What Causes Brown Patch in Lawns?

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What causes brown patch in lawns? In most cases it is a soilborne fungus, Rhizoctonia solani. University of Kentucky extension describes brown patch as “also known as Rhizoctonia blight,” and calls it “a common disease of turfgrass.” The fungus is a common soilborne organism, and the disease appears when conditions weaken the grass and keep the leaves wet. This guide explains the mechanism step by step, from the organism to the conditions that favor it.

Quick answer

The most common cause is Rhizoctonia solani, a soilborne fungus. Rhizoctonia zeae can also cause brown patch in tall fescue under very hot, humid conditions.
The fungus survives in soil and thatch and infects grass when stress weakens the turf.
Humidity and long periods of leaf wetness favor infection, and high nitrogen can encourage lush growth that the disease favors.
Watering pattern, mowing, thatch and crowded growth also raise the risk.

The organism behind the disease

The Kentucky publication says that “Brown patch is caused by Rhizoctonia fungi, which are very common soilborne organisms.” It adds that “In most instances, the pathogen is Rhizoctonia solani; however, Rhizoctonia zeae can also cause brown patch in tall fescue under very hot, humid conditions.” The Nebraska guide confirms the cause: “Cause: Rhizoctonia solani.”

The disease is also known by another name. The University of Florida publication lists “Other Names: Brown patch or Rhizoctonia blight,” and names the pathogen as “Rhizoctonia solani.” The Florida document is titled for a related disease, so its brown patch lines are used here only for naming and the infection trigger.

The Kentucky publication explains the difference between presence and disease: “Rhizoctonia fungi often harmlessly colonize organic matter in the thatch. However, when stressful conditions weaken turf, Rhizoctonia can infect plants and cause disease.”

How the fungus survives and spreads

The fungus has a resting stage that lets it outlast unfavorable periods. The Kentucky publication states that “Rhizoctonia fungi survive adverse conditions as tiny, brown resting bodies (sclerotia) in soil and turfgrass thatch layers.” These resting bodies are the reservoir that the disease returns from each season.

When conditions turn favorable, the resting bodies become active. “Germinating sclerotia produce cobwebby fungal mycelium, which is the infective phase of the pathogen.” The mycelium is the part that grows from the soil into the grass.

The cycle closes once infection has begun. “After colonizing infected tissues, Rhizoctonia forms new sclerotia, thus completing its life cycle.”

Why humidity and leaf wetness matter

The main trigger is moisture on the leaves. The Florida publication states that “Infection is triggered by rainfall, excessive irrigation, or extended periods of high humidity resulting in the leaves being continuously wet for 48 hours or more.”

Weather sets the season. The Kentucky publication says that “Brown patch is most destructive during humid weather and when temperatures are stressful to the grass.” For cool-season grasses, the disease “is most severe under high temperatures (highs above 85°F, lows above 60°F).” For warm-season grasses, Kentucky says the infections “are most severe in humid weather with moderate temperatures (45° to 70°F).” The Nebraska guide places the occurrence in its own window: “Occurrence: July – September.”

The Florida publication gives a timing point for warm-season turf: the disease “is most likely to be observed from November through May when temperatures are below 80°F.”

For a general look at how too much or too little water shows up in plants, see how to tell if you are overwatering or underwatering plants.

How nitrogen and growth feed the disease

The Kentucky publication connects growth to infection. It says that “Cultural practices that encourage lush, succulent plant growth, as well as conditions that create a humid environment within the turf, can favor disease development.” Among the factors it lists is “Applications of high levels of nitrogen fertilizer, particularly during spring and summer.”

The Florida publication gives the same direction: “Excessive nitrogen application during potential disease development periods should be avoided.” The Nebraska guide adds the summer timing: “Avoid high nitrogen fertilizer applications during the summer.”

Kentucky also warns against fertilizing an outbreak, saying that “Do not attempt to cure summertime outbreaks with nitrogen fertilization, as this will simply aggravate the disease.” For a broader view of how soil nutrients work, see how to make garden soil better.

Watering, mowing and thatch

How the lawn is watered changes the leaf wetness period. The Kentucky publication says to “Avoid frequent, light watering, which encourages grass to develop a shallow root system and provides surface moisture for Rhizoctonia infections.” It also advises to “Water early in the day so that leaf blades dry quickly, reducing periods of leaf wetness that would favor infection.”

The Florida publication gives a time window for irrigation: “Irrigation should only occur when necessary and during the early morning hours (between 2:00 and 8:00 a.m.) when dew is already present.” The Nebraska guide says to “Avoid night irrigations.”

Mowing can also spread the problem. Kentucky lists “Mowing when wet” among the factors that raise severity, and it notes that “Keep the mower blade sharp; leaf blades shredded by a dull blade creates an ideal site for infection.” Florida adds that “Diseased areas should be mowed last since mowers can spread this disease.”

Thatch matters because the fungus lives there. Kentucky lists “Excessive thatch” among the factors that increase severity, and it says the fungus can colonize “organic matter in the thatch.” Crowding is also a factor: Kentucky states that “overcrowding favors outbreaks of brown patch.” For a look at how mulch and leaf material are handled in the garden, see how to make mulch from leaves and grass.

What the patches look like

The symptoms are the first clue. The Kentucky publication says that “Affected areas are initially roughly circular (Figures 1 & 2), varying in size from 1 to 5 feet or more.” It also describes a sign that can be seen at dawn: “fine strands of grayish, cobwebby fungal growth (mycelium) may be evident at the margin of actively developing patches.” The Nebraska guide describes tall-cut turf as “Roughly circular patches of blighted grass with a reddish-tan color.”

Kentucky adds that the sign “is generally more evident on closely mowed turf” and “quickly disappears as dew dries.” Patches can also merge. “If an outbreak progresses and diseased patches coalesce, affected areas may lose the circular appearance and become irregular or diffuse.”

Which grasses and lawns are affected

Susceptibility differs by grass. Kentucky says the disease is “usually only destructive in tall fescue and perennial ryegrass,” and that “Fine fescues (hard fescue, creeping red fescue, chewings fescue, and sheep fescue) are all moderately susceptible to the disease.” It also notes that “Occasionally, Kentucky bluegrass lawns can be affected by brown patch, although this grass is less susceptible than others.”

Young lawns need more attention. Kentucky states that “Seedlings of all grasses are more susceptible to infection than mature plantings.” It also says that “While brown patch can appear unsightly in home lawns and commercial turfgrass, it usually does not cause permanent losses except in plantings less than one year old.”

Kentucky explains that “Even if an outbreak of brown patch occurs, crowns and roots of established plants often survive, and blighted turf begins to recover when cooler weather arrives.” For newer seedings, the same publication notes that “infections of crowns and roots sometimes occur, particularly in seedlings, resulting in plant death.”

Frequently asked questions

What causes brown patch in lawns?

Most often it is Rhizoctonia solani, a soilborne fungus. The Kentucky publication says “In most instances, the pathogen is Rhizoctonia solani,” and the Nebraska guide lists the same cause.

Can too much water cause brown patch?

Excessive irrigation can be a trigger. The Florida publication says infection is “triggered by rainfall, excessive irrigation, or extended periods of high humidity resulting in the leaves being continuously wet for 48 hours or more.” Long leaf wetness is the trigger, and the timing of watering affects how long leaves stay wet.

Does nitrogen fertilizer cause brown patch?

The sources say high nitrogen favors the disease. Kentucky lists “Applications of high levels of nitrogen fertilizer, particularly during spring and summer” as a factor that increases severity.

Is brown patch permanent?

Usually not for established lawns. Kentucky states that it “usually does not cause permanent losses except in plantings less than one year old.”

Can mowing spread brown patch?

Yes, according to the sources. The Florida publication says “Diseased areas should be mowed last since mowers can spread this disease.”

How we chose

The three sources were chosen because each is a university extension publication that covers brown patch directly. The Kentucky fact sheet is the most detailed on the organism and conditions, the Nebraska NebGuide covers symptoms and management in short form, and the Florida publication names the pathogen and the infection trigger. All three were opened on 2026-10-27 and returned HTTP 200.

What we could not verify

The sources describe the disease and its conditions, but they do not predict whether a given lawn will show brown patch in a given year. The Florida publication is titled for a related disease, so its brown patch lines are used only where they match the other sources.

What the sources do not say

The sources do not give a single temperature or rainfall threshold that applies to every climate. The Kentucky temperature ranges differ by grass type, and the Florida timing applies to warm-season turf in its own region. None of the sources provides a schedule for a specific lawn.