If you’ve noticed circular patches of thinning, yellowing, or dead grass in your lawn, you may be dealing with one of the common patch diseases that affect cool season turfgrass. The two diseases we most commonly encounter are Summer Patch and Necrotic Ring Spot.
While these diseases are caused by different fungi, they produce very similar symptoms and are often difficult to distinguish without laboratory testing. Fortunately, the management strategies are nearly identical, making diagnosis less important than understanding how to improve lawn health and recovery.
What Are Patch Diseases?
Patch diseases are fungal diseases that attack the roots, crowns, and lower stems of grass plants. Unlike many lawn diseases that primarily affect the leaf blades, patch diseases damage the parts of the plant responsible for absorbing water and nutrients.As a result, affected grass struggles to survive periods of stress, especially during hot, dry weather.
Summer Patch is caused by the fungus Magnaporthiopsis poae, whileNecrotic Ring Spot is caused by the fungus Ophiosphaerella korrae.
Both diseases are most commonly found in Kentucky bluegrass lawns, which make up the majority of residential lawns throughout Minnesota and Wisconsin.
What Does Patch Disease Look Like?
Patch diseases typically appear as:
- Circular or crescent shaped patches of yellow, tan, or brown turf
- Patches ranging from a few inches to several feet in diameter
- Rings or arcs of damaged turf
- “Frog eye” patterns where healthy grass remains in the center of a ring
- Thinning turf that struggles to recover during summer stress
- Areas that repeatedly decline in the same locations year after year
Necrotic Ring Spot is often most noticeable during spring and fall, although symptoms can persist throughout the growing season. Summer Patch symptoms typically become most visible during periods of summer heat and drought stress.
One of the most frustrating aspects of patch diseases is that the fungi may be actively infecting roots long before symptoms become visible on the surface.
Why Do Patch Diseases Occur?
Several factors contribute to the development of patch diseases:
- Environmental Stress – Hot temperatures, drought stress, and inconsistent moisture place additional pressure on already weakened root systems.
- Soil Compaction – Compacted soils restrict root growth and reduce air movement in the soil, creating conditions that favor disease development.
- Shallow Root Systems – Frequent, shallow watering encourages roots to remain near the surface where they are more vulnerable to stress.
- Kentucky Bluegrass Susceptibility – While Kentucky bluegrass creates a beautiful lawn, some varieties are more susceptible to patch diseases than others.
- Lawn Stress – Any factor that weakens turfgrass can increase disease severity, including excessive thatch, poor drainage, nutrient imbalances, and heat stress.
Common Misconceptions About Patch Diseases
- “My lawn was healthy last year, so this can’t be a disease.” – Patch diseases often develop over several years before symptoms become obvious. The fungus may already be present in the soil long before visible damage occurs.
- “The grass is dead.” – Not necessarily. Many affected areas still contain living grass plants with weakened root systems. As environmental conditions improve, portions of the lawn may recover naturally. That said, with patch diseases most of the brown grass will need to be reseeded or resodded.
- “A fungicide will immediately fix the problem.” – Unfortunately, fungicides do not repair damaged roots. Preventative fungicides may help reduce future disease activity, but they cannot reverse existing damage. Recovery requires the grass to rebuild its root system and grow new tissue.
- “The lawn should recover in a few weeks.” – Recovery is often slow. Because these diseases attack the roots, the lawn needs to grow new plants laterally to fill in the dead grass. This takes time.
- “This must be fertilizer burn, grub damage, drought stress, or application damage.” – Patch diseases are commonly mistaken for several other lawn problems. The circular patterns often look similar to drought stress, fertilizer injury, grub damage, pet damage, or even pesticide application issues. In many cases, homeowners first notice symptoms during periods of hot weather and assume the lawn simply needs water. While watering often helps reduce stress and improve appearance, the underlying issue is actually root damage caused by disease. The good news is that patch diseases are not caused by routine lawn care applications. In fact, these diseases are often present in the lawn long before symptoms become visible.
What Can Homeowners Do Once They Have a Patch Disease?
- Water Deeply and Consistently – Proper watering is one of the most important steps in helping reduce the severity of the symptoms. Apply enough water to keep the soil consistently moist while encouraging deeper root growth. Avoid frequent light watering that keeps roots near the surface.
- Raise Mowing Height – Maintain the lawn between 3 and 4 inches tall. Taller grass develops deeper roots and is better able to tolerate stress.
- Reduce Additional Stress – Avoid aggressive dethatching, excessive traffic, or other activities that place additional stress on affected turf during the summer months.
- Overseed Damaged Areas – Overseeding can help fill in thin areas and improve overall turf density. When possible, consider using improved Kentucky bluegrass varieties or perennial ryegrass varieties that show greater disease tolerance.
What Can bioLawn Do Once You Have a Patch Disease?
- Evaluate the Extent of Damage – Our agronomy team can help determine whether patch disease is the likely cause of the symptoms and rule out other common lawn problems.
- Improve Soil Conditions – Core aeration can help relieve compaction, improve root growth, and create a healthier environment for turf recovery.
- Seeding Services – Typically patch diseases kill the grass. bioLawn’s seeding services can be performed to grow new grass in these dead patches.
What Can Homeowners Do to Prevent Patch Diseases?
- Water Properly – Deep, infrequent watering encourages stronger root systems that are better able to withstand disease pressure.
- Aerate Regularly – Annual aeration helps reduce soil compaction and improve root health.
- Maintain Proper Mowing Height – Keeping grass taller reduces stress and promotes deeper rooting.
- Overseed with Improved Varieties – Modern turfgrass varieties often have better disease tolerance than older cultivars.
- Apply Fungicides – Preventative fungicides timed appropriately can help reduce the occurence of patch diseases like necrotic ring spot and summer patch.
What Can bioLawn Do to Help Prevent Patch Diseases?
- Aeration Services – Reducing compaction remains one of the most effective cultural practices for preventing patch disease problems.
- Soil Health Programs – Improving soil structure and biological activity helps create growing conditions that favor healthy turf rather than disease development.
- Preventative Disease Management – For lawns with a history of severe patch disease, preventative fungicide programs may help reduce future outbreaks when applied at the proper timing.
The Bottom Line
Summer Patch and Necrotic Ring Spot are among the most challenging diseases found in cool season lawns because they attack the roots rather than the leaf blades and the symptoms show up long after infection has taken place. While the symptoms can look alarming, recovery is possible with patience and proper lawn care.
The most effective long term strategy focuses on reducing stress, improving root health, relieving soil compaction, and maintaining consistent watering practices. While fungicides may play a role in severe cases, strong cultural practices remain the foundation of managing patch diseases in home lawns. If you’d like to learn more about fungicide options, please visit our webpage on disease treatments.