Diseases On Marijuana Plants

July 2, 2021

Diseases on marijuana plants and infections (cannabis diseases) usually create the worst possible issues for your plants. They have a tendency to be much harder to treat than pests. Diseases in your plants will usually come in one of two major categories: fungal or bacterial.

Fungal diseases are often caused by environments that are too damp or humid, or places which lack airflow. Fungal spores float around in the air looking for a suitably damp place to root down and if the environment is right, that suitably damp place might be on your cannabis plant.

Bacterial infections are often sneakier and harder to notice. They are spread by a number of different possible vehicles, ranging from insects and humans to rain and unclean soil or substrate.

Bacteria can sometimes get into a plant and then leave it mostly untouched unless the plant is weakened by external stresses, at which point they can quickly take down the whole plant.

Proper treatment of a fungal or bacterial disease requires a knowledge of the potential symptoms, which we’ll list below. As always, the best defense against disease is prevention. Do your best to create an environment which is healthy for plants and inhospitable for fungus and bacterial growth.



Hydroponics systems are water-based, and without proper care, they can become havens for algae. Both cannabis and algae thrive in nutrient-rich water environments, but you really don’t want them to be sharing the same space.

Algae will live on the roots of your plants, and thus deprive your cannabis of the nutrients necessary for the plant to develop properly in a hydroponics system.

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Not to beat a dead horse, but your best defense is always preventative. Do your best to minimize the potential for algae growth before it occurs by the taking the proper precautions beforehand. Because algae and cannabis have similar requirements of water and light, it can be difficult to do this.

The easiest way is to make sure that no light reaches the roots of your plants. Use an opaque, dark material to block out the light. If algae begins to develop, clean it out right away. Read more about Algae on marijuana plants

Bud Rot

Bud rot

Bud rot (also gray mold or botrytis) is one of the most damaging of the possible infections your marijuana plants can face. It eats every part of the plant, not bothering to distinguish between buds and flowers or stalks and stems.

The infection can spread very rapidly, so you want to take precautions to avoid letting gray mold take root in your plant’s environment. The disease prefers a cool but relatively humid environment.

Keeping the temperature in your grow room above 70 degrees Fahrenheit will help, and make sure to monitor the humidity constantly so that it doesn’t get too wet.

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Another step can be to change clothes or wear protective gear before going into your grow room. The spores can attach themselves to clothing fibers and then release in the grow room if the environment feels appropriate.

If all else fails and your plants develop gray mold, there a few different soaps and sprays you can use to help mitigate the damage and treat the plant. Marijuana Plant Protector is the best solution. Read more about Bud rot on marijuana plants

Leaf septoria

The yellow spots initially develop on the lowest leaves of the plant and then work their way upwards. In the worst cases of Leaf Septoria, the entire leaf will turn yellow and begin to crumble. Generally, however, the leaves won’t die off, and the plants will survive.

Despite the fact that it isn’t usually lethal, yellow leaf spot can heavily inhibit the growth of the plant and its eventual yield. You can prevent yellow leaf spot by keeping the soil well-aerated and applying a fungicidal compound to your compost.

If you still have trouble with yellow leaf spot, you can apply baking soda as a reactive measure. Read more about Leaf septoria on marijuana plants

Powdery Mildew

Powdery mildew

Powdery mildew on marijuana is a spore based fungal problem that can affect both outdoor and indoor marijuana gardens. The spores are transmissible through the air, and being carried by wind means it can be fairly difficult to protect against.

It’s also a very sneaky fungus because the spores can lie dormant in the soil until conditions are just right for them to start growth. Usually, this is when the environment is warm and highly humid. 

Powdery mildew also occurs frequently when the cannabis plants are too close together, and there isn’t enough airflow. An overcrowded grow room unnecessarily risks damage to your plants.

You can recognize powdery mildew by its white color. It will cover the foliage of the plant and inhibit the photosynthesis process.

If your plants suffer from powdery mildew, there are a number of safe naturally based sprays you can use (including milk and apple cider vinegar) to help deal with the problem. Mold control also treat powdery mildew. Read more about Powdery mildew on marijuana plants



Fusarium on marijuana is another fungal issue. Instead of eating the foliage, however, fusarium damages the root systems of your cannabis plants.

Thankfully, hydroponics systems aren’t affected by fusarium since it requires a soil-based substrate to survive in. Usually fusarium causes wilting in the plant or a more generalized root rot. Like some of the other fungi, fusarium will sometimes lie dormant in the soil for long periods of time, just waiting to strike.

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If fusarium does begin to take hold in the root systems of your plant, there’s very little that you can do to treat it. First of all, it’s very difficult to spot until it’s too late.

Second, although indoor growers can help prevent fusarium by using sterile soil, there’s no way to be sure it isn’t present outside in nature. Read more about Fusarium on marijuana plants

Verticillium wilt

Verticillium wilt

Verticillium wilt on marijuana is fairly similar to  fusarium wilt, except that it is mostly like to appear in soils that are improperly drained or overly full of nutrients.

The first signs of verticillium wilt are drooping and yellowing foliage. The fungus will also cause the stem to turn a dark brown at the base, where it makes contact with the soil. These sorts of fungal diseases are nigh impossible to treat, so it’s of the utmost importance that you practice preventative care.

Try to make sure that your soil or substrate has proper drainage, so that verticillium wilt won’t rear its ugly head. Because there’s really no way to cure or treat verticillium wilt, make sure that you rotate your crop if you have issues with it, otherwise it will just keep coming back. Read more about Verticillium wilt on marijuana plants

Bud Rot

Root rot

Root rot, also known as Pythium, is a fungus that will live on the roots of your marijuana plants. As you can imagine, this isn’t good for the roots, and if it’s not good for the roots, it’s definitely not good for the plant.

Root rot can occur both indoors and outdoors, and in a wide variety of substrates, including the water of hydroponics systems. Initial symptoms include wilting of the plant, and a change in the color of the foliage to brown or yellow.

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Since these symptoms can be caused by a wide variety of issues, you’ll need to check the roots themselves if you want to verify the source.

When a plant is infected by pythium, the roots will begin to change color and eventually the outer layer of the roots will be shed to reveal stringy, weak inner core.

You can help keep your plants root rot free by fastidiously cleaning the hydroponics system and making sure that the substrate drains properly. Remember, preventative care is key! Read more about Root rot on marijuana plants

Damping Off

Damping off

Damping off of marijuana seedlings isn’t actually a disease, instead it’s the plant responding to the presence of a disease.

You’ll notice when you see damping that the plant seems like it’s wilting— it may resemble overwatering, but really it’s the roots which causing the issue. Damping off usually occurs in cannabis seedlings. The plant itself will develop lesions before actually dying entirely.

Again, preventative care is the only real way to deal with damping off and other fungal disease-related problems. Usually, plants won’t be able to bounce back from damping off, and so it’s up to the grower to prevent it from happening in the first place. Read more about Damping off on marijuana seedlings

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible


Marijuana Diseases FAQ

Can Algae Grow on Marijuana Plants?

Hydroponics systems are water-based, and without proper care, they can become havens for algae.

What Causes Bud Rot?

Bud rot (also gray mold or botrytis) is one of the most damaging of the possible infections your marijuana plants can face.

What Causes Fungal Diseases on Cannabis Plants?

Fungal diseases are often caused by environments that are too damp or humid, or places which lack airflow. 

The founder of I Love Growing Marijuana, Robert Bergman, is a marijuana growing expert that enjoys sharing his knowledge with the world. He combines years of experience, ranging from small-scale grows to massive operations, with a passion for growing. His articles include tutorials on growing... [read more]


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      I’m using grow bags and it’s really easy to check for water by weight of the plant in the grow bag. That and an index finger 2 knuckles deep works for me!
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    I suggest you all join our "Support Forum" We have many experienced growers that can help you out.
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      the tops of leaves show silvery patches with microscopic shiny black spheres. I also have leaves get speckled all over with brown,, spots, yellow color and turn brittle.. I remove affected leaves but the symptoms will basically overwhelm the plant.
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    The service you have provided me with this far has been outstanding! The ordering process, the delivery, the quality of the products is also outstanding. My femenized seeds are 15 inches tall at 4 weeks from sprout and all 5 […]Read More
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  18. By john watts ,21 Aug 2017
    I to buy the root pertecter that's really all I need how can we make that happen
    1. By Stacy ,23 Aug 2017
      Hi John, Unfortunately that's not possible, we only offer the root protector as a set together with the mold control and bug blaster. Sincerely, Stacy ILGM
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  23. By Mathew ,24 Sep 2019
    Hi! How do I go about sharing photographs?
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    Does the plant protector stop or control them but rot n powder mildew and the other sicknesses
  25. By zadomino ,27 Apr 2020
    Good article, thanks for sharing! I have found that 1tbs baking soda and 1tbs organic dish soap worked like a charm!
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