Root rot on cannabis plants

Rhizoctonia seen under microscope
Rhizoctonia seen under microscope (Photo: Forestpests)

What is Root rot?

Root rot is a disease that affects many types of plants, including cannabis. It can be caused by different agents like Fusarium, Pythium or Rhizoctonia, which is a genus of fungi named by Augustin Pyramus de Candolle to describe a group of organisms in the order Cantharellales which don't produce spores but hyphae and sclerotia. Rhizoctonia means "root killer".

These organisms are able to survive in the topsoil (and also in plant debris) for long periods of time, causing damage to many types of plants, of course also to marijuana plants. Thus, they don't need host plants to survive, and can attack plants in their surroundings as long as temperature and humidity conditions inside the grow tent - or outdoors  - are suitable for their propagation.

Development of root rot

A number of factors can cause the development of this disease: temperatures ranging between 15 and 24ºC (being spring and autumn seasons the most suitable seasons), a soil too wet or too hot, without sufficient drainage or an excessive use of nutrients for cannabis rich in nitrogen also favor its appearance.

Fusarium can attack our plants if the temperature of the growing media (20-34ºC) or the nutrient solution (more than 24ºC) are too high, especially if we are using soil with high content of potassium and low content of nitrogen. Bare in mind that Fusarium spores can remain in dead plant matter for 6 years, so removing any plant debris from the growing space is crucial.

Pythium produces spores that also remain in dead plant material for long periods of time until they find a live host plant. Once they reach the roots of the plant the spores germinate and form the mycelium, which will soon cause root and stem rot.

Rhizoctonia are saprotrophic fungi that can infect a vast number of plant species. Among many others, this group of fungi can cause different diseases on plenty of plants, like root rot, damping off, black scurf, brown patch,...

Healthy roots (left) and damaged roots (right)
Healthy roots (left) and damaged roots (right)

Symptoms and damages caused by root rot

At first,the growth of the upper part of the plant is notably slowed. The leaves turn yellow and develop chlorosis to finally die. The overall state of the plant is rapidly deteriorated, from the initial wilt to the development of brown spots on the leaves and stems (even the buds or fruits) which will soon die.

White and healthy roots treated with ZHO from Botanicare + Rhizo Blast
White and healthy roots treated with ZHO from Botanicare + Rhizo Blast

Under the soil surface, it’s possible to observe necrosis of the root hairs, which gradually stop absorbing water and nutrients (which causes the aforementioned symptoms of chlorosis). The roots develop brown tones with necrotic tips. The stem base can also show dark or brown staining.

Basically, what we see is a general wilting of the plant, especially during the hottest hours of the day. We will also see that the plant does not recover from that wilting, no matter how much we water it; in this case, many growers could think that the plant needs more water, which far from solving the problem further aggravates it. Plants usually die a week after the first symptoms are shown.

Root rot causes severe damages on both roots and stems
Root rot causes severe damages on both roots and stems (Photo: Scot Nelson)

Curing - or even just trying to manage - an infection of any of these fungi is almost impossible. Keeping this in mind, the only thing we can do is to prevent its appearance in our crops: How? Follow these easy steps to highly reduce the chances of infection:

  • Use a Substrate suitable for cannabis with good drainage and which doesn't retain too much moisture.
  • Don't water too much and avoid to swamp the substrate.
  • Don't water with hot water or during the hottest hours of the day.
  • Don’t over-fertilize the substrate (especially with nitrogen).
  • Whether you use seeds or cuttings, remember that proper hygiene is crucial. Disinfect your utensils and tools before use.
  • Use beneficial fungi and Microbial life for the roots - Trichoderma - which will protect the root system of your plants.
  • Always discard the substrate attacked by fungi.
  • Keep your grow space clean and free from plant debris.
  • Avoid high soil temperatures.
  • Grow in raised beds and disinfect the pots thoroughly, especially if you are using clones or plants with wounded stems.

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

Comments in “Root rot on cannabis plants” (1)

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Tatnic 2020-08-04
I've having some issues with about half of my outdoor plants. This year I used a different soil mix and suspect root problems. I've never experienced this before but this is only my 4th yr growing. The nodes of these affected plants are turning brown--sorta looks like a rotting banana. The brown patches are not soft though. And these nodes which on my plants have 2 to four stems coming from it are dying. I've been cutting off the tops of what should have been some decent colas. First it will be two opposite stems dying that attach directly to this node. The leaves die first and then its pretty obvious the entire stem is gone so I cut them. I'm always worried about watering issues since my grow space is totally exposed to both sun and wind and rain. I don't think I've been overwatering because the pots are not heavy when I lift them. At least that's one way I verify. These plants started with bug problems...little green dots--tiny really but I saw them moving around--little white and gray spots that were obvious bug related. I sprayed a couple of times with neem oil and that got rid of the little fuckers. But I suspect my brown nodes and dying stems are root related. Should I just accept the fact that I'll lose these plants or can I perform some miracle work? Getting kind of late in the game to transplant but they have not started flowering so its still possible. Any thoughts, ideas would be appreciated.

Alchimia Staff

Tim Alchimia 2020-08-04
Hi and thanks for your comment. From what you say, it definitely sounds like a fungal pathogen of some kind. The nearest thing I can find a reference for is Brown Stem Rot, which starts in the roots and spreads upwards throughout the plant. Alternatively, it could be a serious case of Fusarium, the way your branches are dying off sounds very similar. It appears that the pathogen has taken hold within the plant and I'm not sure if there's anything you can do. I'd probably try using some beneficial microbes such as Trichoderma, which may go some way to fighting the infection in the soil, but I fear that once it starts moving up the plant, the battle is already lost. If you know anyone else using the same soil mix, ask them if they've been having similar problems, it sounds to me like the problem could very well lie in that area. It might not be too late for a last-minute transplant to a new substrate, although obviously it's not the ideal moment, it could mean that you save the plants and end up harvesting something even if they are slightly delayed due to transplant stress. Sorry that I can't be more help, but good luck and I hope you manage to harvest something.

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