Bacteria? Virus? Fungi? Diagnosing Common Cannabis Diseases

By Chris Bond
Published: October 18, 2017 | Last updated: May 11, 2021 04:43:47
Key Takeaways

Like marijuana plants that have fallen victim to a pest infestation, diseased cannabis plants will have visual symptoms as well. Your first step is finding out whether that disease is a foliar disease, or a soil-borne sickness. Here's what to look for.

Source: Buds Beginning to Rot / Photo by Green Box Grown

Just like any other crop cultivated, cannabis is subject to a wide range of pests and diseases that can distress your plants, even kill them if left untreated. Before a grower can make an informed decision about how best to solve a problem, it is necessary to be able to identify the cause of the plant stressor by its symptoms.


If you cannabis plants are failing due to a pest infestation, you'll either see the bugs for what they are, or at least notice something has been eating away at your plants. Like marijuana plants that have fallen victim to a pest infestation, diseased cannabis plants will have visual symptoms as well.

Diseases of cannabis come as the result of three main pathogens: bacteria, viruses, or fungi. Control of these is very different for each type, so it is important to be able to identify the symptoms of each. However, the most common plant diseases affecting cannabis plants are caused by fungi.


Fungal outbreaks in the grow room are usually caused by the right combination of poor air circulation, high humidity levels, overcrowding of plants, wrong temperatures, and starting off with unhealthy clones or seedlings (infected stock).

Fungal diseases can also be spread by pests, or contaminates that hitch a ride into the growroom from unsanitary equipment, pets, a grower's clothing, etc.

Here are common symptoms relating to common diseases found in poorly maintained grow rooms.


Foliar Diseases Cannabis Growers Need to Look Out For

Powdery Mildew

Powdery mildew, which is caused by a fungus, is common to a wide range of crops. It is easy to identify and if caught early enough can be treated. It appears as a white or grey, powder-like substance on the surface of leaves or flowers. It often forms at the base of plants first and then moves its way up if untreated.

Yellow Leaf spot

This is a generic term for more than one type of either bacterial-induced or fungal-caused spots on cannabis leaves that start out as yellow spots, but then can either join to form larger blotches or may turn brown. This disease, like powdery mildew, usually starts at the bottom and then moves its way up.


Bud Rot

While bud rot is a disease of cannabis flowers, its symptoms may first appear in the leaves. It is caused by a fungus that rots from the inside out on colas and can appear on one or many buds of a cannabis plant at the same time.

Depending on the environment, the symptoms may appear in a different order or vary in appearance. There will be some combination of white fuzz around the bottom or sides of buds and the bottom leaves of the bud will turn yellow. The inside of the buds can be purple, grey, brown, or black.

Soil-borne Diseases Cannabis Growers Need to Look Out For

Root Rot

Root rot is a catch-all term for any number of pathogens that can cause roots to, you guessed it, rot. Roots of infected cannabis plants will usually be brown to black, soft and mushy and usually stunted. Symptoms usually appear as stunted growth, and then can appear as nutrient deficiencies. Root rot will lead to death in plants and whatever media is playing host to it will need to be purged and/or sanitized before attempting to grow anything else in it.

Damping off

Damping off is a disease that affect cannabis seedlings as it causes them to never mature beyond this stage. It is a disease that blocks the passage of nutrients form the roots to the stems and leaves of young plants. It has a variety of causes but the effect is the same. Seedlings may appear healthy one day, and then have toppled over the next. This disease moves rapidly and can wipe out entire crops if not prevented.


Fusarium is a soil-dwelling fungus that can cause either Fusarium root rot, Fusarium wilt or both. It blocks the flow of nutrients throughout the plant causing systematic death, section by section. If it causes wilt, dark spots will appear on lower leaves. These will quickly become yellow to brown and then the leaves will collapse, but not fall off and the stem they are attached to will droop downwards. This pattern will repeat all the way up the plant.

When Fusarium causes root rot, its symptoms may not be seen until it is too late. It causes cannabis roots to turn red. This coloration will then travel up the stem. The stem will burst in one or more spots, inviting insects and disease in and will eventually lead to the death of the plant.


Verticilium wilt is caused by a fungus and can present symptoms in cannabis plants that look very much like those of Fusarium. Verticilium wilt will first appear toward the bottom of the plant with yellowing leaves. This yellowing will occur in the veins and leaf margins first.

These leaves will then darken in appearance and begin to wilt. The stem will also darken and weaken at the base. Wilt tend to happen in larger sections at once with Verticilium as compared to branch by branch with Fusarium.

Other Issues that Look Like a Disease, but Aren't

These are only the more common diseases of cannabis. Problems may appear to be a disease but could be caused by other vectors such as algae in hydroponic systems that may be interfering with the plant’s oxygen.

Water itself may be causing disease-like symptoms; not enough and your leaves will brown—too much and they will turn yellow. Nutrient deficiencies can also present as a wide range of symptoms that can be mistaken for diseases.

Learning the basic appearance of common diseases and deficiencies will help make informed and proper decisions when it comes time to take action or prevent the return of an unwanted pathogen.


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Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional

Profile Picture of Chris Bond

Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.

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