ascochyta fungus on grass

Ascochyta Leaf Blight: Why Your Lawn is Yellow

Ascochyta leaf blight is a lawn fungus that is active when temperatures turn hot after long periods of wet, cool weather. Because of the rain followed by days of increased temperatures, Ascochyta is prevalent this spring season.

Identifying Ascochyta

Lawns that develop ascochyta turn yellow and often have healthy green grass dispersed within the patches of yellow grass⁠. Sometimes the grass looks like straw. The disease comes on quickly⁠—sometimes it appears as though the lawn turned yellow overnight. The fungus is present throughout the year in most lawns, but the spores flare-up in favorable conditions.

Ascochyta can live on dead grass tips in the thatch layer of the lawn, and spores can spread when they are disturbed by splashing water or mowing.

The disease causes grass blades to turn yellow from the tip to about halfway down the grass blade. Sometimes the grass blades develop a dark line separating the green half of the blade from the yellow half. However, the crowns and roots of the grass are seldom affected, so the grass rarely dies because of the disease.

The fungus that causes ascochyta to show on grass usually enters through tips of the grass that are damaged by dull lawnmower blades. Dull mower blades tear and cut grass unevenly. Instead of cutting the grass blades smoothly, dull mower blades pull at the grass and tear the blades, making the lawn more susceptible to ascochyta.

Treating Ascochyta

It takes two to three weeks for the disease to run its course and for the grass to turn green again. To help the process, do the following:

  • Avoid mowing in really wet conditions.
  • Clean your lawnmower deck frequently to avoid spreading the disease from one part of your lawn to another.
  • When mowing, keep the grass blades about three inches tall.
  • Avoid mowing too often and only cut 1/3 of an inch at a time.
  • Make sure to water well —about 1 to 1.5 inches when temperatures approach 80 degrees.
  • Aerate your lawn yearly to reduce the thatch layer (where the fungus lives on dead grass tips)

Preventing Ascochyta

Ascochyta can spread when spores hiding in the thatch layer of the lawn are disturbed by rain and routine lawn maintenance. To prevent your lawn from developing ascochyta, try the following:

  • Keep your lawn’s thatch layer small (less than half an inch thick is best) to prevent spores from hiding there
  • Cultivate a healthy, thick lawn to discourage fungal growth
  • Fertilize your lawn at even intervals
  • Follow proper watering guidelines
  • Aerate your lawn yearly to allow nutrients and water to penetrate into the soil
  • Sharpen your lawnmower blades periodically to prevent damage to the tips of the grass blades

Following these tips will help decrease the chance that your lawn will suffer from ascochyta. However, even after following these prevention steps, your lawn may still become infected with ascochyta. If it does, following the treatment guidelines above will help control the infection and slow the spread of the fungus.


Stewarts’ specialists can help identify lawn problems like ascochyta. Call or text 801-226-2261 to talk to a specialist and get a free quote.