Necrosis caused by the fungus Pythium was first described in 1930 and is today a widespread problem in many plants. In cannabis it is an increasingly common problem especially in indoor crops.

Cannabis: damage from Pythium spp.

  1. What is Pythium
  2. Pythium reproductive cycle
  3. Asexual reproduction
  4. Sexual reproduction
  5. Ideal conditions for the development of Pythium
  6. What harm does Pythium cause in cannabis?
  7. How to control Pythium in cannabis cultivation
  8. Crop management
  9. Biological control
  10. Chemical control

What is Pythium
















Pythium spp.

It is a fungus belonging to the oomycetes and its morphology includes hyphae without partition; the organ that produces the female gametes is the oogonium with a sub-globose shape and a size of 15-25 micrometers. The organ responsible for generating the male gametes is the antheridium, having 1 or two for each oogonium. The organ responsible for asexual reproduction is the sporangium and has an ellipsoidal shape; Each sporangium has a single oospore.

There are more than 120 species of the genus Pythium cataloged but the most common in indoor cannabis crops are:

  • Pythium aphanidermatum
  • Pythium ultimum
  • Pythium irregulare
  • Pythium debaryanum

Pythium to the electron microscope

Pythium reproductive cycle

Asexual reproduction The asexual reproduction structures of Pythium are the sporangia that normally have a globose shape, although in some species they can be filamentous. When a mature sporangium can result in a hyphae or a gallbladder; It is in the vesicles where zoospores are formed which are released when the gallbladder wall ruptures. The zoospores once released move in the free water that is in the soil or on the surface of the plant propelling themselves by flagella; Once the zoospore is encysted, it germinates emits a tubular hyphae and infects the plant.

Pythium has the ability to reproduce sexually which leads it to evolve and form new populations with the consequent risk of creating resistance to the chemicals we use to control it. The organ used to generate the male gametes is the antheridium and to produce the female gametes is the oogonium: both can appear in the same hyphae or in different hyphae. When both structures come into contact, the antheridium emits a fertilization tube that penetrates the oogonium; The antheridium inoculates its nuclear material, the fusion of both nuclei occurs and the zygote appears. Once the wall surrounding it thickens, the oospore is formed, which is a spore capable of resisting periods of drought of more than 10 years.

Both zoospore and oospora are capable of living in water and air.

Ideal conditions for the development of Pythium The development of Pythium, both in soils and substrates is favored by long wet periods caused by environmental conditions such as lack of light, poor ventilation, excessive irrigation, rainfall or poor drainage. Under these conditions the oospores can germinate produceda vesicle with zoospores which move looking for a host plant. Once found, zoospores move towards the tips of the roots, by boosting flagella in response to chemical and electrostatic stimuli caused by root exudates.

In the case of indoor cannabis cultivation, the conditions of high humidity, waterlogging of substrates, poor oxygenation of these, high temperatures (20-30ºC), poor ventilation and the lack of organisms that compete with pathogens create an ideal environment for the development of this type of fungi. The ideal temperature conditions for the fungus to show its full destructive potential are 30-35 ºC during the day and above 20 at night, although the fungus can also develop between 13 and 18ºC. Pythium can be found in the substrate in the form of zoospores or oospores being the viability of the first very short.

What damage does Pythium cause in cannabis The genus Pythium includes species capable of living in a wide variety of ecosystems. Most behave like saprophytes feeding on decaying organic matter and in aquatic environments such as lakes and canals or ferticultural irrigation systems, but other species behave like ferocious plant pathogens causing disease, reducing their yields and quality. They mainly infect seedbeds of different crops causing the death of seeds and seedlings in pre and post emergence what is known as “damping off” affecting both the embryo and the hypocotyl and the radicle. In adult plants it causes a reduction in growth, symptoms of nutrient deficiency and reduced yield by decreasing root biomass due to the destruction of secondary roots and root hairs. They preferably infect young tissues, with moisture and succulent organs of plants that are in contact with the soil or substrate, especially the areas of root elongation.

As we have seen before, in cannabis the main species of Pythium are:

  • Pythium aphanidermatum (Edson) Fitzp. In the seedbeds we will appreciate the death of seedlings in pre-emergence; after the emergence, the disease manifests itself as wet rots that cause the fall of the seedlings. In adult plants, growth delays with progressive wilting and leaf rolling towards the beam, defoliation and rapid death of the plant are observed. At the root, generalized browning and destruction of the bark.
  • Pythium ultimum Trow. If the attack occurs in pre-emergence the seedling dies; if it occurs in post-emergence it causes the collapse of the seedling by attack on the neck. Infected seedlings usually have moist and hydrotic lesions. It can cause necrosis in the roots producing delays in growth and decrease in production.
  • Pythium irregulare Buis. It mainly attacks nursery seedlings producingecrosis in the rootlets and causingthem to die.
  • Pythium debaryanum. Pre-emergence infection causes the death of the embryo;post-emergence infection causes the leaves to yellow and fall off. At the base of the stem appear brown lesions and in the roots appears an aqueous and dark decomposition detaching lto epidermis easily. The lateral roots rot, and sometimes the root system collapses.

Damage caused by Pythium

How to control Pythium in cannabis cultivation

Crop management

  • Avoid watering at the beginning of the dark period, especially with high temperatures.
  • Maintain good drainage of the substrate.
  • Control nutrition, especially nitrogen, in hot weather.
  • Avoid high temperatures when making the seedbed or removing the cuttings.
  • Use quality substrates.
  • Disinfect irrigation water with hydrogen peroxide.
  • Disinfect planters and growing utensils with water and bleach.
  • Use lighting that introduces ultraviolet light into its light spectrum.

Biological control

  • Consider the use of Trichoderma harzianum.

Chemical control The most effective active substances against Pythium are:

  • Etridiazole
  • Propamocarb
  • Capture
  • Fosetyl aluminium
  • Metalaxyl
  • Mefenoxam

We must take into account the resistance to chemicals that the fungus can develop, so we should not apply metalaxyl after mefenoxam and vice versa.