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How to Treat Pythium Blight

Pythium blight or cottony blight, is a common turfgrass disease in cool season grasses. Although it can also be found in bermuda grass (a warm season grass), the disease is often less severe. 

Pythium blight impacts the quality, growth and development of turfgrass. Pythium was not a problem for turfgrass until 1954. More cases indicated that multiple grass types develop the disease over time. Golf courses, athletic fields and lawns experience this problem even today. 

Pythium spp. is a common mold found in the soil that causes many different turfgrass diseases. Pythium blight, on the other hand, occurs when the pathogen attacks the turfgrass tissue. However, when the roots and crowns are affected, the disease is called Pythium root rot. 

This article will focus on one species of Pythium in particular—Pythium aphanidermatum (or Pythium blight)—the most commonly spotted species in turfgrass.

What does Pythium blight look like?

The following lists a few key characteristics of Pythium blight.

Dark Patches

The turfgrass will develop large dark patches on the surface when Pythium blight is present. It’s best to start working on disease control at this initial stage. 

The grass illness can spread quickly and damage large surface areas, and for this reason, treating it before the disease expands saves you time and money spent on disease control applications as well as keeping more of your lawn unaffected by the disease.

Greasy Gray-Colored Layers

The second most common symptom of Pythium blight is a greasy, gray layer coating the grass blades. It can follow a long streak-like pattern and spread across surfaces in a short time. 

Light Brown Surface Formation

Once the disease progresses, it becomes less visible and the grass can develop light brown patches that may have a reddish color around the border of affected areas.

What are the symptoms of Pythium blight?

Below are some symptoms that can help you know if your lawn is facing growth issues due to Pythium blight.

Cotton-Like Growth

Most homeowners dealing with Pythium blight see cotton-like surfaces forming on the turfgrass surface. You will see fluffy white mycelium because of the infected grass foliage. This growth can increase with high humidity levels.

Wet Surfaces

The Pythium blight grass diseases increase with wet or damp surfaces. The mycelium produced by Pythium spp. and Sclerotinia homoeocarpa (dollar spot) are somewhat similar, which leads us to the next symptom—multiple species infestation.

Multiple Species Infestation

Many homeowners often report seeing multiple kinds of fungal build-up on turfgrass when dealing with Pythium blight. However, the aphanidermatum is the most destructive for turfgrass. 

Impact on Other Nearby Lawns

Finding the best lawn fungicide for Pythium blight is imperative because, as previously mentioned, the disease can spread fast and contaminate other landscapes nearby.

Summer Outbreaks

Homeowners also see Pythium blight forming on leaves of both cool season grass and bermuda grass during the summertime. It can also appear during humid or damp weather conditions that may occur any time of the year

Patchy Surfaces

Pythium blight creates patches on the turfgrass surface, impacting its appearance. These patches can increase at high rates and cover large lawn areas rapidly. They start out as small, circular sunken patches with black or purple spots that can expand into larger irregular areas. Infected turfgrass can become brown or tan after drying completely.

Oily Feel

The easiest way to identify Pythium blight disease is to take a small part of the turfgrass and rub it between your fingers. The surface should feel a little oily if Pythium blight is present. Be sure to wash your hands afterwards.

What causes Pythium blight?

Pythium blight is a grass fungus that can occur through various Pythium diseases. Some common examples of these include:

  • Pythium graminicola
  • Pythium Aphanidermaturm
  • Pythium Ultimum
  • Pythium Vanterpoolii

The infection is a persistent issue for season turfgrass types in colder and areas with humid weather.

According to Cardinal Lawns, “Pythium blight is easily spread by moving water and mechanical equipment. It can travel on the blades of a mower and on the soles of shoes. The fungus thrives in water-logged, poorly drained grass. It also loves over-fertilized, alkaline (pH over 7) soil with high levels of nitrogen and low levels of calcium.”

When does Pythium blight appear in lawns?

Pythium blight appears during warmer seasons ranging between late spring and early summer. The disease needs moderate weather surroundings to prevail properly. 

As previously mentioned, it typically appears during extended periods of humid or damp weather conditions. Pythium outbreaks are more common when temperatures reach or exceed 90 degrees Fahrenheit, however, outbreaks can still occur when temperatures are at 68 degrees Fahrenheit or higher.

What types of grass does Pythium blight affect?

Several cool season grass types, including the perennial ryegrass, creeping bentgrass and annual bluegrass, have high chances of developing Pythium blight issues. 

Grass types like Kentucky bluegrass and tall fescue have a higher resistance against Pythium blight. They can still become infected, but damage is much less severe. The same can be said about bermuda grass, a warm season grass also used as putting greens.

 Pictured above from left to right: Perennial ryegrass, Kentucky bluegrass, annual bluegrass and tall fescue.

How do I treat Pythium blight?

Lawn disease control can be tricky for any disease outbreak, and for this reason, homeowners should identify cultural and chemical treatments before moving forward. Take a look at our lawn disease treatment strategies for Pythium blight below.

Cultural Treatment

Following the best irrigation practices is the easiest way to steer clear from Pythium blight. Homeowners should water their grass early during the day so that the sunlight has plenty of time to dry the lawn. 

Damp, dark locations are optimal environments for disease outbreaks. When the lawn is watered at the end of the day, it creates this ideal environment for disease outbreaks and growth. 

Most lawns typically require 1 inch of water per week including rainfall. You can conduct a simple and inexpensive irrigation audit to make sure the various areas of your lawn are receiving appropriate amounts of water.

In addition to proper irrigation management, gathered water also causes damage to the lawn. Thus, it is imperative to provide good drainage options. Some simple options include regularly aerating or leveling the lawn.

A Lot Of Standing Water In Green Grass

Thatch build-up on the surface can promote Pythium blight formation. Thus, it’s imperative to dethatch the turfgrass regularly. 

Providing your lawn with essential nutrients is vital to maintaining a healthy, strong lawn. Sod Solutions recommends the Lawnifi® Fertilization Program. Lawnifi gives your lawn what it needs in the form of nano-sized nutrients, exactly when it needs it in the form of easy-to-apply monthly applications from March through November.

It’s important to note that even with the use of an improved fertilizer program, the timely application of fungicide is still necessary for overall lawn health. If you fertilize your lawn with nitrogen before applying a fungicide and presently have a lawn fungus, that fungus will expand exponentially, which can be disastrous for your lawn.

Chemical Treatment

Some chemical lawn fungus treatments that can come in handy for dealing with Pythium blight include those with the active ingredients Azoxystrobin (Heritage) or Fluoxastrobin (Fame).

If problems persist, consider rotating fungicides with different active ingredients as the Pythium blight may have built up an immunity to one active ingredient over time. Be sure to read product labels thoroughly before application.

How do I prevent Pythium blight?

Preventing Pythium blight from the get-go is the easiest way to avoid all these complicated control methods. We suggest the following preventive measures for it.

  • Maintain a strong, healthy lawn with our care guides,
  • Choose varieties of turfgrass with higher Pythium blight resistance,
  • Reduce shade and humidity levels by pruning back trees and shrubs,
  • Increase air circulation by pruning back trees and shrubs,
  • And apply a disease control product preventively to keep Pythium blight from infecting your lawn,
  • Use a slow release fertilizer—especially during the summers,
  • Aerate and dethatch regularly,
  • And water grass in the early mornings.


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