Summer Lawn Diseases

All living things are susceptible to diseases, and lawns are no different.

This spring, the cool, wet weather that affected most of the U.S. caused a greater than normal increase in lawns afflicted with gray snow mold, a lawn disease that thrives when those type of conditions are present.

The majority of lawn diseases, however, occur in the summer months. Some are basically harmless, while others can wreck an entire lawn or golf course. Among groundskeepers, they are simply known as turfgrass diseases.

A turfgrass disease is any infectious disease caused by microscopic fungi that invade the grass. Fungi need a “host” in order to survive – whether it is dead or alive – and in the case of infectious turfgrass disease, the host plant in this case is the grass. The fungi also need favorable conditions, such as moisture level and temperature, in which to thrive.

Let’s take a look at some of the more common lawn diseases that occur in the summer months, when and why they are likely to occur, as well as how to recognize them. And finally, we’ll give you some tips on what you should do to your lawn to prepare it against turfgrass disease invasion.

Leaf Spot

Leaf spot, or helminthosporium turf diseases, can become a problem in the summer because the brown, dry grass it leaves behind is often confused with drought conditions. It is a warm weather disease that mainly occurs in susceptible varieties of Kentucky Bluegrass and Bermudagrass. It thrives in warm, humid weather.

Look for leaf lesions, that are purplish-brown spots on the surface of the blade. Eventually, the lesions will run together and the blade will turn entirely brown and die – it may appear that the plant is rotting away. Watering the area will only compound the problem.

Brown Patch

Brown patch will affect all grass types in temperate climates. The disease is favored by excess thatch, high temperatures (75° F – 95° F) high humidity (99 – 100%), excess moisture, and lush growth caused by excess nitrogen. Closely mowed turf and particularly the bentgrasses of golf courses are especially susceptible and easily damaged by brown patch. In addition, it can be extremely destructive to St. Augustinegrass from fall to early spring.

If brown patch is present in the lawn, it will first appear as brown patches in circular patterns a few inches across. As the disease spreads, the affected areas will grow to several feet in diameter. The center of the spots may recover, leaving a ring of infected grass.

Fairy Ring

Fairy ring gets its name from many centuries ago, when it was believed that they appeared as the work of fairies or gremlins. Fairy ring occurs in all grass types as a result of organic matter, such as old tree stumps or wood, that is decaying in the soil below. In lawns where fairy ring resurfaces every year, major reconstruction must be done to eliminate the problem.

Fairy ring often starts with the appearance of mushrooms, in a ring or circle. The fungi growing from this decaying material release nitrogen, which creates a dark green circle in the lawn.

Fusarium Blight

Fusarium blight is mainly a problem in lawns throughout the Midwest and mid-Atlantic regions, affecting Kentucky bluegrass, red fescue and sometimes centipedegrass and bermudagrass. Lawns with compacted soil or heavy thatch that receive long hours of intense light are susceptible to fusarium blight. Hot, dry weather that follows extended periods of rain creates favorable conditions for this infectious turfgrass disease.

Infected turf will appear pale green in patches, which fades to a wilted purplish green and eventually to a dull, straw color. It typically stays green in the center, surrounded by a ring of dead, brown-colored grass. The disease will spread when temperatures are near 90° F during the day and 70° F at night.


A turfgrass disease that resembles its name. Rust affects all grass types and appears as orange flecks on the grass blades – basically, it looks like rust. Typically, it goes away without much harm. However, prolonged periods of rust can result in eventual loss of turf. During hot, dry weather rust will cause the blades to fold inward as if suffering from drought. Eventually, the infected blade shrivels and dies.

Red Thread

Can occur at any time of year, but is most prevalent in late Summer and Fall and is only likely to appear on water-logged lawns, and particularly those that are deficient in nitrogen. Bright pink fungal threads grow among the grass creating a pinkish hue. Later, the grass blades redden and then look bleached. It rarely forms patches larger than 12 inches across.

Is it a disease or not?

Keep in mind that there are a handful of problems that may cause brown or dying grass in a lawn. Insects, dog urine, fertilizer burn, gas spills, and drought could be the cause. For proper identification of lawn problems, take a specimen to a professional garden center, lawn care specialist, or contact your local County Extension Service. Collect two samples from your lawn, one from the area of the disease, and another from a spot nearby where the grass appears to be growing healthy.

Prevention Tips

The following are some tips to keep your lawn healthy and disease-free.

  1. Resistance or susceptibility to diseases is a genetic characteristic in plants. Plant a grass type and variety that is adapted to your climate and resistant to diseases known to occur in your area.
  2. Timing fertilizer applications can be critical in avoiding lawn disease. To keep your lawn growing healthy and vigorously, follow a fertilization schedule that fits your grass type and growth cycle. Lawns under many shade trees need to be fed at least three times a year to replace the nutrients taken by the tree.
  3. Lawns that are watered deeply and less frequently usually have fewer disease problems. Water during the early part of the day so grass blades will not remain wet for extended periods. Warm wet grass in poorly drained soil promotes many lawn diseases by activating fungus spores. Conversely, some trees are moisture robbers and grass growing in their root zones may need additional watering.
  4. Avoid mowing when the grass is wet, as this may help spread any fungi on the turf. Mow at the proper height for your grass type. As a general rule, the chances for disease damage are increased as the height of mowing is decreased. Mowing with a sharp mower blade on a more frequent schedule will produce a healthier lawn.
  5. Check thatch and remove if necessary in Fall or Spring. A heavy layer of thatch provides a spot for disease spores to thrive and over-winter and can slow entry of water into the soil. Thatch restricts the movement of air and fertilizers thereby weakening the lawn and making it more disease prone. It can also absorb large amounts of fungicides when they are applied, helping to protect the disease from the fungicide.
  6. Correct compacted soil by aerating with a plugging tool that removes cores of soil. Compacted soil prevents water from properly reaching the roots of the grass where it is needed the most.
  7. Thin out branches of trees and surrounding plants to allow more sun light to reach the grass and to improve air circulation. This will facilitate the evaporation of water from leaf surfaces and the drying out of disease spores. It will also reduce the competition for nutrients from other plant roots in the soil.
  8. When a disease does infest your lawn or as a preventative treatment, you may need to use a control, such as  lawn fungus control or lawn disease control. Gardeners recommend that you attempt to properly identify the fungus before applying any fungicide to your lawn.