Plant Problems
Mold on pot almost always means bud rot, the fungus Botrytis cinerea eating a cola from the inside. The first sign is one sugar leaf going yellow and dry on an otherwise green bud; pull it and the base of the bud beneath is gray, brown or mushy. Cut it out today. It spreads overnight.
This page is the diagnosis and the triage. The second wave of mold happens in the drying room, and that has its own chapter at drying and curing. This page covers the plant while it is still standing.
What Mold on Pot Looks Like: How to Spot Bud Rot
Bud rot starts inside the cola, at the stem, where the tissue is densest and stays wettest. From outside you see one thing first: a single sugar leaf, or a small tuft of them, going yellow then brown and papery while everything around it stays green. Pull the bud apart at that spot and the interior is gray to dark brown, soft, and either dusty or slimy depending on how far it has gone.
Fresh rot smells musty and faintly sweet, like wet leaves left in a bag. Older rot grows a gray fuzz you can see without a loupe, and the whole section crumbles when you touch it. The pistils on the infected section go dark and shrivel early, and the calyxes lose their shape. On outdoor plants the first infected cola is usually the biggest one, at the top, where rain sits.
What Causes Botrytis
Botrytis cinerea is a gray mold that lives on dead plant matter everywhere. It needs three things to take a living bud: spores, which are always present; water on the tissue for several hours; and still air so the water stays. It spreads by air and by touch. UC IPM's gray mold page puts its sweet spot at high humidity and moderate temperatures, roughly 15 to 25 C, which describes late summer and fall almost everywhere weed is grown outdoors.
Indoors the causes are humidity above 55 percent in late flower, fans that move air across the canopy but not through it, and colas so dense the inside never dries after the lights-off dew. Big, tight, indica-leaning colas in week seven and later are the usual victims. A dead pistil, a caterpillar wound, or a snapped branch gives the spore its way in.
How to Confirm It: The Calyx Pull Test
Find the yellow leaf. Grip it at the base and pull straight out. On a healthy bud the leaf resists and comes away clean and green at the stem end. On rot it slides out with no resistance, the base is brown, and a gray or brown smear comes with it.
Then open the bud at that point with two fingers. A brown or gray interior with soft tissue confirms it. Smell the opening: healthy flower smells like the strain, rot smells like a damp basement. Check every cola on the plant the same way, the same day, because you almost never have just one.
How to Fix It: What to Do With an Infected Plant
Cut, do not spray. Nothing you can apply kills botrytis inside a bud, and anything sprayed on a flower two weeks from harvest ends up in the jar.
- Sterilize scissors with alcohol. Cut out the infected section plus 2 to 3 cm of clean-looking tissue on each side. Cut into a bag, not over the plant, because every cut throws spores.
- Bag and bin the cuttings. Do not compost them near the garden.
- Check the remaining colas daily. A second spot within 48 hours means the plant will not finish clean; harvest what is sound now.
- If more than about a quarter of the plant is infected, or the strain has three weeks to go and the weather is not changing, chop the whole plant, trim out every affected bud under a bright light, and dry the rest fast and spread out. Early harvest beats no harvest. The harvest chapter covers taking a plant early.
Is Moldy Weed Safe?
No. Do not smoke it, vape it, cook with it, or wash it and hope. Mold spores and the compounds they produce are not removed by drying, and heat does not make them harmless. Trim out the affected bud and throw it away. If a whole jar has gone gray or smells like a basement, the jar goes too. Moldy weed is trash, and not trash you leave near the plants.
How to Prevent Bud Rot
Humidity under 50 percent in the last three weeks of flower, air moving through the canopy and not just over it, and enough defoliation that no cola sits in its own shade. The VPD chapter has the numbers by stage. Outdoors, pick strains that finish before the fall rains where you live, shake water off the colas after rain, and harvest at the first dry window rather than the last. Space plants so a hand fits between colas.
Looks Like This but Is Not
Bud rot vs powdery mildew vs normal ripening
| Feature | Bud rot | Powdery mildew | Normal ripening |
|---|---|---|---|
| Where | Inside the cola, at the stem | On leaf surfaces, top side first | Pistils, on the outside of the bud |
| Color | Gray to brown, dead tissue | White, flour-like dust | Pistils orange to brown, calyxes stay green |
| Texture | Soft, then crumbling | Powder that rubs off with a finger | Firm and sticky |
| Smell | Musty, damp basement | Little or none | The strain |
| Spread | Cola to cola in days | Leaf to leaf over a week | Even across the whole plant |
| Action | Cut out or harvest now | Remove leaves, drop humidity, treat | None |
Two more pages cover things that get mistaken for mold on pot.
- Pests: caterpillars leave black frass and a bored hole in the cola, and rot follows through the wound. Find the caterpillar and you have found the rot.
- Heat stress: foxtailing and bleached tops look wrong, but the tissue underneath is dry, firm and green. No smell, no gray.
The Mistake Most Growers Make
Waiting. A grower sees one brown leaf in a cola, hopes it is a dead sugar leaf, checks again in three days, and loses the top of the plant. Judging cups, I have smelled that basement note in entries that were otherwise beautiful, and it disqualifies them every time. Cut on the day you see it, and if the rot has already reached three colas, harvest the plant that day too.
Next Step: Dry It Without a Second Infection
The plant you save now still has to dry, and drying is where most second infections start. Read drying and curing before you chop. Keep the leaf symptom chart handy for the leaf-borne lookalikes and the plant problems index for everything else. Cultivation is for adults where it is legal.
Sources
- UC IPM: Botrytis blight, gray mold (used for the humidity and temperature conditions that favor Botrytis cinerea)
