Quick answer
Powdery mildew on weed plants looks like white or gray powder spread across leaf surfaces, stems, and sometimes buds. It starts as small round spots, then merges into blotches. The powder sits on top of the plant tissue, so it smears when you rub it. It rarely looks fuzzy or hairy like bud rot.
how to treat powdery mildew on cannabis plants
Before you start
- Use white light or natural daylight. Colored LEDs hide the gray tone.
- Have a 10x to 30x loupe or a phone macro lens.
- Wash your hands or wear fresh gloves before touching plants. Powdery mildew spreads by spores.
- Work in a clean area and avoid moving from a suspect plant to a healthy one.
What powdery mildew looks like
Powdery mildew has a clear visual signature once you know what to check.
how to treat powdery mildew on cannabis plants
- White to light gray powder, not bright white like flour.
- Round patches 1 to 5 mm wide that join into larger areas.
- Yellow or pale spots on the leaf under the powder.
- Leaf edges that curl, twist, or dry.
- Powder on stems, petioles, and sometimes flower buds.
- New growth that looks stunted or distorted.
Where it shows up first
Powdery mildew often starts on the upper leaf surfaces of lower and middle canopy leaves. Those leaves sit in shade and humid air. It also appears on crowded plants and on the side of the tent with less airflow. Check the leaf surface facing the light first. Then check stems and the area around bud sites.
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How to tell it apart from lookalikes
Trichomes
Trichomes are plant hairs, not powder. They stick out from the leaf and sparkle under light. Powdery mildew lies flat and dull. A loupe shows trichomes as clear or amber stalks with round heads. Powdery mildew shows as chains of oval spores on a web of threads.
Dust and residue
Dust wipes off and leaves clean tissue. Powdery mildew wipes off and leaves a pale or yellow mark where the fungus fed. Dust also settles on the top of leaves in a uniform layer. Powdery mildew grows in patches.
Spider mites
Spider mites leave fine webbing and tiny moving specks. Webbing looks like thin strands, not powder. Mite damage causes stippled, yellow leaves. Powdery mildew causes white patches with yellow spots underneath.
Bud rot
Bud rot is brown, gray, or black and looks slimy or fuzzy inside the bud. Powdery mildew stays on the surface and is white to gray. Bud rot smells sour or musty. Powdery mildew has a mild, earthy or fungal smell.
Step-by-step identification check
- Turn off grow lights and use a white flashlight or daylight. Look at the plant from the side so light hits the leaf surface.
- Check upper leaf surfaces on the lowest and middle branches first. Look for white or gray spots.
- Rub one suspect spot with a clean finger or a cotton swab. Powdery mildew smears and leaves a faint mark. Dust wipes clean.
- Use a 10x loupe on the spot. Look for flat, powdery colonies with tiny round spores. Trichomes look like stalks, not flat patches.
- Check the undersides of the same leaves. Powdery mildew can grow there too, though it favors the top.
- Inspect stems, petioles, and the base of buds. Powdery mildew often spreads to these areas.
- Compare a healthy leaf from another plant. Powdery mildew patches look out of place against clean green tissue.
- Look for yellowing, curling, or stunted growth near the powder. These signs support a powdery mildew diagnosis.
Signs that confirm it is powdery mildew
- White powder that smears on touch.
- Patches that spread over days.
- Yellow spots under the powder.
- Powder on stems as well as leaves.
- Spores visible with a loupe.
What to do after you identify it
Separate the plant from healthy plants. Remove affected leaves with clean scissors and bag them. Improve airflow and lower humidity. Do not compost infected plant material. Check the rest of the garden every day for new spots. Treat according to your local rules and product labels.