pythium blight lawn

How to Treat Pythium Blight (Fungus Control)

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Written By: Mark Marino

a Massachusetts Core Applicator License holder and owner/operator of Lawn Phix,

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Sometimes referred to as cottony blight, pythium root rot, or grease spot, pythium blight is a fungal lawn disease that affects various types of turf grass all over the country, but especially warm-season grasses in the southern U.S. Wet, humid weather can cause pythium diseases. Grass blades affected by Pythium blight will appear wet or oily. With disease development, cottony white tufts will begin to appear on infected grass. If you’re concerned about pythium blight, use the following information to help identify and control pythium outbreaks. 

What Is Pythium Blight?

Pythium blight, or cottony blight, is most common during hot, humid weather. It primarily affects poorly drained grass of golf greens, athletic fields, and residential lawns. Once pythium blight appears, it can spread quickly, especially during consecutive days of humid weather. Prolonged leaf wetness of grass and excessive soil moisture favor disease development. Most fungus thrives in damp conditions. Pythium blight is known to affect perennial ryegrass and bentgrass

What Causes Pythium Blight in Lawns?

There are various contributing factors to pythium blight. First, the pythium species must be present in the turf grass roots zones or even thatch. The most common pythium species associated with common disease development traits and pythium root dysfunction is Pythium aphanidermatum, but other species can cause the same disease development and foliar blight. 

Pythium blight can break out if the turf grass has remained wet for more than 12 hours, daytime temperatures exceed 82 degrees Fahrenheit, and night temperatures exceed 68 degrees Fahrenheit. With continued hot, humid weather, foliar symptoms of pythium blight are likely to worsen and become quite deadly to turf grass. Soils with high nitrogen content can also be more conducive to pythium fungus outbreaks.

Pythium blight often occurs on highly maintained turf grasses like putting greens, golf courses, and home lawns. It often follows water tracks. Unfortunately, the pythium species can be easily spread via mechanical equipment like lawnmowers and even shoes. 

Pythium Blight Prevention

Gardeners can take care to prevent pythium blight by reducing conditions that support pythium blight outbreaks. For instance, remove thatch where the fungal spores may be present in order to improve air circulation for grass and soil. Since pythium blight loves over-fertilized soils, avoid applying too much fertilizer, especially during the peak summer season when the temperatures are hot and humid. Be sure to correct any subsurface drainage problems in your lawn, and pay close attention to environmental conditions; when it’s hot and humid, water your grass deeply and sporadically in the early morning, giving the grass a chance to dry out before nighttime. 

Pythium Blight Control and Treatment

If you identify pythium blight, take steps to treat it right away. In fact, any turfgrass diseases should be identified quickly, so you know what you’re dealing with. The sooner you treat pythium blight, the more likely you can save affected grass from root rot. Pythium blight is a fast spreader. Without proper pythium blight control measures, the disease can spread to healthy areas of your lawn in mere days.

Try to treat pythium blight in its earliest stage–the greasy or grease spot stage. Once the lawn turns brown, it may be harder to save from the mycelium produced by the disease. To control pythium blight, treat affected areas with a fungicide that includes the active ingredient Mefenoxam, such as Subdue Maxx . Chemical treatments are also available in generic form. 

Also, avoid using mowing equipment on lawns that are affected by pythium blight. If you must, be sure to wipe its blades thoroughly, so you don’t spread the pythium species to other parts of your landscape. Be sure to avoid walking across areas affected by pythium blight. Close disease sections of putting greens or rope off areas of your lawn after you treat them for a time to ensure that the disease isn’t spread.

Pythium Blight FAQs

How do you treat pythium blight?

The best way to treat pythium blight is with the use of a fungicide formulated for treating this fungal lawn disease. 

How do you identify pythium blight?

Pythium blight can be identified in early stages by its grease spots and cottony mycelium. Later, the disease causes grass to turn brown and die.

What causes pythium fungus?

Pythium fungus refers to several–about 20–types of pythium species that cause pythium root rot and foliar symptoms. 

What type of disease is pythium?

Pythium blight is a lawn disease. Pythium is a fungus that can cause root rot in turfgrasses such as perennial ryegrass, creeping bentgrass, Kentucky bluegrass, and other warm and cool season grasses. 

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Author's Note: this piece has been updated for accuracy since its first publication on
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Author: Mark Marino
My name is Mark Marino, and I am the founder, owner, and operator of Lawn Phix. With a passion for lawn care and turf nutrition for over a decade, I've dedicated countless hours to correcting soil and perfecting lawns. Today, my expertise, backed by formal courses at UMass Extension Pesticide Education, allows me to offer top-tier lawn care services and advice. I am a fully licensed and insured lawn care applicator in Massachusetts, specializing in comprehensive turf nutrition, weed control, and lawn pest control. My license number is AL-0053865. Contact me at [email protected] or +1 (508) 500-8402.

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