping. No dead corners. Readings across the tent should sit within about 3°F and 5 to 8 points of humidity of each other. Wider than that, find the dead spot.

3. Hold the humidity steady, nights included. Run the lane for the stage, and in late flower hold 40 to 50% with the lights off too. The risk is the high 60s held for hours, not a short spike, and the lights-off jump is when mildew gets its chance.

4. Scout on a schedule. Look under the leaves once a week in veg, and twice a week in flower. Use our rule: one spot on one leaf is a watch; a second leaf or any new growth earns a move.

5. Then treat, if it keeps moving. If it's still spreading once the room is fixed, talk to us about what fits the stage she's in. Anything that goes on the plant has to suit her stage, and once there are flowers, the list gets short.

Late in flower

You can't wipe it off a flower. Late in the run the job is the same: stop the spread, take off what's hit, keep the nights in the lane, and get her to the finish. At harvest, our budwash guide covers what washing can and can't do. Anything with gray fuzz or brown inside the flower goes in the trash, not the wash.

What not to do

Don't spray on a guess. Dust, residue and mites can all look like mildew at a glance, and a spray picked before you know what it is can hide the evidence you needed.

Don't mist the leaves or push the humidity up at night. That's the room mildew wants.

Don't strip her bare to get ahead of it. A big cut is its own stress. Targeted leaves, then air.

Don't walk infected leaves through the house uncovered. Bag them first.

When it's something else

• Gray fuzz or mush inside dense flowers: see Bud Rot Survival Guide

• Specks that move, webbing, or silvery streaks: see Tiny bugs on my plants: what are they, and what do I do first?

• Humidity that won't come down: see What humidity should I run, and what if I can't hit it?

• Mildew on the flowers at harvest: see the budwash guide

On our shelf in Worcester: Mildew controls, organic options first, are in Pest & plant protection, and clip fans are in Climate, air & controls. Call (508) 459-9609 or get directions.

Keep going

What's on our shelf for mildew, and what we reach for first: see our pest plan shelf guide.

Air through the canopy, steady nights, one move at a time. The whole system for a steady grow is written down in the free Stability Playbook. Get the free Stability Playbook

More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.