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Pythium Root Rot Control in Seedlings

and Cuttings Starts with Prevention

        By Dr. Nicole Gauthier

Pythium spp.

Industrial hemp (Cannabis sativa x indica) in Kentucky is grown both outdoors and in greenhouses.  Under wet soil conditions in either system, Pythium root rot and damping off diseases pose threats to seedlings, cuttings, and young plants.  Indoor-grown hemp is most often infected, however.

Pythium root rot and damping off are caused by several species of Pythium, a water mold pathogen.  Pythium are soilborne, fungus-like oomycetes.  The term water mold is often used because these pathogens require free water in order to complete their life cycles (reproduce and spread from plant to plant).  Under wet conditions such as rain or irrigation, propagules (particularly swimming zoospores) increase dramatically, disseminate to healthy roots, and infect.  Disease often develops within a week or two after infection.

Root rot and lower stem decay cause above-ground symptoms such as marginal leaf scorch, stunting, poor vigor, and/or inconsistent stands.  These symptoms are the result of lack of water and nutrient uptake as root and vascular tissue is damaged.  Disease is more severe under wet soil conditions, high soluble salts, and plant stress.

Root symptoms begin as necrosis (death) of root tips and young feeder roots.  Infection progresses upward to infect larger roots.  Severely infected roots lose turgidity, become brown or black, and are soft and slimy.  The root cortex (outer tissue) often sloughs off, leaving the stele (vascular tissue) intact.  Healthy roots, on the other hand, are usually white, plump, and rounded with root tips intact.  Inspection of roots begins with careful washing and examination using a lens or magnifying glass.  However, a trained plant pathologist is usually required to identify certain spore structures and confirm the causal pathogen.

Damping off may occur before or after seeds germinate.  If seeds are infected soon upon planting, they may not germinate at all or the shoot will die before emerging from soil.  Post-emergence damping off results in seedlings collapsing soon after emergence.  Symptoms and rate of disease development depend on time of infections and severity of disease. 


Understanding the basic biology of Pythium is important to preventing introduction into greenhouses and maintaining low populations in fields.  Soil moisture, presence of pathogens, and plant susceptibility are all major factors in the disease cycle.  Thus, understanding the relationship is vital to prevention and management of Pythium root rot and damping off.  (Refer to Hemp Disease 101 Back to Basics for more details).

Pythium root rot and damping off can be caused by one of many Pythium species.  Many of these species are ubiquitous, meaning that they are ever-present in soils.  Thus, pathogens can be introduced from outdoor soil, plant debris, groundwater, and pond water.  Some of these species have wide host ranges and can use other plants as a “green bridge” between growing seasons and hemp plantings.  Multiple Pythium species may be present in the same location or field.

Initial infections usually are initiated from movement of propagules (active zoospores, zoosporangia, or inactive survival structures called oospores) to healthy plants. 

The most common methods of spread to greenhouses include:

Contaminated soil – shoes, used containers, dirty tools.
Contaminated water – surface runoff, rain or irrigation splash, contaminated ponds.
Infected plants – movement of infected plants from one area to another or from one greenhouse to another.
Carryover from the previous year – improperly sanitized greenhouse, failure to empty/sanitize greenhouses before introducing a new crop.

Spread within fields is often through surface runoff, water splash, or soil moved through farm implements. 

Upon introduction, Pythium species require susceptible host plants and favorable environment in order to infect and establish.Different Pythium species have different optimal temperatures.However, excess water and high soluble salts are favored by all species.As long as free water is available in soils, the pathogen can replicate quickly and spread rapidly.

 Once Pythium establishes, it can survive for long periods in the absence of host plants.Survival structures (oospores) can endure adverse conditions such as drought and excess heat; they germinate when ideal conditions return.Many Pythium species can also survive as saprobes, living on decayed plant matter until a more favorable host becomes available.Additionally, the pathogen can survive on other host plants, which may serve as reservoirs or green bridges between hemp crops.

Pythium root rot and damping off are best managed by preventing introduction and infection. Cultural practices can help prevent introduction of propagules.Further, cultural practices can also help reduce disease spread and eliminate pathogens if they become established.

 The following cultural practices can be used to prevent and to manage Pythium diseases in greenhouses:

  • Manage soil moisture by allowing plants to dry out between waterings. Maintain a well-drained porous medium and avoid overwatering. 
  • Use clean plants.  Do not bring in plants from contaminated areas or poorly-managed producers.
  • Monitor soil and nutrients.  Maintain low soluble salts in potting media, avoid excess nitrogen, and prevent soil compaction.
  • Maintain a good sanitation program.  Use a footbath.  Keep tools and hoses off of the floor.  Never reuse soil.  Thoroughly clean used pots, tools, and supplies between uses.  Disinfest floors, benches, and greenhouses between crops.
  • Prevent predisposition to disease by avoiding physical damage, promoting plant vigor, and preventing insect damage (e.g. fungus gnats and root aphids).

Once Pythium infects hemp plants, there is no cure.  Discard infected plants and quarantine nearby plants that may be infected.  Do not risk establishment of disease and development of survival structures.  Sacrifice of a few infected or unhealthy plants can prevent losses of large investment. 

There are no synthetic fungicides labeled for hemp, and no botanical “open label” products have been proven effective against disease.

Follow Dr. Nicole Ward Gauthier and her University of Kentucky Twitter page for Diseases of Fruit Crops, Ornamentals, and Hemp for the latest alerts and interesting finds.

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