Large Patch Disease in Zoysia Lawns: What St. Charles County Homeowners Need to Know

If your zoysia lawn has big tan or straw-colored patches showing up now, or you noticed them at spring green-up last year, the most likely culprit is large patch disease. It is the most common disease of zoysia lawns in Missouri, and it does most of its damage in the fall and spring, not during the peak heat of July. The good news is that a lot of it is preventable, and the window to act opens soon.

What Is Large Patch Disease?

Large patch is a fungus caused by Rhizoctonia solani AG 2-2 LP. It attacks warm-season grasses, mainly zoysia, and it can also hit St. Augustine and centipede grass. Here in St. Charles County, zoysia is the warm-season lawn you see most often, which is why this disease keeps coming up.

The fungus lives in the thatch and in infected stolons and rhizomes. When conditions turn favorable, it starts killing the grass at the base of the leaf blades, near the soil line. That is why the damage shows up in big patches instead of in tiny spots.

What Large Patch Looks Like

The name tells you most of what you need to know. The patches are large, usually a few feet across, and they can merge into irregular shapes that cover big sections of a yard. The grass turns tan or straw-colored, and at the active edge of a patch you can sometimes see a rust-colored or orange border. Turf people call that the “smoke ring.” It is a good sign that the disease is still spreading.

Pull at the edge of a patch and the leaf blades come loose from the base. The roots usually hold. Compare that with grub damage, where the whole turf lifts up like a loose carpet.

When It Shows Up in Missouri

Large patch is a cool-ish weather disease for a warm-season grass. It is most active when soil temperatures are roughly 60 to 80 degrees. In practical terms for St. Charles County, that means two windows:

• Fall, roughly September through November, as soil starts cooling
• Spring, roughly April through June, as the grass comes out of dormancy

A zoysia lawn that looked fine all summer can start showing patches in late September. And in the spring, the classic scene is a lawn that greens up unevenly, with big dead-looking areas that never wake up. That is large patch damage that happened the previous fall or over winter.

Large Patch vs Brown Patch vs Dormancy

This is where homeowners get confused, and I don’t blame them. The names are similar, the patches look similar, and if you have fescue in one part of the yard and zoysia in another, you could see both diseases in the same season.

Sign	Large patch (zoysia)	Brown patch (fescue)	Dormancy (zoysia)
Grass affected	Warm-season: zoysia	Cool-season: tall fescue	Warm-season: zoysia
When	Fall and spring	Summer heat, humid nights	Winter, after first hard frost
Patch size	Large, feet across	Variable, often rings	Whole lawn evens out
Blade damage	Pulls loose at base	Lesions with dark border	Intact, just straw colored
Recovers	Greens up in patches in spring	Recovers when weather cools	Greens up evenly in spring

Dormancy is the big one to rule out first. Zoysia turns straw-colored after the first hard freeze in Missouri, and that is normal. If the whole lawn went tan at the same time and greens up evenly in spring, you’re fine. Large patch makes irregular patches, and they tend to appear while the rest of the lawn is still green.

Our lawn fungus identification guide walks through the other common diseases in the county, and the brown patch article for fescue covers the cool-season version in detail.

What Favors Large Patch

The fungus is already in the soil and thatch of most zoysia lawns. Whether it turns into visible damage depends on conditions. These are the things that push it over the edge:

• Too much nitrogen in the fall. This is the number one trigger. Fast-release fall fertilizer on zoysia grows lush, tender leaves right when the fungus is waking up.
• Heavy thatch. A thick thatch layer holds moisture and gives the fungus a place to live.
• Poor drainage and compaction. Wet feet stress the grass and help the disease spread.
• Mowing too low. Scalping zoysia weakens it and opens up the canopy.
• Wet leaves. Long dew periods and overwatering in fall extend the window of infection.

What You Can Do Without Chemicals

Start here. A lot of large patch damage is driven by how the lawn is managed, and you can change that this fall.

1. Hold off on fall nitrogen for zoysia. This is the opposite of what you do for fescue. Our fall lawn fertilizer guide explains why fescue wants a fall feeding. Zoysia does not. If you have a mixed lawn, that makes things trickier, and it is worth getting local advice before you spread anything.

2. Keep mowing height up. Mow zoysia at the higher end of its range through fall, and keep it there in spring. Taller grass means deeper roots and a healthier canopy.

3. Water less in fall. If the lawn is not under drought stress, skip the irrigation. Early morning watering beats evening watering, because the leaves dry out faster.

4. Aerate if your soil is compacted. Core aeration helps drainage and breaks up thatch over time. For clay-heavy St. Charles County yards, this pays off. See our article on common summer lawn care mistakes for more on why compaction sneaks up on people.

5. Remove heavy leaf cover. A thick mat of wet leaves in October is basically a blanket for the fungus. Keep the zoysia canopy clear in fall.

Fungicide Timing: The Fall Window

If your lawn has had large patch before, or you know the disease is present, a preventive fungicide application in the fall is the standard move. The timing matters more than the product.

For St. Charles County, the target window is usually September to early October, once soil temperatures start dropping toward 70 degrees. A second application about 28 days later is common in higher-pressure situations. Some lawns also get a spring application, but fall prevention does the heavy lifting.

Products for large patch are usually labeled for it on the bag. Look for active ingredients like azoxystrobin, pyraclostrobin, fluxapyroxad, or propiconazole, and check that the label lists large patch (or Rhizoctonia) and zoysia. Follow the label rates exactly. Underapplying just wastes the money and teaches the fungus nothing.

Two honest cautions here. First, a single spray in late October is not a cure-all if the lawn already has heavy damage and thick thatch. The cultural stuff matters too. Second, fungicide timing is weather-dependent, and the window shifts a little every year. If you are not comfortable reading soil temperatures and label instructions, this is a reasonable job to hand to a local pro.

What About Spring?

If you missed the fall window, the spring window is late April into June, when the grass is greening up. Spring applications are more of a salvage play. The better plan is to note it now, set a reminder for September, and treat preventively.

When zoysia is coming out of dormancy and you see big areas that never green up, that is large patch damage. The grass crowns died over winter. Do not rush to overseed zoysia with fescue without thinking it through; you are changing the whole character of the lawn. If the patch is small, the surrounding zoysia stolons will often fill it in over the growing season. If it is big, talk to someone who can look at the yard before you spend money on seed.

Why Fall Is the Time to Decide

Here is the thing about large patch in St. Charles County: the damage you see in spring mostly started the previous fall. The fungus is active in September and October, and that is when the decision gets made. So August is actually the perfect month to check your zoysia, remember what spring looked like, and line up a plan.

If your zoysia lawn had patches last spring, mark September on the calendar. Whether you do a preventive treatment yourself or hire it out, the timing is the part you cannot buy at the store.

Getting Local Help

Zoysia is a small slice of the lawns around here, and not every mowing crew is comfortable with its disease issues. A provider who works with warm-season grass will ask about your thatch, your fall fertilizer, and your mowing height before they ever talk about fungicide. That is the order it should go in.

If you want help finding a local provider who knows zoysia, that is exactly what Midwest Lawn Care does. Tell us what you are dealing with and where you are in the county, and we will point you toward vetted companies who work in your area. No pressure, just a local match. Request lawn care help here and we will take it from there.

Last updated: August 2026. Timing guidance based on Missouri climate and standard turf pathology recommendations for large patch (Rhizoctonia solani AG 2-2 LP). Always read and follow the label on any fungicide you apply.