Fusarium in Cannabis: How to Spot, Prevent and Fight Fusarium Wilt

Fusarium is a soil-borne fungus that attacks cannabis through the roots and can wilt and kill branches, or the whole plant, in a matter of hours or days. Once it takes hold it's very hard to treat, so the real fight is prevention: clean, healthy growing medium, sterilized tools, careful watering and good control over heat and humidity.

One of the fungi every grower should know is the genus Fusarium.

Spotting Fusarium on your weed plants means trouble: the infection can take out branches or the entire plant in hours or days.

The losses can be huge, but there are several ways to prevent it and fight back.

What is Fusarium?

Fusarium is a type of fungus that goes after the roots and leaves of cannabis plants. It loves warm, tropical climates and can lie in wait for years until conditions are just right to strike. So if you grow outdoors, it could be anywhere in your garden without you knowing. It often spreads through running water and through gardening or grow tools.

For a lot of growers, Fusarium is a massive threat once it wakes up, because it causes wilting or root rot. The worst part: once an infection starts, the affected plants can be extremely hard to treat. For many growers, dealing with it is a real headache.

Since Fusarium oxysporum mostly lives and thrives in soil, hydroponic growing is a good way to lower your odds of meeting it. For the most part, keeping your grow Fusarium-free means working hard to make the environment less welcoming to fungi.

How do you get rid of Fusarium on cannabis?

It's tough to save plants once Fusarium hits. There are a few things you can do to limit the damage, but it usually makes more sense to toss the affected plants and start over. That's why prevention is the top priority with this fungus. 

The best way to beat pests and diseases is to grow from high-quality seeds. Good seeds usually come with strong genetics that make it harder for fungi to creep in through the roots.

Preventing Fusarium beats trying to kill it

Since Fusarium thrives in soil, we can protect our plants with healthy, Fusarium-free growing medium. That includes using natural fungicides. You can also use compost or compost tea to boost the beneficial bacteria that crowd out the bad fungi.

To feed the soil, you can add bacterial food like molasses to support the good bacteria living in your medium. Clay soil helps raise the pH of the medium, and many growers lean on that to make the soil less friendly to harmful fungi. Growing other healthy plants alongside your cannabis also improves soil quality and helps keep the problem in check.

To make your soil even safer, a smart move is to grow in containers instead of straight in the ground. That limits the infection area and makes it easier to stop the fungus from spreading.

What causes Fusarium in cannabis?

Like any fungus, Fusarium on cannabis develops in warm, humid environments. So you have a better shot at preventing it if you can control humidity and temperature yourself. The best way to do that is to grow indoors, where you can monitor and tweak lots of variables.

Diseases can travel from one spot to another on garden tools that have touched the fungus. So make sure you clean and sterilize your grow gear, along with your watering can and/or hose.

Besides cutting off oxygen to the roots, overwatering also encourages fungal growth. Overwatering happens when you water too often or give too much water at once.

Keeping an eye on your plants is a huge part of caring for them. We can't stress enough how important observation is. Without it, you won't know if your plants are dealing with a Fusarium infection. So what does an infected plant look like? The two main symptoms to watch for are wilting and root rot.

How can you tell if your cannabis plant has Fusarium?

The most common symptom of Fusarium in cannabis is wilting. It's a sign the fungus has already started colonizing the plant, and it's time to act: if you can, cut off the affected parts with clean tools and get them out of the grow area.

Earlier symptoms vary depending on which Fusarium is hitting your crop. They include spots on the stem, lesions and a worn-out look that doesn't match the plant's space or feeding schedule.

Keep in mind that once a plant shows Fusarium symptoms, it's harder to treat than most other fungi, and the infection moves fast.

The Fusarium life cycle

Fusarium produces three kinds of spores: microconidia and macroconidia, made of one cell and many cells respectively, and chlamydospores, super-tough spores that can survive 5 to 15 years in the soil.

Fusarium spores are released and can grow on living or dead plant tissue. They get into the plant through damage to the stem, leaves or roots.

The fungus develops at temperatures between 10 and 30 °C (50 to 86 °F), with optimal reproduction at 20 to 25 °C (68 to 77 °F) on 12-hour light and dark cycles, which makes both indoor and outdoor grows potential targets. On pH, Fusarium is hard to corner: it can survive anywhere from a soil pH of 2.2 to 9.0, so adjusting pH helps at the margins but won't keep it out on its own.

What happens if you smoke buds with Fusarium?

The big question a lot of people ask is whether you can smoke weed with Fusarium. First off, this genus of fungi is so widespread that it can even live in our bodies without causing any problems. However, people with weakened or compromised immune systems can end up with different kinds of infections.

Even so, as with any moldy weed, smoking it isn't recommended, since in many species the spores can survive high temperatures or solvents.

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