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Pythium Blight


Pythium Blight

Family: Pythiaceae

Pythium Blight Pythium Blight

About Pythium Blight

Life Cycle

Pythium blight is caused by organisms that are similar to fungi and are sometimes referred to as water molds. This disease is mainly caused by 2 different Pythium species: Pythium aphanidermatum and Pythium ultimum. However, there are still numerous other species that are capable of causing Pythium blight, they are just not as common. More severe cases are often associated with P. aphanidermatum. These pathogens can reproduce both asexually and sexually. During asexual reproduction, they create structures similar to sacks where they can germinate when mature. Zoospores form inside and when the wall of the sack ruptures, these spores can move freely using flagella in water on plant tissue or in the soil. When they find a suitable host, they remain there and germinate, forming a germ tube that will infect the host plant. Under unfavorable conditions, zoospores will not live long. This pathogen will reproduce sexually when common strands come into contact with one another. Egg fertilization occurs and a zygote is formed. The egg cell walls will thicken to create an oospore with the zygote inside. These oospores can survive in dry environments and can live for as long as 12 years. Oospores can be found in the roots and leaves of turfgrass. During wet conditions the spread of this disease occurs quite easily. Spread can also occur when infected equipment, soil, or mower clippings are moved to uninfected areas.

Pythium Blight Identification and Habitat


When plant tissue first becomes infected, tissue is darker green and sometimes purple in color. The infected spot will appear to be soaked and will form patches on the turf grass, either irregular or circular in shape. These patches can be anywhere from under 1 inch to over 8 inches in diameter. Infected plant tissue will not form any spots or lesions; they mainly just feel slimy when touched. The infection can enlarge and form one massive patch on a lawn, which can cause severe damage, especially for field turf and golf courses. As the pathogen kills the plant tissue, the grass will shrivel and eventually fade to brown or gray. If the pathogen infects a water source and is spread to a new area, the patches may appear as streaks through the lawn; this can also happen when the mowing equipment is infected. This pathogen can also cause root and crown rot. Pythium blight may also show up as fluffy cotton-like growth on foliage, especially when leaves remain wet for a substantial amount of time.


Pythium blight favors weather that is warm and humid and tends to be most prominent when the turfgrass tissue has been moist for a minimum of 12 hours as this promotes infection. For this disease, a preferred day temperature is above 82 degrees Fahrenheit and a preferred night temperature is above 68 degrees. Overall, weather conditions that include hot temperatures, cloudy skies and either rain or humidity (greater than 90 percent) is ideal. When conditions are optimal, this pathogen has the ability to destroy an entire turfgrass stand in less than a week. It is fairly common to develop first in low areas of the lawn and on cool-season turfgrass varieties. Other conditions that help this disease thrive are soil types with high salinity. When this is the case, Pythium blight is able to survive and favors lower temperatures and humidity levels than usual. High soil salinity can actually facilitate Pythium infection in conditions of drought. Additionally, when fertilizer is used with high nitrogen or calcium levels, this can also make plants more susceptible to Pythium blight.

Pythium Blight Management and Control Methods

Cultural Control

To effectively manage Pythium blight, closely monitor irrigation practice. If irrigation is completed in the morning, foliage has time to dry, which will keep the plants dry during the night. Additionally, ensuring turfgrass areas have proper drainage to avoid areas where water can pool can reduce the severity of Pythium blight. Encourage good air flow and remove thatch that is greater than half an inch. If there is a chance your mowing equipment is contaminated, make sure to wash it before entering an uninfected area. If overseeding is needed for species of cool-season turfgrass, wait until early fall when evening temperatures are below 65 degrees Fahrenheit. Applying moderate amounts of fertilizer can also assist with control. Warm-season grasses are not as susceptible to Pythium blight.

Chemical Control

Fungicides are often necessary when weather conditions are favorable for the Pythium pathogens. Application should be done at the first sign of symptoms. Carbamates, phenylamides, aromatic hydrocarbons, dithiocarbomates, and phosphonates have been known to have effective results. If your area is highly susceptible or has a history of this disease, fungicides used for preventative measures should also be considered. Examples of preventative fungicides include mefenoxam and metalaxyl. Always be sure to carefully read the label for cautions and proper application before use. Also, when multiple applications are done, make sure to alternate between chemical groups to limit Pythium’s ability to grow resistant to the chemicals used. There is also seed available that has been treated with preventative fungicide.

Alternative Pythium Blight Names

  • • Pythium disease
  • • Pythium root rot
  • • Cottony blight
  • • Grease spot