Root Rot in Cannabis: Symptoms, Causes and What to Do

Root rot is a fungal (or fungus-like) infection that kills a cannabis plant's fine feeder roots, so the plant can no longer take up water or nutrients. The telltale sign is curled, droopy, sickly-looking leaves that don't bounce back after watering, which is why it's so often mistaken for overwatering. Left alone, it can kill a plant in about a week. 

Root rot is one of the most common problems below the soil line. It can have several causes, but the best-known culprits are Fusarium, Pythium and Rhizoctonia, the last being a genus of fungi that doesn't produce spores but spreads through hyphae and sclerotia. Rhizoctonia literally means “root destroyer.”

Root rot shows symptoms that look a lot like an overwatered plant. That said, every infection looks a little different.

Root rot symptoms in cannabis

Each infection looks a bit different, but the most common symptoms are:

1. Above ground, the first thing you'll notice is an overall slowdown in the plant's growth. The leaves wilt and go chlorotic (they turn yellow), then brown, and eventually drop. The foliage gets thinner and paler by the day. The plant's overall health goes downhill quickly: from that initial wilting to brown tones on the leaves and stems (even the buds), which eventually die off.
2. Below ground, you'll see the root hairs dying off. In other words, rotten roots that gradually stop absorbing water and food (which is what causes the symptoms you see up top). The roots turn brown with necrotic tips that keep breaking down, which can open the door to new problems. The base of the stem may also darken or turn brown.

Bottom line: what you're looking at is general wilting of the whole plant. And no matter how much you water, the leaves and stems won't firm back up. That's why so many growers assume the plant is thirsty at this stage and water even more, which only makes things worse. Once you're there, be ready to lose the plant about a week after the first symptoms show up.

Symptoms of Rhizoctonia

The main signs of this fungus in cannabis plants are rotting seeds, rot in the upper roots all the way up to the root collar (where stem meets soil), and damage to the foliage.

What to look for

• Leaves on affected plants come in smaller and gradually turn yellowish.
• The fungus slows the plant's growth way down until it eventually wilts.
• Dry brown spots show up on the root collar and on the leaves.
• The roots rot progressively and a sticky, gel-like layer forms on them.
• If humidity is very high, threads can appear on the soil and the infection can spread all the way up to the foliage.

Symptoms of Phytophthora

Here, too, the plant's growth slows down. The symptoms are similar to Pythium's but hit harder.

What to look for

• Leaves wilt, yellow and drop.
• Brown or reddish spots appear on the root collar.
• Brown/reddish discoloration shows up on the stem's outer layer and on the roots.
• When you pull the plant out of the soil, you'll clearly see the roots have gone dark.
• It spreads fast through any plant debris left in the grow.

Symptoms of Fusarium

Fusarium hits the roots, the root collar and sometimes the leaf sheaths hard, covering them in a brown/reddish rot. Plant growth slows down as the infection takes hold.

Symptoms of Pythium

Pythium slows down growth in infected plants.

What to look for:

• An overall loss of vigor, followed by the plant gradually wilting.
• Dark, wet rot at the crown.

What causes root rot in cannabis?

Humidity is one of the factors that sets up root rot, and so is overwatering. Grow room temperature can also take a toll on your plants, since these fungi thrive between 20 and 34 °C (68-93 °F). Root rot can also hit hydroponic grows, especially when the reservoir runs warm.

The goal is to keep it from happening in the first place, and there are a few things that help. Keep temperature and humidity in the grow room dialed in, keep the space clean, and don't leave dead organic matter like fallen leaves and branches near the plants. Your growing medium should also be airy so roots can grow healthy, and go easy on the watering. Adding perlite to the mix helps with drainage. You can also bring beneficial microbes into the soil by mixing in some compost. 

Root rot is tough to treat because the spores of the pathogens behind it are tiny and nearly impossible to spot. That's why prevention is where you should focus.

How do you prevent root rot?

Since these fungi can already be hanging around before you even start growing, it's crucial to thoroughly clean the grow room and every surface and tool you'll use to set up your grow.

Keeping pests in check also helps prevent root rot. Fungus gnats can carry root rot, so keeping them away is key. The best way to avoid gnats is to nail your watering: let the top layer of the medium dry out a bit before watering again and you'll avoid drowning your plants.

In hydroponics you have to be obsessively clean. Adding beneficial bacteria to the system can also help keep root rot fungi from taking hold. 

If root rot has taken over a plant, toss it. The fungi can spread and infect the rest of your grow, which is why prevention and cleanliness always win.

Healthy roots are vital for every process going on in your cannabis plant. That's where nutrients and water get absorbed, so the best advice is to be patient and keep a close eye on your plants.

How do you treat root rot?

Unfortunately, these diseases are very hard to wipe out completely. If you follow the tips above, you can slow their arrival and spread. But if the infection is severe and has spread across the whole grow, your best bet is to pull every plant and start a fresh grow. If you don't, you risk losing everything.

Here are some approaches you'll come across:

Biological treatment:

Garden centers and nurseries carry biofungicides made from beneficial fungi or bacteria that protect plant roots against fungal attacks.

You'll find products based on the bacterium Streptomyces griseoviridis, which works against most of the pathogens behind root rot. It can also be used preventively. Depending on how bad the infection is, you can reapply every 2 to 5 weeks.

Chemical treatment:

Chemical treatments usually don't do much once the infection has spread through the grow. In that case, we recommend pulling all the plants and throwing them out. Then clean the grow space and all your gear and apply a preventive antifungal treatment. It's the only way to make sure your next grow starts clean.

If you're just getting started, check out our grow courses (in Spanish, from Argentina).  Solid training goes a long way 🙃

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