The first step in dealing with a lawn disease is to diagnose the problem correctly. Close observation is essential. Brown patches that appear in a lawn may be caused by environmental factors such as drought injury, needlessly low mowing height, compacted soil, improper watering, or even dog urine. First, examine the affected turf to rule out insect problems such as sod webworms, chinch bugs or grubs.
To determine if the problem is a disease, try to pull up the damaged grass plants. Unlike insect-damaged lawns whose roots are destroyed, grass that has died from disease tends to remain attached to the soil. Exceptions include root and crown rots caused by Fusarium blight or Helminthosporium leaf spot diseases, which soften the grass so that it pulls up easily.
Each Grass Has Its Own Fungal Disease
While there is a little overlap, in most cases, each fungal disease that can hit a lawn is connected to a specific species of lawn grass, such as Tall fescue as opposed to Kentucky bluegrass. If you have a mixture of grasses in your lawn, then this fact is not important, but if your lawn is composed of just one species of grass, then this list can be helpful in your determinilng which disease is presenting a problem. The following northern grasses tend to be vulnerable just to those fungal diseases listed after each name:
Fine fescue: Dollar spot, Fusarium patch, leaf spot, powdery mildew, red thread.
Tall fescue: Brown patch, leaf spot.
Perennial ryegrass: Brown patch, dollar spot, rust, leaf spot, red thread, typhula blight.
Kentucky bluegrass: Dollar spot, fusarium blight and patch, rust, leaf spot, powdery mildew, red thread, stripe smut.
Temperatures Affect Fungal Disease
Because individual fungal diseases require certain specific environmental conditions to thrive, identification is easier. You can rule out those that do not prefer cooler or warmer temperatures.
| Temperature vs. Disease | ||
|---|---|---|
| Cool | Moderate | Warm Temperatures |
| Temperatures | Temperatures | |
| (40° to 75°F) | (60° to 80°F) | (70° to 100°F) |
| Leaf spot | Dollar spot | Brown patch |
| Melting out | Necrotic ring spot | Fusarium blight |
| Fusarium patch | Red thread | Summer patch |
| Pink snow mold | Leaf smut | Pythium blight |
| Grey snow mold | Rust | |
Stress Encourages Fungal Disease
It is important to remember that lawn diseases are often caused by an underlying environmental condition that is stressing the grass plants. A healthy lawn can defend itself against the insect pests and disease organisms that normally inhabit the lawn’s ecosystem. But when stressed, grass plants are less able to resist the disease organisms whose growth is encouraged by abnormal conditions.
Aside from extreme weather conditions, too much or too little water or fertilizer, improper mowing height, soil compaction, uneven grading, accumulated thatch, overuse of lawn pesticides, or any combination of these may stress grass plants.