Fungi That Affect Cannabis

Written by Ryan Scott

What is a Fungus?

There are many types of fungi that can infect humans and animals, as well as plants. A fungus is defined as “any of  a  diverse  group  of  eukaryotic  single-celled  or  multinucleate  organisms  that  live  by decomposing  and  absorbing  the  organic  material  in  which  they  grow”. [1] Most types of fungi prefer an environment that offers darkness, humidity, and warmer temperatures. [2]

Types of Cannabis Fungi

Because cannabis plants thrive in fairly moisture-rich environments, it’s easy for them to develop fungal infections. This fact, coupled with high indoor temperatures and the twelve-hour minimum of darkness to trigger flowering hormones, means that fungi can easily proliferate and destroy an entire crop. [3]

The most common types of fungi that affect cannabis plants are: [4]

  • Grey Mold- or Botrytis cinerea, is a pathogenic fungus that mainly affects cannabis flowers during the flowering phase and is often the result of high humidity, and a lack of proper air ventilation. Common indicators of Botrytis contamination include grey or brown leaves and stems, in addition to necrosis from rotting flowers.
  • OidiumOidium, or white or powdery mildew (PM), is another common fungus affecting cannabis plants. It often shows up on leaf blades, then gradually moves to other parts of the plant, including flowers. PM prefers high-humidity environments, while its spores are easily knocked loose to settle on other parts of the plant, consuming the nutrients stored in affected areas. Signs of PM include a powder-like, white substance on leaf blades, stems, and other parts of the cannabis plant.
  • PythiumPythium, or root rot, is another common fungus that attacks the root system of cannabis plants. The cause of root rot is often due to overwatering of plants in soil, and a lack of proper aeration and drainage of the growing medium. Stagnant water in and around the root system stifles the oxygen the roots need in order to maintain the health of the entire plant. Symptoms of root rot include chlorosis of the leaves, dark-colored root systems, wilting, and death.
  • Fusarium Fusarium are fungi that reside in the soil of cannabis plants. Beginning at the roots, Fusarium can block the flow of nutrients throughout the plant, resulting in systematic death, segment by segment. Symptoms of Fusarium include plant wilt, necrosis on leaves and stems starting at the bottom of the plant, and the appearance of insects that feed on the dying sections.

Cannabis Fungi Treatment

Chemical fungicides can be employed for treating fungal infections of cannabis. These chemicals include sulfur and copper sulfate. However, the use of chemicals for the treatment of fungal infections can also damage cannabis plants, especially during the flowering period.

Using a sulfur spray to stave off a PM infection cannot be used during the flowering phase as it will contaminate precious cannabis flowers. Similarly, copper sulfate cannot be used during the flowering period, and has been shown to stunt plant growth and maturation if overused. [5]

The recommended treatment for cannabis fungal infections is facilitating prevention. Sterilized grow equipment, optimizing growing-environment parameters, and keeping watchful eyes on all aspects of the plants’ life cycle will help maintain healthy crops while ensuring fungal infections never proliferate in the grow room.


  1. Merriam-Webster’s Medical Dictionary; Page 249; 1995 Edition
  2. Nicholas P. Money; Fungi: A Very Short Introduction; Oxford University Press, 2016.
  3. Ayres, P. and Boddy, L. Water, Fungi, and Plants: Symposium of the British Mycological Society; Cambridge University Press, 1985.
  4. McPartland, J. et al Cannabis Diseases and Pests: Management With Emphasis on Organic and Biological Control, an Advanced Treatise; Amrita Press, 2009.
  5. Pegg, G. and Ayres, P. Fungal Infection of Plants: Symposium of the British Mycological Society, Cambridge University Press, 1987.

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Ryan Scott

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