Cannabis Fungal Diseases: 8 Humidity-Loving Fungi Every Grower Should Know

The most common fungal diseases in cannabis are botrytis (bud rot), powdery mildew, downy mildew, fusarium, pythium, rust, sooty mold and root rot, and almost all of them thrive on high humidity, warmth and poor airflow. Sooner or later nearly every grower runs into fungi in the garden, and if you don't catch it in time you can lose the whole harvest you worked months for.

What are fungi, and why do they love your grow?

Fungi are simple eukaryotic organisms (members of the kingdom Fungi) that reproduce through spores and tend to thrive in warm, dark, damp environments. Spores float around in the air waiting for the right conditions to germinate and grow, and unfortunately those conditions (darkness, warmth and humidity) overlap a lot with what your plants need. That overlap is exactly why grow rooms are such easy targets.

Some fungi reproduce shockingly fast and can wipe out an entire grow in a matter of days. That's why prevention matters so much. And if prevention fails and fungus hits your cannabis, early detection and fast action are what keep the damage contained and save the harvest.

Botrytis (bud rot)

Botrytis, also called gray mold or bud rot, often gets a foothold after caterpillars have been chewing on the plant. The holes they bore into the buds make it easy for the fungus to move in. Buds rot from the inside out, turning brown and looking like they're still holding all their moisture inside. It's especially common in strains harvested late in the season, when autumn rain and humidity roll in (October onward in the Northern Hemisphere, April onward in the Southern).

Once it's in your garden, it can take out a big chunk of the harvest. Even if you remove everything that looks affected, its spores have probably spread almost everywhere. It moves easily even through dried flower, so when you spot mold on your buds, the standard move is to cut the whole branch to stop it spreading. Smoking it is a real health hazard: the spores go straight to your lungs and can cause serious respiratory problems.

Powdery mildew on buds

Powdery mildew is one of the most common fungi you'll run into on cannabis plants.

It's pretty easy to spot, because it first shows up as a white powder, usually on the leaf blade, and gradually spreads to the stems and flowers. Once it's widespread, it causes yellowing leaves and leaf drop.

This fungus loves humidity and high temperatures. It settles on the leaves and uses its hyphae (the thread-like filaments fungi grow from) to suck nutrients out of the plant.

It isn't especially deadly, but it's very common, and if you don't get it under control before it reaches the buds, those buds have to go in the trash. Just like buds with botrytis, they're harmful to your health and can't be consumed in any form.

Downy mildew

Downy mildew is well known and feared by cannabis growers all over the world. It's a very aggressive organism that thrives in grows where the temperature stays below 26 °C (79 °F) and humidity sits above 50%.

It can show up in both veg and flower, but it prefers young leaves. Downy mildew appears as yellow and brown blotches on the leaves, and as it spreads, the whole crop turns pale.

It spreads at breakneck speed and, after stripping the plant of its leaves, kills it. It develops on the plant material in the grow itself, and the best way to avoid it is prevention.

Fusarium

Fusarium is a fungus with serious destructive potential. It's more common in indoor grows and greenhouses, where temperatures run higher.

It usually reaches the plant through water, the growing medium, contaminated tools or infected clones.
It attacks the xylem, the plant tissue in charge of moving fluids, blocking the flow of nutrients and seriously damaging the plant.

It starts with yellowing of the lower leaves, then moves on to interveinal chlorosis, necrosis and leaf drop, and finally the plant wilts. Sometimes fusarium only affects part of the plant, depending on where it got in. It's a very fast process that can kill plants in no time.

You can confirm fusarium by checking the inside of the stems, which turns a reddish brown.

Pythium

Pythium, a water mold, sets up shop in the root system and almost completely cuts off the plant's ability to feed. Little by little it weakens plants until it kills them. It can hit plants of any age and size, and young ones are usually too weak to survive it.

Its spores travel through contact with infected growing media, or even on mycelium tracked in on your shoes. It develops in environments between 20 and 30 °C (68-86 °F) with high humidity.

Pythium causes severe weakness and deficiencies because it wrecks the root system. In setups where the roots are exposed, like hydro or aeroponics, you can actually see them change color, going from healthy white to dark brown or even reddish.

One effective way to fight it is to boost the microbial life in your growing medium by adding trichoderma and mycorrhizae. That can stop the spread, especially if you build up that microbial life before pythium ever reaches your grow.

Rust

Rust is one of the fungi that spreads most easily on weed plants, since it gets through the leaf's epidermis and cuticle, so simple contact is enough to pass it along.

It shows up as yellowish spots on the upper side of the leaves, starting at the tip and moving up. Once it's fully established, the underside of the leaves shows red and brown dots, followed later by slight leaf deformities.

Destructive as it is, if you treat it in time you can stop it spreading. It appears when your grow sits in a high-humidity spot without enough ventilation. Your best bet is to get rid of affected leaves or plants and apply a fungicide to keep it from spreading.

Sooty mold

Sooty mold grows on the sticky substance (honeydew) that some insects, like aphids and whiteflies, excrete. That molasses-like goo is what lets the fungus take hold.

You can spot sooty mold with the naked eye, since the leaves get covered in black patches. It makes it hard for the plant to photosynthesize, with all the growth problems that come with that: slower growth, loss of vigor and a smaller harvest.

Root rot

Root rot, as the name says, is a fungal disease that attacks the roots, turning their healthy white color light brown and then darker and darker as the disease progresses.

Symptoms include chlorosis and yellowing leaves, leaf drop, wilting, overall weakness and, eventually, the death of the plant.

It usually comes from overwatering, poor drainage, or small insects and other organisms damaging the roots and kicking off the rotting process.

How to prevent fungus in an outdoor cannabis grow

• Keep your garden clean: sweep up the leaves and weeds that pile up on the ground.
• If you grow in containers, move your plants under cover when it rains during flowering so the buds don't stay wet.
• If you live somewhere cold with short summers, pick fast-flowering genetics like Jamaican Dream so harvest doesn't drag into autumn rain and bad weather that invite fungi.
• Check your plants regularly for signs of fungus.

How to prevent fungus in an indoor cannabis grow

• Don't let pockets of humidity build up in your grow, and make sure it has proper ventilation.
• Keep your grow clean and your tent's trays free of dead leaves.
• Leave enough space between plants for air to circulate properly.
• Keep humidity between 40% and 50% during flowering.
• Inspect your plants regularly.