Powdery Mildew

Powdery mildew is a fungus that grows on the surface of the leaf rather than inside it. It spreads by airborne spores, moves through a tent in days, and if it reaches the buds in flower it can end the crop.

What you are seeing#

It starts as small circular patches of white or pale gray dust on the upper surface of a leaf, usually a lower or inner leaf in a shaded, still part of the canopy. A spot is about a quarter inch across at first and looks like a smear of flour.

The confirmation is simple: wipe it with a fingertip. Powdery mildew comes off on your finger because it sits on the surface. Spots that will not wipe off are something else.

Left alone, the circles enlarge, merge and coat whole leaves, and the leaf underneath yellows and dies. In flower the white dust gets onto sugar leaves and then into the buds, which is the point at which the flower is no longer usable.

Check the leaves getting the least light and air: the inner canopy, anything touching a tent wall, and the lowest layer.

Why it happens#

High humidity plus poor airflow plus a crowded canopy. Powdery mildew does not need standing water the way most fungi do, which is what makes it different. It germinates happily at 55 to 70 percent relative humidity and especially likes big swings, such as a warm dry day and a cool humid night at lights off.

The spores are everywhere, arriving on clones, air and clothing. Assume exposure and manage conditions rather than trying to keep them out.

Beginner causes: no exhaust fan or an undersized one, plants packed shoulder to shoulder, a canopy that was never defoliated, and a tent sitting at 60 percent humidity through flowering because nobody put a hygrometer at canopy height.

How to fix it#

1. Remove the infected leaves today. Bag them inside the tent, seal the bag, carry it out. Do not shake or brush them, which throws spores into the air.
2. Drop humidity below 50 percent and hold it there. In flower, target 40 to 45 percent. This is the control that matters most.
3. Increase exhaust and add a circulation fan so every leaf is moving slightly. Still air is where it grows.
4. Defoliate to open the middle of the plant, removing leaves that lie flat on other leaves or block airflow.
5. Spray with potassium bicarbonate at label rate, lights off, covering tops and undersides. It changes the surface pH and kills the fungus on contact, and it leaves little residue.
6. Repeat every 7 days for 3 rounds. Spores the first spray did not wet germinate a few days later. A single application always looks like it worked and never is.
7. Keep the lights-off temperature drop under 10F so you do not condense moisture onto the leaves, and wipe the tent down with dilute bleach between grows.

How to tell it apart#

Versus bud rot. Powdery mildew is white, sits on the outside, and wipes off. Bud rot is gray and grows from the inside of the bud outward, so the bud is dead and brown at the core when you open it. Break a suspect bud apart: white on the surface is mildew, gray fuzz and brown mush inside is rot.

Versus spider mites. Mite damage is a fine scatter of pale dots in the leaf, not a raised white film on top of it, and it does not wipe off. Mites also bring webbing, silk strung between leaf tips rather than a flat dusty patch.

Versus leaf septoria. Septoria makes distinct round yellow spots with tan or gray centers, does not wipe off, and starts strictly on the lowest leaves.

Versus hard water and spray residue. Mineral deposits and dried foliar sprays also leave a white film, but it is evenly scattered where droplets sat and does not form growing circles. Mark a spot with a pen and look again in two days: mildew enlarges, residue does not.

How long until it looks better#

New spots should stop appearing within 5 to 7 days once humidity is under 50 percent and the first spray is down. The environment change does more of the work than the spray.

Affected leaves do not recover and should be removed rather than watched. Judge success on whether new spots appear on clean leaves over the following two weeks.

Be honest about the worst case. If powdery mildew has reached the buds in late flower, that flower is not usable, and washing or trimming does not make it so. Harvest what is clean and treat the next run as a humidity and airflow problem from day one.