Fusarium: Do You Know The Danger?

Fusarium is dangerous to both people and plants it was once proposed as a weapon of biological warfare in the international War on Drugs.

May 27, 2016

In the giant world of fungi, not all are beneficial for plants. Certain fungi are absolutely essential for plant health. Mycorrhizal fungi, in particular, are a must for healthy soil and top-notch plants. But, other fungi are better kept as far away as possible. One such fungus is fusarium. The nightmare of every grower, fusarium will kill your plants in a hurry. Here’s everything you need to know about the effect of fusarium on cannabis. 

What is fusarium?

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Fusarium is a genus of soil-borne fungi. There are many different species of fusarium, and they’re common all over the world. The big danger? When fusarium gets out of hand, it is a pathogen to crops around the globe. This includes cannabis. Fusarium can survive dormant for years inside the soil. It’s actually really quite common. When the perfect conditions arise, however, it will take over and destroy everything in its path. This is hell for your crop.

Fusarium tends to impact plants that we humans like to eat and use. Part of the problem may be our current agricultural practices and how we’ve learned to manage our soil. Back in 2012, The Agricultural Biotechnology Council of Australia reported that there may be a link between increased use of certain herbicides like Glyphosate and increased risk of fusarium outbreak.

What effect does fusarium have on marijuana plants?

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It’s simple: excess fusarium kills your plants. It causes what’s known as “fusarium wilt”. Your leaves will begin to droop, turn yellow, and die. Some fusarium species can also cause root rot, during which your roots will turn red. This red color begins to creep up the stem of your plant. You’ll be able to tell right away that your plant is diseased. Here are some signs to look for:

In plants:

  • Wilting
  • Red stem
  • Red roots
  • Swollen stem
  • Stem that’s broken open

In leaves:

  • Yellowish-brown colored leaves, particularly lower leaves
  • Brown spots on lower leaves
  • Wilt
  • Leaves that are curled upward

Cannabis that has been contaminated with fusarium will make you very sick. It’s important to keep a close eye on your plants to ensure that infection doesn’t happen. If someone with a compromised immune system consumes fusarium contaminated cannabis, it can be extremely damaging to their health. Fusarium is powerful enough that it was once

Fusarium is powerful enough that it was once proposed as a weapon of biological warfare in the international War on Drugs. You don’t want to mess with this stuff.

How to prevent fusarium wilt

Cultivate aerobic conditions

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You don’t want to prevent all fungus from growing in your soil. Your plant needs fungi to help create nutrients and protect itself from diseases. The trick is cultivating the right kinds of fungi, and minimizing the opportunity for harmful fungi to take over. There’s a simple way of doing this: make sure your soil is getting enough oxygen.

Some harmful fusarium varieties can grow in anaerobic conditions. This includes fusarium oxysporum, which has been known to impact hemp in the United States“Anaerobic” means in soils with a lack of oxygen. While other fusarium species can grow in aerobic (oxygen containing) conditions, ensuring that your soil is well aerated mitigates the risk of pathogenic anaerobic varieties. Give your soil the chance to breathe.

Turn on the fans

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Two things really encourage fusarium growth: heat and moisture. This is bad news for grow rooms, where hot lights can heat up soil. In grow rooms, you need to pay close attention to heat and humidity. Turn on some fans and make sure there’s enough ventilation.

Creating a nice cross breeze is nice. Invest in a hygrometer and keep track of the rooms humidity. When you first start your plants, the humidity can be at around 70%. Then you need to gradually decrease it by about 5% each week until you hover around 40%.

Ensure that your soil has proper drainage

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Now, for moisture. One of the most common mistakes new growers make is overwatering. Overwatering creates the perfect conditions for undesirable fungal growth, as well as other potential pathogens. The more water in the soil, the less oxygen. Fortunately, there’s a simple solution. Water only when your soil is completely dry, then drench, then let it completely dry again. Repeat until your plant is fully grown and ready to harvest.

Aerating your water ahead of time will also make sure that when you do water, you’re also introducing oxygen into the soil. For a quick discussion on how to aerate your water, check out this post.

Cultivate healthy soil microbiology

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This is where balance comes in. You don’t want one specific species of fungi to dominate your soil. You need a fairly balanced ratio of healthy bacteria to healthy fungi. Anything skewed in either direction is not good.  In order to keep fungi in check, make sure you’re cultivating healthy bacteria in your soil. You can do this a couple of ways:

  1. Inoculating your plant with a bacterial dominated compost tea, or
  2. Adding bacterial foods such as diluted molasses

If you create healthy soil conditions from the start, you’re far less likely to see exponential fungal growth. This is like nature’s own system of checks and balances. Diverse soil microbiology will help create the optimal soil food web, which means that the microbes will work to create balance for you. When we interfere too much, we tend to create conditions for imbalance, which increases the opportunity for disease.

A study published in Applied Soil Ecology back in 2014 compared the resilience of organic soil with healthy microbiology and conventional soil. They looked specifically at how well the two types could fend off flax infection from fusarium. Guess what? The organic soil won. The organic soil had healthier soil microbiota, making it better able to suppress fusarium than the non-organic soil.

Rotate your crops

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If you’re growing outdoors, avoid planting your plants in the same place every single time. Just like in regular ol’ agriculture, if you’re only growing one crop, lack of crop rotation really puts your harvest at risk. Basically, you’re creating a marijuana monoculture which makes your plants more susceptible to disease over time. This includes susceptibility to fusarium fungi.

One other way to minimize disease susceptibility is to plant marijuana in a permaculture guild.You plant everything, everywhere. Guild planting is more complicated than can be fully explained by this post. The basic idea is that you plant your marijuana with other companion crops that work to preserve water, increase soil fertility, and work together in a natural, ecological system that is more or less self-managing.

This option is not suitable for the standard marijuana grower, but might be a neat option for casual home growers. You can find more information about permaculture here.

Manage your pH

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While fusarium fungi can grow in a pH of six, the optimal pH for marijuana plants, making sure your pH doesn’t get too low will discourage excess fungal growth. If you’re worried about fusarium, you may even consider maintaining a pH that hovers slightly above 6, as high as 6.3 to 6.5. This is not a fool-proof plan, but it will definitely help check back fungal growth. Higher pH levels encourage bacterial growth, lower pH levels are more suitable for fungus.

How to stop fusarium wilt

Trim away diseased parts of your plant immediately


Notice a little mold on your bud? Get rid of it. As soon as possible. Chop that thing off. Sterilize any tools you use on the diseased plant before using them again.

Check your plants regularly for signs of disease, especially if you’re growing indoors where moisture and heat can become a problem. If you notice a branch has become infected, cut it off. Remove the diseased plant from your grow room, and try to salvage it somewhere else.

If you’re growing your plants in extremely close quarters, you may just want to sacrifice the diseased one. Fusarium can spread to other plants in the room, quickly creating chaos for your entire harvest. You won’t want to risk that.

Get rid of your soil

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Don’t throw this soil into your compost. You need to throw it somewhere else, take it offsite, dump it in a yard waste container. Or, rake it out into a thin layer and let it bleach and dry out completely in the hot sun. Obviously, you won’t want to use this soil again.

If you have a raised bed, you can “cook” the soil by covering it with black plastic and letting it sit out in the sun after you’ve pulled the plants out. This cooking process will kill off much of the fusarium and allow you to use the soil again in the future.

Change growing locations

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If you’re outdoors and your plants have become infected, your harvest is pretty much lost. It’s good to plan ahead and split your crop into two different locations from the get-go. Or, have some growing in containers or indoors where it’s easier to control in the event of an outbreak.

Unfortunately, if you’ve planted directly in the ground outdoors and you’re hit with fusarium, there’s not a whole lot you can do. You’ll have to try starting up in a different location until you can address the soil issue or plant again in raised beds or containers.

When it comes to fusarium, it’s all about prevention. Start with high-quality soils, maintain healthy soil microbiology, and minimize conditions that will allow the fungus to breed. A fusarium outbreak can cause you considerable stress and once it takes off, it’s difficult to get back under control. Smart growing practices can drastically reduce the risk, the headache, and the financial damage caused by this nearly invisible fungus.

Do you have any fusarium control or prevention tips? Share them with us on social media or in the comments section below. We’d love to hear them!

May 27, 2016