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Pythium Blight: Causes, Symptoms, and Treatment

Posted on June 16, 2023 by Lawn Doctor

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Pythium blight is a fungal disease that affects turfgrass and can cause significant damage to lawns and golf courses. It is caused by the pathogen Pythium aphanidermatum, which thrives in warm, humid conditions and can spread rapidly in wet grass. Symptoms of pythium blight include greasy, water-soaked patches of grass that can quickly turn brown and die.

Outbreaks of pythium blight are most common in the summer months when temperatures are high and humidity is elevated. It can be particularly damaging to cool-season grasses such as Kentucky bluegrass and perennial ryegrass, but can also affect warm-season grasses like Bermuda grass. Prevention is key to avoiding pythium blight, and measures such as proper watering and mowing practices, improving soil drainage, and avoiding over-fertilization can help to reduce the risk of an outbreak.

Pythium Blight: Causes and Symptoms


Pythium blight is a fungal disease that affects cool-season turfgrasses such as tall fescue and annual bluegrass. The disease is caused by the pathogen Pythium species, which thrives in wet and humid weather conditions. The disease development is most conducive when night temperatures exceed 65°F for cool-season turf and 50°F for warm-season turf, and when leaves are continually wet for 12 to 14 hours for several consecutive nights.

Poor surface drainage, overwatering, and compacted soil can also contribute to the outbreak of Pythium blight. Soil moisture plays a crucial role in the disease development, and overwatering can lead to Pythium root rot, which is a more severe form of the disease. Calcium deficiencies in the soil can also make the grass more susceptible to Pythium blight.


The first signs of Pythium blight are circular patches of infected grass that are usually 1 to 3 feet in diameter. The patches may appear greasy or water-soaked and can quickly expand, killing the grass. The mycelium of the fungus can be seen on the grass blades and soil surface in the morning, giving it a cottony or web-like appearance.

As the disease progresses, the infected grass turns yellow and dies, leaving behind a brown patch. The disease is most destructive when temperatures are between 85° and 95°F, and it can be mistaken for brown patch.

Mowing can also spread the disease, as the blades of the infected grass can stick to the mower and transfer to other areas of the lawn. Vertical mowing and topdressing can help prevent the disease by improving surface drainage and reducing soil compaction.

Fungicide applications can also be effective in controlling Pythium blight, but good drainage and proper irrigation practices are the most important factors in preventing the disease.

Diagnosis and Treatment


Pythium blight can be identified by examining the affected turfgrass. The disease typically presents as small, circular patches of wilted or water-soaked grass that can quickly spread and merge together. In warm and humid conditions, the patches may develop a greasy appearance due to the presence of fungal spores.

To confirm a diagnosis of Pythium blight, a sample of the affected turfgrass can be sent to a laboratory for analysis. The lab will examine the sample for the presence of Pythium aphanidermatum, the most common cause of Pythium blight.


Chemical control is an effective way to manage Pythium blight. Fungicides containing mefenoxam or metalaxyl are commonly used to control the disease. These fungicides are classified as Group 4 and Group 28 fungicides, respectively, based on the FRAC code classification system.

A preventive fungicide program is recommended to control Pythium blight. Fungicide applications should be made before the onset of disease symptoms, typically in late spring or early summer. In areas with a history of Pythium blight, fungicide applications may need to be made every 14 days during periods of high disease pressure.

In addition to chemical control, cultural practices can help manage Pythium blight. These practices include proper irrigation and fertilization, reducing shade and improving air circulation, and avoiding excessive thatch buildup.

In cases where Pythium blight has caused extensive damage, overseeding with a more resistant turfgrass species may be necessary.


Diagnosing and treating Pythium blight requires a combination of cultural practices and chemical control. A preventive fungicide program, along with proper irrigation and fertilization, can help manage the disease and prevent extensive damage to turfgrass.

Cultural Management Practices

Cultural management practices are essential for preventing and managing Pythium blight in turfgrass. These practices include nutrient management, water management, mowing practices, and cultural practices.

Cultural Practices

Cultural practices that can help prevent Pythium blight include regular aeration to reduce soil compaction, overseeding to promote healthy turfgrass growth, and topdressing to improve soil quality. It is also important to avoid overwatering, as this can create humid conditions that are ideal for Pythium blight to thrive. Additionally, mowing practices should be adjusted to ensure that grass blades are not cut too short, as this can weaken the turfgrass and make it more susceptible to disease.

Nutrient Management

Nutrient management is critical for maintaining healthy turfgrass and preventing Pythium blight. Nitrogen applications should be carefully timed to avoid promoting excessive growth, which can increase the risk of disease. It is also important to ensure that the soil is not deficient in calcium, as this can make turfgrass more susceptible to disease.

Water Management

Water management is key to preventing Pythium blight. It is important to water turfgrass early in the day to allow the blades to dry before nightfall, as this can help prevent the growth of Pythium blight. Additionally, low areas and poorly drained areas should be addressed to prevent the buildup of moisture.

Mowing Practices

Mowing practices should be carefully managed to prevent Pythium blight. It is important to avoid mowing turfgrass when it is wet, as this can spread disease. Additionally, lawnmowers should be cleaned regularly to prevent the spread of disease between areas of the lawn.

Overall, cultural management practices are critical for preventing and managing Pythium blight in turfgrass. By implementing these practices, it is possible to maintain a healthy lawn and prevent the spread of disease.

Why You Should Hire a Professional to Remove & Prevent

Pythium blight is a common disease that affects turfgrass and can cause significant damage to lawns. If you are dealing with an outbreak of pythium blight, you may be tempted to try to remove and prevent it on your own. However, there are several reasons why hiring a professional to handle the job is a better option.

Professionals have the expertise and knowledge to accurately diagnose the problem. This is crucial because there are several diseases that can mimic the symptoms of pythium blight, and misdiagnosis can lead to ineffective treatment. A professional will be able to identify the specific disease and recommend the appropriate treatment.

Professionals have access to specialized equipment and products that are not available to the general public. They can use these tools and products to effectively remove and prevent pythium blight from spreading. This can save you time and money in the long run, as you won’t have to purchase expensive equipment or products that may not be effective.

Professionals can provide ongoing maintenance and monitoring to prevent future outbreaks. They can develop a customized plan for your lawn that includes regular inspections, fertilization, and other treatments that can keep your lawn healthy and free from disease.

Hiring a professional to remove and prevent pythium blight is a wise investment for any homeowner. They have the expertise, equipment, and knowledge needed to accurately diagnose and treat the disease, and can provide ongoing maintenance to prevent future outbreaks. Don’t hesitate to reach out to a professional if you are dealing with pythium blight in your lawn.


Pythium blight is a common disease that can cause significant damage to turfgrass. It is important to take preventive measures to avoid an outbreak of the disease. The following are some key takeaways from this article:

  • Pythium blight is most common in warm, humid weather.
  • Perennial ryegrass and annual bluegrass are most prone to the disease while creeping bentgrass and tall fescue are more resistant.
  • Proper watering and fertilization practices can help prevent an outbreak of the disease.
  • Fungicides can be used to treat an outbreak of pythium blight, but they should be used as a last resort.

By following these tips, homeowners and turfgrass managers can help prevent an outbreak of pythium blight and keep their lawns healthy and disease-free.

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