Most Common Kentucky Bluegrass Diseases

Kentucky Bluegrass

Kentucky bluegrass is commonly found in athletic fields, golf course fairways, tees, and rough. This dense, bold-colored turfgrass is popular even in residential lawns and, in certain regions, is widely planted for pastures. One of its downfalls is its sensitivity to certain diseases. Here, we’ll touch on the most common turfgrass diseases affecting Kentucky bluegrass.

What is Kentucky bluegrass?

Kentucky bluegrass is a perennial, cool-season turfgrass. It spreads by rhizomes and tillers to form a dense sod. This turfgrass will tolerate some shade but prefers full sunlight and well-drained, fertile soil. It recuperates well from foot traffic, which is why it is so popular in athletic fields and golf courses. 

Although newer cultivars have become hardier and more disease-resistant, Kentucky bluegrass is considered a high-maintenance turfgrass. This is due to its moderately high potential for disease. It also requires frequent mowing for optimal health. In home lawns, it’s typically mixed with another species like tall fescue.

Common diseases

Fairy ring

Signs and symptoms:

Fairy ring grows in thatch and soil. It does not infect the turf but indirectly causes symptoms to the turf. It feeds on organic matter found in the soil.

Fairy ring can produce various symptoms depending on the fungal species infecting the soil. Typically, you’ll find dark green or brown circles or semi-circle rings in your turfgrass ranging from one- to 25 feet or more in diameter.

Rings can turn brown and die, or grass within a dark green ring may grow vigorously. You’ll often find mushrooms growing along the perimeter of rings. A white, fungal mat may also show up in the thatch area of the ring. It often produces a musty smell. 

Dark green turfgrass can be found when the fungus releases nitrogen into the soil. This happens while it is decomposing organic matter, giving the turfgrass a boost of an essential nutrient. Brown grass occurs when the fungus forms a layer in the soil, preventing water from reaching the turfgrass’s root system. 


Fairy ring can grow over a wide range of temperatures and can be seen during any season, but particularly spring through early summer. 

It is favored by moist soils, but can be found in various soil moisture conditions. In prolonged dry periods, your turfgrass may go brown.


  • Improve water penetration in the root zone of Kentucky bluegrass by using a wetting agent. Aerate and water the infested area deeply. 
  • Increase fertilization inside the ring to encourage new growth.
  • Remove the infected soil and replace with new soil, and sod or seed. 
  • Rototilling or verticutting the soil are also effective ways to renovate your lawn after a fairy ring infestation. 
  • Fungicides can be used as a preventative treatment.

Use resistant cultivars of Kentucky bluegrass to prevent fairy ring.

Summer patch

Signs and symptoms:

Summer patch can be found in turfgrass in late July and August. It appears as crescent-shaped, dark, wilted spots. The spots can range from just a few inches to a few feet in diameter. 

Outer edges of the circle or patch turn brown, with tufts of healthy turfgrass sometimes appearing in the center. Individual plant roots dry out and decay. 


The soil-borne fungus Magnaporthe poae causes summer patch, infecting roots in warm, wet soils. The fungus can develop in poor quality, compacted soil and excessive thatch. 

Close mowing, frequent irrigation and a high nitrogen content in the spring and summer provide optimal conditions for the summer patch fungus to develop.


  • Raise the mowing height significantly to promote better root development and healthier turfgrass. It also provides nutritional reserves that can help during stressful periods.
  • Water to a depth of three- to four-inches in the summer, allowing the soil to dry out between waterings. 
  • Use a slow-release fertilizer. Heavy nitrogen fertility in the spring and summer can promote summer patch causing fungal development. All or most of Kentucky bluegrass’ annual nitrogen fertilizer should be applied in autumn and early winter.

Renovate the damaged areas with resistant turfgrass cultivars to prevent summer patch. Perennial ryegrass is not affected by summer patch and can be inter-seeded with Kentucky bluegrass.

Powdery mildew

Signs and symptoms:

Powdery mildew shows up as white or gray spore masses on the surface of leaves. The turfgrass wears thin and its leaves turn yellow in severe cases. This is most likely due to it being a weak turf and more vulnerable to stresses such as powdery mildew and other diseases. 


Optimal conditions for powdery mildew to develop include moderate to high humidity and shade as well as areas of poor air circulation.


  • Reduce shade in affected areas.
  • Increase air circulation by removing or transplanting surrounding plants that inhibit it.
  • Use resistant cultivars of Kentucky Bluegrass to prevent powdery mildew.


Signs and symptoms:

Rust can pop up on Kentucky bluegrass in the late summer and early fall, sometimes even in the early spring. It starts as small, yellow flecks on the turf’s leaves, often in shaded areas like at the base of evergreens or next to large structures.

The flecks become rust-colored raised spots called pustules. These pustules rub off on your shoes when walking through infested areas of turfgrass. That’s a sure sign that you’ve got a rust problem. Your turfgrass will turn yellow and thin out.


Poor quality, compacted soil is ideal in order for rust to develop. Moderate to wet foliage is as well. 

Dry soil promotes slow leaf growth, making its speed of recovery poor when damaged from rust.


  • Add Nitrogen fertilizer in mid- to late-summer. This will promote leaf growth, allowing for regular mowing, in a time where Kentucky bluegrass is typically slow-growing. The turfgrass then has the ability to outgrow rust’s slow infection cycle.
  • Water the turfgrass to promote growth.
  • Aerate your soil if it is compacted.
  • Top off your soil with compost to improve its poor quality.
  • Use resistant cultivars of Kentucky Bluegrass to prevent rust.

Leaf spot

Signs and symptoms:

Symptoms of leaf spot can develop in spring, summer, and fall. Initially, small dark spots will appear on the leaves. As the spots grow, brown or purplish-red borders appear around tan centers. Then, the spots grow so big that the entire leaf turns tan or brown.

During the melting out stage, the crowns and roots become so diseased that they turn brown or black and the turfgrass thins out severely.


Prolonged wet conditions lead to leaf spot in Kentucky bluegrass. It typically occurs in cooler temperatures. 

Infrequent mowing at low cutting heights, light and frequent watering, and excessive nitrogen applications in early spring can all spur leaf spot’s development. 


  • Raise the mowing height and mow often to avoid stressing your turfgrass.
  • Avoid applying too much nitrogen.
  • Water less frequently to avoid wet turfgrass.
  • Apply fungicide containing iprodione, chlorothalonil, mancozeb, fludioxonil, azoxystrobin, or penthiopyrad. This should be done in spring or summer, during early stages of leaf spot. It is usually not effective to apply fungicides in the melting out stage.
  • Select more resistant cultivars of Kentucky bluegrass to prevent leaf spot.

Red thread

Signs and symptoms:

Red thread can occur in spring and fall or in summer when rain persists.

In its early stages, symptoms appear as small, blighted areas on Kentucky bluegrass leaves. In the advanced stages, leaves become covered with red thread-like strands. Eventually, the turfgrass will dry out and fade to a bleached color.


Excessive moisture and inadequate nitrogen content, along with poor quality soil, make great conditions for red thread to develop and grow. 


  • Watch for nitrogen and phosphorus deficiencies in your turf.
  • Aerate and water properly to promote growth. Ensure that Kentucky bluegrass doesn’t remain wet for too long.
  • Use resistant cultivars of Kentucky bluegrass to prevent red thread.


Q: Is Kentucky bluegrass prone to disease?

A: When stressed by extreme temperatures, lack of water, poor soils and maintenance practices, Kentucky bluegrass can be prone to disease.

Q: Will lawn fungus go away on its own?

A: No. Depending on the fungus, treatment may vary. However, practicing good turf management helps prevent fungal diseases.

Use this guide to help you in determining which turfgrass disease has taken over your Kentucky Bluegrass. Don’t forget that your Lawn Love lawn care professionals are just a call away and waiting to help with any of your lawn care needs.

Main photo credit: Wikimedia | CC-BY-3.0-US

Madeline Hoppe

Born and raised in Tampa, FL, Madeline Hoppe is a customer service expert with a deep respect for the written word. In her down time, she enjoys low-key nights watching movies with her family or heading to one of Tampa Bay's local beaches on a summery day.