What Large Patch Is
Large patch is caused by a strain of the fungus Rhizoctonia solani that infects the base of the leaf sheath in warm-season grasses. St. Augustine and zoysia are its main hosts in our area; when it hits zoysia it is sometimes called zoysia patch. Bermuda grass is rarely affected. The fungus is present in most lawns all year and does nothing while the grass is growing hard in summer heat. It becomes active when soil temperatures drop to about 70 degrees Fahrenheit, which on the Gulf Coast means October or November, and it flares again in cool, wet weather in spring before the grass has started growing again.
What It Looks Like
The first sign is a circular or oblong area of light yellow grass, from a few feet across to much larger, that turns orange and then tan as the infected leaves rot at the base and pull free with a gentle tug. The patch spreads outward like spilled tea, often with a ring of orange at the advancing edge and small islands of green grass surviving in the middle. Patches are perennial: without treatment they come back in the same places each year and grow larger.
It is easy to confuse with two other problems. Brown patch is the same pathogen on cool-season grasses like fescue, a summer disease that does not concern a Florida lawn. Chinch bug damage in St. Augustine also makes expanding yellow and brown patches, but in summer, in full sun, and the grass is dry and dead rather than rotted at the base; the soap flush test in our sod webworm guide brings chinch bugs up. If you are unsure, the county Extension office will diagnose a sample.
What Brings It On
The University of Florida’s Gardening Solutions puts it plainly: the best way to treat large patch is also the best way to prevent it, which is lawn care that fits the season. The conditions that invite it are too much nitrogen on warm-season grass in fall and spring, irrigation the lawn does not need, poor drainage, a thick thatch layer, and mowing too low. Every one of those is a management choice.
Preventing It
Stop nitrogen fertilizer in the fall; the grass is not growing enough to use it, and the fungus is. Feed according to a soil test rather than a schedule, with potassium where the test calls for it. Water only as needed once the weather cools, early in the morning so the blades dry, and never in the evening. Mow St. Augustine at 3 to 4 inches and zoysia at 1 to 2, with a sharp blade. Dethatch and aerate lawns that have built up a spongy layer, and fix low spots that hold water; where a low area floods every time it rains, sub-surface drainage or regrading beats replanting grass in it.
Treating It
Fungicides labeled for large patch work best applied preventively in the fall, when soil temperatures first reach 70 degrees, and repeated per the label through the risk period; they are far less effective once the rings are established. Treated patches do not green up until the grass starts growing again in spring, so a fall application is an investment in next year’s lawn. Where a patch has killed the grass outright, the fix is to correct the drainage and mowing, then re-sod the area once the weather warms.
Bayside Sod’s Turf Care plans include disease prevention and control on a fall schedule for Manatee and Sarasota County lawns, with the soil test that keeps the fertilizer from feeding the fungus. For sod to replace a patch that is past saving, request a quote or call (941) 702-1746. Sources: UF/IFAS Gardening Solutions, large patch; Sod Solutions, how to treat large patch.