Necrotic Ring Spot in Missouri: The Root Disease Killing Kentucky Bluegrass Lawns

If your Kentucky bluegrass lawn has dead, straw-colored rings or arcs that keep getting bigger every summer, especially in late July and August, you are probably looking at necrotic ring spot. It is a root disease caused by the fungus Ophiosphaerella korrae, and it is one of the most destructive problems bluegrass lawns face in Missouri. It attacks the roots and crowns below the soil, which is why it is so hard to fix and so slow to recover.

What Is Necrotic Ring Spot?

Necrotic ring spot is a fungal disease that infects the roots, rhizomes, and crowns of Kentucky bluegrass. The fungus lives in the soil and on infected plant debris, and it moves from plant to plant by growing along the surface of the roots.

University of Missouri Extension lists necrotic ring spot among the common turf diseases in the state, and it is the most destructive disease of Kentucky bluegrass in several states. The fungus is active when the soil is cool and damp, which in Missouri means spring and fall. But here is the twist: you usually do not see the damage until summer.

The fungus colonizes the roots in cool weather, and then the damaged root system cannot take up enough water when the heat and drought stress hit. That is why the dead rings show up in July and August, even though the infection started months earlier.

How to Identify Necrotic Ring Spot

Necrotic ring spot has a distinctive look once you know what to check for.

Common signs:

• Circular patches or arcs of dead, straw-colored grass, from a few inches to several feet across
• A “frog-eye” or doughnut shape: dead ring with healthy grass regrowing in the middle
• Patches that appear in late spring or late summer and keep expanding in successive years
• Grass that looks drought-stressed even when you are watering
• Roots that are dark brown or black and rotting when you dig them up (the telltale sign)

The root check is the difference-maker. Cut a small square of sod from the edge of a dead patch, shake off the dirt, and look at the roots. Healthy bluegrass roots are white and firm. With necrotic ring spot, they are dark, mushy, and rotting. A hand lens may show dark fungal threads running along the root surface.

Necrotic Ring Spot vs Summer Patch vs Fairy Ring

Necrotic ring spot gets confused with a few other lawn problems, and the fixes are different enough that it is worth telling them apart.

Problem	What it looks like	When it shows	Key difference
Necrotic ring spot	Large frog-eye rings, dead arcs	Late spring, late summer	Roots and crowns black and rotting
Summer patch	Smaller frog-eye patches, more numerous	Midsummer, after heat	Very similar; lab test confirms
Fairy ring	Mushroom circles, dark green or dead rings	After rain, summer	Mushrooms present; roots mostly fine
Drought stress	Large irregular brown areas	Hot, dry spells	Roots white and healthy; recovers with water

Summer patch is the hardest to tell apart. The two diseases look almost identical, and even the root symptoms are similar. The practical difference: summer patch patches tend to be smaller and show up in the middle of summer, while necrotic ring spot rings can be several feet across and appear in late spring and again in late summer. If you need certainty, a plant diagnostic lab can confirm it. Our lawn fungus identification guide walks through the common Missouri diseases side by side.

Why It Shows Up in Missouri

Necrotic ring spot is mostly a Kentucky bluegrass disease, and that matters in St. Charles County because of what kind of grass people plant.

Most lawns in the county are tall fescue, which is resistant to necrotic ring spot. But bluegrass shows up in a few places: older neighborhoods where bluegrass was the standard seed mix, new subdivisions that were installed with bluegrass sod, and premium lawns where homeowners wanted that dense, dark green look.

A few local conditions make the disease worse:

• Compacted clay soil. Missouri clay packs down hard, especially on new construction lots. Necrotic ring spot is most severe in compacted soil because the roots are already stressed.
• Sod-installed lawns. The disease is most common on lawns that are two to four years old and were established from sod. That describes a lot of St. Charles County subdivisions.
• Excess watering and nitrogen. Overwatering and heavy spring nitrogen applications both make necrotic ring spot worse. Both are common habits here.
• Bluegrass at the edge of its range. St. Charles County sits in the turf transition zone, and bluegrass is already stressed by Missouri summers. A stressed lawn is a vulnerable lawn.

If you are not sure what kind of grass you have, our Kentucky bluegrass guide for St. Charles County explains how to tell it apart from tall fescue and whether bluegrass is the right choice for your yard at all.

Will It Kill Your Lawn?

It can, and that is what makes it different from diseases like red thread or dollar spot.

Necrotic ring spot attacks the roots and crowns, which are the parts of the grass plant that keep it alive. When the roots die, the grass cannot take up water or nutrients. In a hot Missouri August, that combination is usually fatal for the affected patches. The dead rings can expand year after year, and because the roots are gone, recovery is slow even after you fix the problem.

The good news: the disease does not spread from blade to blade by mowing, and it does not move across the yard the way some leaf diseases do. It spreads mainly through infected sod and contaminated equipment, and the fungus grows outward slowly from where it started. Left completely untreated, some lawns actually improve over several years as other soil organisms build up and crowd the fungus out. But that is a slow, unreliable path, and most homeowners want their lawn back sooner than that.

How to Fix It

You cannot cure necrotic ring spot overnight, but you can manage it well enough to stop the spread and bring the lawn back. These steps work for most Missouri home lawns.

1. Aerate every fall. Core aeration punches holes through compacted clay, relieves thatch, and helps the roots recover. It is the single most important cultural fix for this disease. For bluegrass in Missouri, fall aeration is the window — our core vs spike aeration guide explains which method to use.
2. Stop overwatering. Deep, infrequent watering is the rule — about an inch a week, in one or two soakings, early in the morning. During the worst of the summer heat, an infected lawn may need light extra waterings to survive, but frequent shallow sprinkling all season makes the disease worse.
3. Ease off the spring nitrogen. Heavy quick-release nitrogen in spring feeds the fungus and stresses the roots. Most of the bluegrass fertility should go in the fall. If you fertilize in spring, use a slow-release product and keep the rate modest.
4. Mow higher. Keep bluegrass at 2.5 to 3 inches in summer. Taller grass grows deeper roots and tolerates the root damage better. Our Kentucky bluegrass mowing guide has the full height schedule.
5. Overseed the damaged areas. Perennial ryegrass is not affected by necrotic ring spot, and it matches bluegrass closely enough that most people cannot tell the difference. Overseeding dead patches with ryegrass, or with a resistant bluegrass blend, fills the lawn back in while you fix the underlying issues.
6. Consider a preventive fungicide in spring. Products with DMI active ingredients (like propiconazole or myclobutanil) can suppress the disease when applied preventatively in spring as soil temperatures reach the mid-50s to mid-60s. Timing is critical, and most effective products are professional-grade, which is why this is usually a job for a licensed applicator.

What NOT to Do

A few common reactions make necrotic ring spot worse.

• Do not keep watering shallow and often. It feels like the right response to dying grass, but it keeps the soil wet and promotes the fungus. Water deep and rarely instead.
• Do not dump nitrogen on it in summer. Fertilizing a sick lawn in July to “green it up” feeds the disease and burns the damaged roots.
• Do not bag the clippings and worry about mower spread. The fungus lives in the roots, not the blades. Mowing is not how it spreads. Leaving the clippings on a recovering lawn actually recycles nitrogen.
• Do not rip out the sod and replace it with the same bluegrass. If you do not fix the compaction, drainage, and watering habits, the new sod gets infected the same way. That is why the disease keeps coming back in the same spots year after year.

Missouri Seasonal Calendar for Necrotic Ring Spot

Necrotic ring spot follows a predictable yearly cycle in Missouri. Knowing the cycle tells you when to act.

Season	What is happening	What to do
Spring (Mar-May)	Fungus colonizes roots in cool, damp soil	Hold off on heavy nitrogen; apply preventive fungicide as soil hits 55-65°F if history is severe
Early summer (Jun)	Root system weakens, lawn looks stressed	Water deep and rarely; keep mower height up
Midsummer (Jul-Aug)	Dead rings and frog-eyes appear	Identify and confirm; light extra watering only to keep plants alive; do not fertilize
Fall (Sep-Oct)	Root recovery window opens	Core aerate; overseed with ryegrass or resistant bluegrass; apply fall fertility
Winter	Fungus survives in dead roots and thatch	Plan next year’s prevention; avoid fall overwatering

When to Call a Professional

Call someone in if the rings are expanding year after year, if you are not sure whether you are looking at necrotic ring spot, summer patch, or something else, or if the lawn is already thin enough that you are considering renovation.

This is one of the few lawn diseases where a professional can genuinely save you money. The fungicides that work are mostly professional-grade, the timing window is narrow, and a misdiagnosis wastes a full season. A local pro can confirm the disease, test the soil, aerate on schedule, and set up a watering and fertility plan that fits your yard.

The providers on our network know St. Charles County lawns: the bluegrass neighborhoods, the compacted clay, the summer heat that makes root diseases show their hand. They can look at your lawn and tell you straight whether you need a treatment plan or a fall renovation.

Get connected with a local lawn care provider who can look at your lawn and put together a plan.

And if you are not sure what your lawn needs month by month, grab our free St. Charles County seasonal lawn care checklist — it walks through mowing, watering, aeration, and overseeding timing for every month of the Missouri year.

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Jerry Bennes is the founder of Midwest Lawn Care, a referral platform connecting St. Charles County homeowners with trusted local lawn care providers. Last updated: August 6, 2026. Sources: University of Missouri Extension (IPM1029), Penn State Turfgrass Pest Diagnostic Lab, Colorado State University Extension.