Pythium Blight: How to Identify and Get Rid of It

Pythium blight in a lawn

Pythium blight is a fungal lawn disease that infects residential lawns and commercial locations, like golf courses, alike. It thrives in hot, humid summers, and extreme heat and humidity can cause Pythium blight to kill large areas of grass in a couple of days. 

If you see small, circular, recessed patches of water-soaked grass with dark spots, you could have Pythium blight in your lawn. These spots are initially about 1 to 2 inches in diameter and grow to about 1 foot wide. Other symptoms include a cottony white material in the early morning. This article will tell you how to identify and get rid of Pythium blight before it kills your lawn.

How to identify Pythium blight

Pythium blight close up
Photo Credit: NC State Cooperative Extension

Pythium spp. fungi occur naturally in the soil and can exist without causing disease. However, many Pythium species can cause plant diseases under certain conditions. The pathogen Pythium aphanidermatum is the most common species of Pythium found in turfgrass. It causes the disease Pythium blight in hot, humid weather. 

If the grass looks like it has grease spots, it may have a case of Pythium blight. Patches of grass that are greasy and water-soaked are symptoms of the early stages of this turfgrass disease. Be mindful that these spots can spread rapidly and kill the grass, so it’s mandatory to act ASAP to treat the disease. 

Pythium blight, also called cottony blight, looks different depending on your grass type. Cool-season turfgrasses, like Kentucky bluegrass and perennial ryegrass, are more susceptible to Pythium blight than warm-season grasses. But warm-season grasses, like Bermudagrass and Zoysiagrass, can succumb to the lawn disease, too.

Pythium blight in cool-season grass: In cool-season grasses, Pythium blight starts as little circular patches that are recessed. Spots start about 1 to 2 inches in diameter and can expand to about 1 foot wide. These spots show up in hot, humid weather. 

Pythium blight in warm-season grass: Pythium blight appears as small purple or black spots in warm-season grasses. These spots can grow into irregularly shaped large patches. Extended periods of cloudy, humid, or warm weather increase the risk of expansion. 

If left untreated, the disease worsens, and the infected grass turns yellow and dies, creating a brown patch.

You may see a cottony substance on grass blades in the early morning. This cottony substance is called mycelium. It develops later in the disease process. You may also see mycelium when grass blades are wet or when it is incredibly humid. Mycelium can cause the affected areas of grass to eventually wilt, desiccate, and die.

In summary, the symptoms of Pythium blight are:

  • Cottony gray or white substance on grass blades. The substance is more prevalent when grass blades are wet in the early morning after it has rained the night before.
  • Little, circular, recessed patches with purple, dark green, or black spots.
  • Water-soaked grass leaves.
  • Light brown patches or dead grass that is tan or brown.
  • Oily feeling grass when rubbed between your fingers. (Wash your hands after handling the grass.)

How to get rid of Pythium blight

Person holding grass infected with Pythium blight
Photo Credit: NC State Cooperative Extension

Homeowners can send samples of the infected grass to a local cooperative extension laboratory to test for Pythium aphanidermatum. Even if it tests positive, there is hope for your lawn. While overseeding with a turfgrass more tolerant to the fungus may be necessary in cases of extreme damage, you can typically gain control of Pythium blight with fungicides. 

Fungicides containing the following chemicals are commonly used against Pythium blight:

  • Metalaxyl
  • Mefenoxam 
  • Propamocarb 

To keep Pythium spp from developing tolerance to fungicides, use fungicides consisting of different chemicals or combine them.

Preventive fungicide programs are also effective. These products should be applied before home lawns are infected. The best time is usually in late spring or early summer. If you have a history of outbreaks, you may need to apply the fungicide every 14 days when weather conditions are most conducive for the disease.

Mefenoxam can be used to prevent and eliminate Pythium blight.

Here are some examples of fungicide products you can use on Pythium blight:

The problem with these fungicides, aside from the fact that they contain harsh chemicals, is that they’re very expensive and can be difficult to obtain. If you must use fungicides, you’re probably better off hiring a lawn care professional to treat your lawn. 

Note: Applying fungicide to a fungus-infected lawn after fertilizing the lawn with nitrogen fertilizer will severely increase the fungus. Be mindful of fungicide application and lawn fertilization timing. 

Causes of Pythium blight

Too much of anything is a bad thing. Overfertilizing, especially with nitrogen, and overwatering are two major causes of Pythium blight. Pythium blight prefers lush lawns and soil high in nitrogen, and excessive nitrogen promotes lush grass. 

Pythium is transferred through water movement. Outbreaks occur due to poorly draining soil. Overwatering can result in a more severe form of Pythium blight, called Pythium root rot.

Wet, dark environments promote diseases, so watering too late at night creates ideal conditions for pathogens.

The combination of high temperatures and high humidity encourages Pythium blight outbreaks. It is most conducive when foliage remains wet for 12 to 14 hours for several nights consecutively. The disease spreads quickly, especially during humid and hot summers. 

It is most prevalent in cool-season grasses when temperatures are greater than 65 degrees Fahrenheit and daytime temperatures exceed 86 degrees Fahrenheit. Pythium blight typically appears in the morning following a summer thunderstorm that occurred the previous day in the late afternoon or early evening. 

In warm-season grasses, it is most prevalent when night temperatures are above 50 degrees Fahrenheit.

In general, the fungal disease is at its worst when daytime temperatures are greater than 82 degrees Fahrenheit and night temperatures are above 68 to 70 degrees.

Pythium blight prefers compacted soil. The fungus is also attracted to thatch build-up and calcium-deficient grass. Lawn locations without good air circulation also promote the disease.

Once Pythium blight is a problem, mowing proliferates it. The mower transfers infected grass blades to other areas of the lawn, especially when a wet lawn is mowed. It can also be spread by the bottom of shoes.

Pythium blight prevention

Pythium blight close up
Photo Credit: NC State Cooperative Extension

Let’s look at some cultural practices that will go a long way in keeping Pythium blight out of your yard.

  • Water your lawn appropriately: Water the grass in the early morning and avoid overwatering, as excessive humidity can cause Pythium blight. Water in the early morning to give the grass’s foliar surface time to dry. Overnight leaf wetness can lead to lawn diseases, including Pythium blight.
  • Mow grass properly: Cut grass to the correct lengths to keep it strong. Mowing wet grass can proliferate disease, so only mow when the grass is dry. The mower redistributes grass to other areas of the lawn, so clean your lawn mower regularly to remove infected grass stuck to the blades.
  • Improve drainage: Since excessive soil moisture encourages outbreaks, improving soil drainage in low areas and poorly drained areas can help prevent occurrences. 
  • Follow proper fertilization protocols: Fertilize the lawn in the correct amounts and intervals, especially in the summer. Excessive nitrogen fertility can increase the risk of Pythium blight. If using nitrogen-based fertilizers, apply slow-releasing products. 
  • Aerate and dethatch regularly: Aeration loosens the soil and prevents thatch accumulation. Dethatching will remove excess thatch that has already built up. Compact, poorly draining soil and excessive thatch can both contribute to a Pythium blight infection.
  • Choose optimal grass types: Select turfgrass varieties that are more resistant to Pythium blight, including improved Bermudagrass cultivars, Kentucky bluegrass, and tall fescue. 
  • Improve air circulation and decrease shady areas: Prune shrubs and trees to let more sunlight reach the soil and to improve airflow in your yard. 
  • Keep the soil pH slightly acidic by adding sulfur to the soil or with an organic acidic mulch like pine needles or oak leaves. 
  • Ensure soil is not calcium deficient. A soil test will let you know if your lawn has any nutrient deficiencies. If your soil is lacking calcium, you can add lime for lawns, bone meal, wood ash, and other calcium-rich soil amendments to improve it. 

FAQ about Pythium blight on grass

Which grass types are more tolerant of Pythium blight?

The cool-season turfgrasses tall fescue and Kentucky bluegrass are more resistant to Pythium blight. However, “more” is the keyword. Although they are more resistant, they can still become infected. But the good news is that these grass types will incur less injury. 

The warm-season grass Bermudagrass is another grass type that typically experiences less severe damage, especially with the improved cultivars. 

Which grass types are most vulnerable to Pythium blight?

These grass types are most susceptible to injury from outbreaks of Pythium blight:

  • Annual bluegrass
  • Perennial ryegrass
  • Creeping bentgrass
  • Rough bluegrass

What grass types are best for overseeding in extreme cases of Pythium blight?

Affected areas can be overseeded or reseeded with tall fescue or Kentucky bluegrass. 

Most fungicides with Fosetyl-Al are not permitted for use on home lawns. 

Final Thoughts

The most effective way to manage Pythium blight is to reduce your lawn’s risk of developing an infection. A healthy lawn is the best defense. Routinely practice lawn care habits that encourage healthy root growth, like aeration to loosen the soil and improve air circulation, watering at the appropriate times and infrequently, and ensuring appropriate nitrogen fertility.

Are you wondering if it’s Pythium blight or dollar spot? The cottony substance mycelium produced by Pythium spp is similar to what is produced when grass is infected with dollar spot. You want to make sure you know what you’re looking at so that you can treat it properly. Lawn Love can connect you with lawn care professionals in your area to help you identify and get rid of anything that may be ailing your lawn. 

Main Photo Credit: NC State Cooperative Extension

LaShonda Tucker

LaShonda Tucker’s passion for maintaining a healthy lifestyle through organic herbs, fruits, and veggies leads her to research and learn about plants and insects. She loves sharing her knowledge to help others achieve their lawn care and landscaping goals.