Large patch

What is large patch?

Large patch is caused by a strain of the fungus Rhizoctonia solani. Missouri Extension identifies it as the AG 2-2 LP group. It is a close relative of the fungus that causes brown patch in cool-season lawns, but this strain attacks warm-season grasses. NC State notes the disease used to be called brown patch too.

NC State ranks centipedegrass and seashore paspalum as most likely to get it, then zoysia, then St. Augustine. Bermudagrass is rarely hurt much and bounces back fast. Purdue says there is little or no known resistance among common zoysia varieties.

The fungus attacks at the base of the leaf sheaths while the grass is going into or coming out of winter dormancy. It does not kill the roots or stolons, so new shoots eventually fill back in. But UC IPM notes zoysia can take many weeks to recover, and Missouri Extension says weeds often move into the thinned areas.

Symptoms and signs

• Round, slightly matted, straw-yellow patches that show up in fall as the grass slows down
• Patch edges turn bright orange or tan-orange while the disease is active
• Patches can grow from about a foot across to more than 20 feet
• Reddish-brown to black rot at the base of the leaf sheaths; leaves above it turn yellow-orange
• In spring, the same spots stay brown and sunken and green up much later than the rest of the lawn
• Weeds often move into the thinned patches

Large patch identification: what to look for

• Symptoms: In fall, round, matted, straw-yellow patches. Purdue says they start up to 12 inches across. Missouri Extension says active patches can top 20 feet, with bright orange edges.
• Symptoms: Reddish-brown to black rot at the base of the leaf sheaths. The blade above the rot turns yellow-orange. Leaves themselves usually have no spots.
• Signs: Missouri Extension says cinnamon-brown fungal threads are easiest to see in the leaf sheaths while the disease is active.
• Timing: Starts in fall when soil temperatures drop to about 70°F. Missouri Extension says the fungus infects between about 50 and 86°F. Damage shows again in spring, often through May.

Disease cycle

The fungus lives through summer and winter in the thatch as tiny resting bodies. It makes no spores. In fall, as soil cools to about 70°F, it starts attacking leaf sheaths, and keeps going while soil stays above about 50°F and moist. It goes quiet in the cold of winter, then picks up again in cool, wet spring weather before hot summer weather shuts it down. Patches grow outward each year. Infected clippings, sod and aeration cores carry it to new spots, which is why new patches often cluster around old ones.

When to look: Look from late September into November as the grass heads into dormancy, and again in spring from green-up through May. Hot summer weather usually stops it.

Large patch look-alikes

A few things get mixed up with it:

• Brown patch (Rhizoctonia solani): A related fungus strain, but brown patch hits cool-season grasses like tall fescue in hot midsummer weather. Large patch hits warm-season grasses in cool fall and spring weather.
• Spring dead spot (Ophiosphaerella korrae): Also shows as dead patches at spring green-up, but mostly on bermudagrass. It rots roots and stolons black, while large patch rots the leaf sheaths.

How to manage large patch

Cultural control

• Do not fertilize warm-season grass in fall or early spring. NC State says to avoid nitrogen within 6 weeks before fall dormancy or within 3 weeks after green-up starts.
• Feed zoysia in summer. Purdue says zoysia needs little nitrogen and suggests applying it between May and mid-August.
• Water only when needed in fall and spring. Do not water in late afternoon or evening.
• Fix low, soggy areas. NC State suggests subsurface drainage where water sits.
• Aerate or verticut in June or July to cut thatch. Do not aerate in early spring or while the disease is active.
• Mow at the recommended height for your grass, and avoid mowing early in the morning when the thatch is wet.

Chemical control

Fungicides only work well when applied before symptoms show. NC State says to start on zoysia in fall once soil temperatures drop to about 70°F for five days in a row; Missouri Extension points to mid- to late September. Purdue found a single spring spray before the lawn is half green, or a fall spray about four weeks before first frost, both worked well. Extension guides list strobilurin (QoI), SDHI and DMI groups among the options; Strobilurins and flutolanil gave excellent control in Purdue's trials. Sprays after damage shows work less well, though Missouri Extension says they may keep patches from growing. Use only products labeled for this use; the label is the law.

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Where cool-season grass can be grown, NC State notes fescues and bluegrasses are immune.
• Do not plant warm-season grass in low spots that stay wet.
• Map damaged areas in spring so problem spots can be watched or treated in fall.
• Keep thatch thin and avoid compacting the soil.

When to call a pro

If several patches on a zoysia or other warm-season lawn are growing fast, Purdue suggests having a lawn care company apply a well-timed fungicide. See our pesticide applicator license guide. If the patches line up with low, wet ground, fixing the drainage is a long-term fix.

Related guides: Weed identification guide · All pests and diseases

Questions about large patch

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