Brown Patch, Take-All Root Rot and Gray Leaf Spot

Three diseases, and why to be sure before treating

St. Augustine grass in Polk County gets three fungal problems often enough to have names every lawn company knows: large patch, usually called brown patch, which shows up in cool weather; gray leaf spot, which comes with the summer rains; and take-all root rot, which rots the roots in the wet months. They get searched together because yellow and brown patches look alike from the sidewalk, but they appear at different times, attack different parts of the plant and call for different care.

University of Florida turf pathologists stress that diseases are the exception and not the rule for lawns, and that injuries and disorders, including pesticide damage, scalping and nutrient shortages, should be ruled out first. The sections below describe each disease from UF/IFAS Extension publications. We can describe what the university says, but we cannot diagnose your lawn. If you are unsure, the last section explains how to get a sample checked.

Brown patch (large patch)

A note on names: UF/IFAS Extension publications call this disease large patch, caused by the fungus Rhizoctonia solani, and say it is sometimes called brown patch. UF Gardening Solutions adds that the name brown patch more accurately refers to a separate turfgrass infection that occurs in summer, which is why you may see the terms used differently. The St. Augustinegrass publication from UF/IFAS uses the combined name large (brown) patch, and that is the disease people usually mean when they search for brown patch on a Florida St. Augustine lawn.

Symptoms and timing. Large patch begins as round, discolored patches that expand over time. The border of an active patch is darker and can look orange, and the center is usually brown and sunken. UF says it is most often seen when temperatures drop below 80 degrees, usually from November through May. It often becomes a problem in fall, the patches keep expanding in spring, summer may bring regrowth, and untreated disease returns in fall. That makes fall, from about November, the time to start watching. UF names a simple field check: pull a blade from the edge of a patch. If it comes away easily from the runner and the base is dark and smells of rot, a fungal disease is likely.

Causes and prevention. Moisture is the main driver. Excess irrigation, rain and humidity all favor the disease, and the UF Gardening Solutions page advises keeping the lawn damp as little as possible by watering only when necessary and only in the morning. Excess nitrogen makes it worse, so use only slow-release nitrogen, sparingly, following the UF/IFAS recommendations. The St. Augustinegrass publication also warns that excessive topdressing encourages large (brown) patch. When mowing, do the diseased areas last and clean the blades afterward so the fungus is not carried to healthy turf. Cultural care can stop the spread and encourage regrowth, and if the turf is otherwise healthy it should recover naturally over one or more growing seasons.

Take-all root rot

Symptoms and timing. Take-all root rot is caused by the fungus Gaeumannomyces graminis and affects all warm-season turfgrasses. UF/IFAS describes it as a root rot: the fungus does not attack the leaves, and leaf symptoms are the result of damaged roots. Above ground you see irregular yellow or light green patches from a few inches to a few feet across. Dig a few plugs and look below: UF describes early root damage as thin, off-white roots with isolated black lesions, and later roots that are very short, black and rotted, with black lesions on stolons too. In severe cases the plants die and bare patches develop. It is observed in the summer and early fall, when Florida gets most of its rain.

Why it is hard to catch. By the time leaf symptoms show, UF says, the fungus has been working on the roots for at least two to three weeks, probably longer. The pathogens are very common on warm-season turf roots, so the disease needs stress and prolonged wet weather to take off. Any stress can worsen it, and the disease is very difficult to control once symptoms appear above ground.

Prevention. UF/IFAS puts the emphasis on reducing stress. Mow at the correct height and remove no more than a third of the leaf at a time, because scalping damages the growing point. Balance nitrogen with an equal amount of potassium, use slow-release sources and avoid quick-release urea. Extra potassium in late summer and early fall may help. Do not apply lime to the turf. St. Augustine is especially sensitive to herbicides, so use them only according to the label and manage weeds culturally where you can. A soil test shows pH and potassium, which helps with several of these choices.

Gray leaf spot

Symptoms and timing. Gray leaf spot is caused by the fungus Pyricularia grisea. In Florida, St. Augustinegrass is the only warm-season turfgrass it affects. The key sign is an oblong spot on the leaf blade, with a straw-colored or olive-green to brown center and a dark brown border. After long warm, humid spells the center can carry a gray, felt-like growth of spores. Heavily affected lawns look thin and unthrifty, and spots may merge until whole blades turn brown. UF says it develops during warm rainy spells from May through September, when leaves stay wet for 12 hours or more and air temperatures hover between 80 and 90 degrees. The St. Augustinegrass publication adds that it is primarily a problem on new growth.

Causes. The disease develops faster on lush new growth. Excess nitrogen can cause rapid flushes of susceptible growth even in mature lawns, and stress makes grass more vulnerable. UF lists summer applications of the herbicide atrazine, soil compaction, nutrient imbalances, improper irrigation and other pests among the stresses. No St. Augustinegrass cultivar sold today is immune, though some are less susceptible than others, which matters if you are choosing sod. Our guide to the best grass for Central Florida covers the grass choice itself.

Prevention and what to expect. Keep nitrogen moderate and avoid pushing lush growth in the rainy season. UF suggests watering in the early morning, but Polk County's current allowed hours are the ones in our guide Polk County Watering Days & Irrigation Rules (12:01 a.m. to 4 a.m. or 8 p.m. to 11:59 p.m. at the time of writing), so follow those, and avoid late afternoon or evening watering that leaves blades wet through the night. UF describes gray leaf spot as chronic but not typically damaging in established, well-managed home lawns. It can become severe in stressed lawns, in poor shady or damp locations, or when unusually long warm wet spells persist. Because Polk County Utilities customers are limited to one watering day a week at present, the practical point is to keep your watering inside those allowed hours and avoid adding extra wet hours in the rainy season. Polk County Watering Days & Irrigation Rules has the details.

Look-alikes: chinch bugs, drought and others

Most yellow and brown patches in a St. Augustine lawn are not one of these three diseases. The quickest comparison is time of year, pattern and the blade-and-root check.

Chinch bugs cause yellowish to brownish patches that appear as the weather warms, first along sidewalks, near buildings and in water-stressed, full-sun areas. UF/IFAS describes the coffee can test, with the ends cut out of the can and the can pushed into the soil at the margin of the damage, then filled with water. Chinch bugs float to the surface within about five minutes. A patch that keeps spreading in summer heat with insects in the can is chinch bugs, not take-all, although the two can look the same from a distance. Our guide to chinch bugs in Florida lawns covers the test and the damage in detail. Drought stress shows as folded blades, a blue-gray color and footprints that stay visible, follows the sprinkler pattern, and usually perks up after a good watering. Large patch gives round patches in cool weather with a rotten blade base. Gray leaf spot gives spots on the blades in summer rain. Take-all gives patches in the wet months with black, short roots.

Other things can also imitate a disease: herbicide damage, a fuel leak, scalping from mowing too short, nematodes, shade, compaction and over- or underwatering. Ask what has been sprayed on the lawn recently, because pesticide damage is the first thing UF/IFAS says to rule out. If the lawn has patches in more than one season, do not assume a single cause.

Prevention that works for all three

The cultural controls overlap, which is good news. Water only when the lawn shows it needs it, in the early morning, and apply about half to three-quarters of an inch when you do, as UF/IFAS advises. Mow at 3.5 to 4 inches for standard varieties, never removing more than a third of the blade, with sharp, clean blades. Fertilize on a schedule that suits the grass, with slow-release nitrogen, balanced with potassium, and keep to the Polk County fertilizer ordinance. The UF/IFAS Polk County guide to it lists a limit of one pound of slow-release nitrogen per 1,000 square feet per application and a 10-foot buffer from water bodies. Polk County's ordinance restricts fertilizer near water and before heavy rain; some cities and nearby counties add a summer ban, so check your city.

Keep thatch in check, since thatch builds fastest in grass that is overfertilized or overwatered, and the St. Augustinegrass publication says a layer over an inch can be removed by vertical mowing in early spring to midsummer, only while the grass is actively growing. Fix drainage and compaction where water sits. Keep trees pruned if shade is the problem. A lawn that is stressed less often gets these diseases less often and recovers faster when it does.

Fungicides: what UF/IFAS says, and what we leave to the label

We do not recommend a specific fungicide or a rate, and no one should buy one without a confirmed diagnosis. UF/IFAS makes three points that are worth knowing. First, fungicides are a backup to cultural care and do not replace it. Second, timing matters: for take-all root rot they are best used preventively, starting at least a month before symptoms normally appear, because they are not as effective once symptoms show. For large patch, UF says treatment works best on areas that had the disease in previous years, applied about two weeks before symptoms usually appear, which is around the end of September in North Florida and the end of October in South Florida, so Polk County falls somewhere in between; ask Extension for the timing that fits your lawn. Third, not every fungicide is labeled for residential lawns, results can be erratic because applications are hard to make properly, and rotating chemistries helps limit resistance.

UF also writes that the label is the law, and that for pesticide questions you can contact the UF/IFAS Pesticide Information Office. For gray leaf spot, UF says that if the problem is serious and persistent, homeowners should consider a professional lawn care firm, and that product availability changes, so a local Extension agent can give up-to-date advice. Read the label for rate, site restrictions and how close you can apply to water, which matters in a county with so many lakefront yards.

When to call a pro or Extension

Contact your county UF/IFAS Extension office when you cannot tell which problem you have, when a patch keeps spreading despite better watering and mowing, or when the same spot fails year after year. UF/IFAS says disease samples can be sent to the UF/IFAS Extension Plant Diagnostic Center for diagnosis, through your county office, and publishes sample submission guidelines. Take a photo of the whole lawn as well as a close-up, and note when the patches appeared, what was applied recently, how often you water, and what the soil test says if you have one.

Call a lawn care company when the lawn is large, the damage is severe, or you need a fungicide applied correctly. Ask what they found, how they confirmed it, whether they will treat only affected areas, and what they will change in your watering, mowing and fertilizing so it does not come back. A company that names a disease without looking at the blades and roots, or that proposes a calendar of sprays with no diagnosis, is worth questioning. If dead areas need re-sodding, fix the cause first; our guide to how to lay sod in Florida covers the replacement.