Most common fungi that can attack the roots of marijuana plants: Fusarium, Pythium and Rhizoctonia

By: Contributor Grow

Everyone with a few growing cycles behind them knows that growing marijuana involves many challenges (including pests, fungi, predators, inclement weather, etc). However, one of the most stealthy and lethal dangers are the pathogens that affect the roots of the plants. These fungi are a silent enemy: in many cases, by the time the grower detects their presence it’s already too late.

The correct development of the root mass is essential for marijuana plants to thrive and produce good yields. The pathogens that damage the roots prevent the proper absorption of water and nutrients, causing serious problems in the development of these plants.

Early identification of the fungi described in more detail below will be crucial to ensure the survival of your grow. Therefore, we have created this guide to show you how to detect the most common fungi that can attack the roots of your cannabis plants and how to prevent them from settling into your crop.

Types of fungi that can affect the roots of cannabis plants

Some of the most common fungi that can damage the roots of marijuana plants are the following:

  • Fusarium: This is one of the fungi that produces more obvious symptoms, as it causes cannabis plants to wither in a really characteristic way. From one day to the next, the leaves appear decayed and spotty, with the branches also beginning to wither a few days later. This pathogen is certainly lethal and acts quickly! Another sign that your plants have been infected with Fusarium is that the roots turn brown, with the stem also exhibiting a dark brown/reddish colour on the inside. Fusarium starts in the soil and ends up colonising the entire plant through the xylems (i.e. the ducts through which the sap flows), blocking its flow and making the plant sick until it finally dies.
  • Pythium: This fungus is responsible for root rot. The roots of infected plants turn a dark colour instead of the usual white seen in healthy plants. Another symptom that will tell you that Pythium has infected your grow is malnutrition, stunted growth, and a progressive yellowing of the leaves. As the roots of the plants deteriorate, they have greater difficulty assimilating nutrients until the leaves end up drying and eventually falling off.
  • Rhizoctonia: This fungus can affect both the base of the stem and the tender roots of young plants. It is responsible for the death of many seedlings in what is commonly known as ‘damping off’. Once symptoms appear, seedlings soon collapse and die. And leaves can also be affected with the appearance of brown spots or lesions. This pathogen doesn’t produce the typical spores caused by other fungi, which in turn complicates its classification and study.
Most common fungi that can attack the roots of marijuana plants: Fusarium, Pythium and Rhizoctonia
Symptoms of root rot in cannabis plants grown outdoors

How to prevent the fungi that cause root rot

Now that you know the main enemies of cannabis roots, it’s time to act so they don’t sneak into your grow. Prevention is the best method to keep these fungi at bay, since once they settle in, the prognosis for the affected plants is pretty gloomy.

Therefore, it is essential to follow certain steps from the very beginning:

  • Choose fungi-resistant cannabis strains.
  • Check your grow regularly. Watch your plants for the following signs: changes in appearance, yellowing of the leaves, or a blockage of growth for no apparent reason. Check if the substrate is constantly moist or waterlogged, or if mould has developed on it.
  • Keep your grow space well ventilated.
  • Sterilise your cutting tools (scissors, scalpels, etc).
  • Keep your grow free from organic debris (i.e. leaves on the ground or rotting leftover plant matter).
  • Always use a sterilised substrate that facilitates the aeration and drainage of the roots.
  • Avoid excessively high temperatures in the substrate: it should be kept at a temperature of around 22ºC. The fungi that affect the roots of marijuana plants can remain dormant for a long time and become activated with the arrival of hot weather.
  • Avoid overwatering. Your watering plan should allow the soil to dry to some extent before rehydrating your plants.
  • Avoid nitrogen over-fertilisation, which can promote the arrival of these invading pathogens.
Most common fungi that can attack the roots of marijuana plants: Fusarium, Pythium and Rhizoctonia
A healthy cannabis plant at about 16 weeks in full flowering (A) and symptoms of root rot ranging from early stages to death by desiccation.

Beneficial fungi: the best vaccine for your marijuana plants

Some fungi act symbiotically with the roots of cannabis plants, thereby promoting their development and protecting them from fungi like Fusarium or Pythium:

  • Mycorrhizae: These fungi act as a network of filaments that promote nutrient absorption and root development. Mycorrhizae not only help protect your plants from the fungi that cause root rot, but also improve their overall heath, helping your grow to produce abundant and high-quality yields.
  • Trichoderma: This beneficial fungus detects and eliminates the presence of other invasive fungi in the substrate. It acts through mechanisms such as competition for nutrients and space, direct parasitisation of other fungi, and induction of resistance in plants. Therefore, applying it to the substrate at the beginning of the growth cycle will be effective in preventing future headaches.

What to do if the fungi that attack cannabis roots appear in your grow

Unfortunately, there are no effective remedies to save your plants once these fungi sink their claws into them. For this reason, prevention plays a key role.

Most common fungi that can attack the roots of marijuana plants: Fusarium, Pythium and Rhizoctonia
Illustration of root rot which you can find in Kannabia’s Bestiary of Cannabis Pests. Download it for free by following this link!

However, this doesn’t mean that you should sit back and do nothing if you find out that any of these fungi have infected your plants. On the contrary, you should act immediately:

  • Get rid of both the affected plants and the substrate in which they have been growing. Make sure you don’t compost infected matter. After removing them, closely watch the healthy plants on a daily basis for any symptoms.
  • Keep your grow space extremely clean to prevent the disease from spreading.
  • If you’re growing indoors, adjust the environmental conditions, ensuring proper ventilation and a temperature below 28ºC, whilst also avoiding excessive humidity (especially in the soil).
  • In some cases, chemical or biological fungicides may be needed to control these fungi. These should be used following the recommendations of an expert and in accordance with local regulations.
  • Once you’re done with that cycle, disinfect the entire grow space and get rid of the substrate.

Proper prevention and treatment are key to avoiding the fungi that attack the roots of marijuana plants. Now that you know all the secrets to keeping your grow healthy and productive, you’re definitely on the right track to fight this silent enemy and ensure abundant and top-quality crops.

Kannabia Seeds Company sells to its customers a product collection, a souvenir. We cannot and we shall not give growing advice since our product is not intended for this purpose.

Kannabia accept no responsibility for any illegal use made by third parties of information published. The cultivation of cannabis for personal consumption is an activity subject to legal restrictions that vary from state to state. We recommend consultation of the legislation in force in your country of residence to avoid participation in any illegal activity.

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