Brown Patch Risk

What is brown patch?

Brown patch is a fungal disease caused by Rhizoctonia solani, one of the most widespread soilborne pathogens in turf. It is active throughout most of the United States but is most destructive in the transition zone — the mid-Atlantic, the Midwest, and the upper South — where midsummer conditions routinely produce the combination of hot days, warm nights, and high humidity that the pathogen needs to infect.

Tall fescue is the home-lawn species most severely affected. Perennial ryegrass, Kentucky bluegrass, and fine fescue are also susceptible. Warm-season grasses (bermuda, zoysia, St. Augustine) can get brown patch but are generally more tolerant and rarely suffer the stand-thinning damage that tall fescue does in a bad year.

Unlike dollar spot, which can damage turf at any fertility level, brown patch is most aggressive when nitrogen is high and growth is rapid. A lawn that has been pushed with quick-release nitrogen going into a stretch of hot, humid nights is at significantly higher risk than one on a moderate fertility program.

Why the weather predicts it

Rhizoctonia solani is a heat-adapted pathogen. Its infection cycle depends on two conditions that must coexist: sustained warmth through the night (the fungus slows dramatically when temperatures drop below ~70°F) and a leaf surface wet enough to allow the fungal hyphae to colonize leaf tissue. When nights stay warm and moisture — either dew, rainfall, or evening irrigation — keeps leaves wet for extended periods, the pathogen can complete an infection cycle within hours.

By the time the characteristic smoke-ring patches appear in the turf, the infection cycle that caused them happened 24–72 hours earlier. Monitoring favorable weather conditions gives you lead time — enough to shift irrigation timing, adjust mowing height, and dry out the canopy before the next infection cycle begins.

The threshold model

This tool counts how many of the past 5 days have been “favorable” for brown patch — days where all three biological thresholds documented in the turfgrass pathology literature are met:

Daily high ≥ 85°F (29.4°C)
Overnight low ≥ 70°F (21.1°C)
Daily mean relative humidity ≥ 90%

The temperature thresholds come from Fidanza, Dernoeden, and Zhang (1996), who studied brown patch incidence in tall fescue across multiple seasons and identified the 70°F overnight minimum as the critical biological driver — the temperature below which Rhizoctonia activity slows substantially regardless of daytime conditions. The humidity threshold reflects NC State University Extension guidance that sustained high relative humidity (conditions that promote overnight leaf wetness) is required alongside warm temperatures for active infection.

A rolling 5-day window is used rather than a single-day reading because brown patch infection requires sustained conditions, not a single favorable night. A count of 0 favorable days in 5 means the disease window hasn't recently opened; a count of 3–5 means conditions have been consistently favorable and infection pressure is high.

This is a threshold indicator, not a probabilistic model. Unlike the Smith-Kerns dollar spot model — which outputs a calibrated percentage from a validated logistic regression — this tool counts favorable days using documented biological thresholds. The risk labels (Low / Elevated / High) describe the weather pattern, not a precise infection probability. The 70°F and 85°F thresholds have strong empirical support; the specific count boundaries (2 favorable days = Elevated, 3 = High) are reasonable operational cutoffs, not validated cutoffs from a controlled study.

What to do when risk is elevated or high

Cultural management is the first line of defense for most homeowners — it addresses the conditions that enable infection rather than the pathogen itself:

• Shift irrigation to early morning. Evening or nighttime watering extends the leaf wetness window through the warmest part of the night, exactly when Rhizoctonia is most active. Early-morning irrigation gives leaves the full day to dry. This single change is the most effective cultural response to brown patch.
• Raise your mowing height. A taller canopy has better airflow, which reduces dew persistence and shortens the leaf-wetness window that favors infection. If you're mowing at 3 inches, raising to 3.5–4 during high-risk periods is a meaningful change.
• Hold off on quick-release nitrogen. High nitrogen drives the lush, succulent growth that is most easily colonized by Rhizoctonia. If you were planning a nitrogen application and the 5-day forecast shows persistent favorable conditions, defer it. Slow-release nitrogen (IBDU, sulfur-coated urea, methylene urea) is a lower-risk alternative when fertility is needed during warm humid periods.
• Remove morning dew. Dragging a garden hose or a bamboo pole across the turf early in the morning disrupts dew and guttation fluid, cutting the leaf wetness period by an hour or more.
• Avoid aerating or dethatching during active periods. Mechanical disruption spreads Rhizoctonia hyphal fragments — wait until a dry, cooler period if these practices are needed.

Fungicide is an option when cultural practices are not sufficient and disease is actively progressing. Azoxystrobin, propiconazole, myclobutanil, and chlorothalonil have activity against Rhizoctonia, but product selection and timing require reading the label and understanding your situation — this tool does not make fungicide recommendations.

How the tool works

1. Your ZIP code is geocoded to a latitude/longitude using Zippopotam.us (free, no account required).
2. Hourly 2-meter air temperature and relative humidity for the past 10 days and the next 7 days are fetched from Open-Meteo (free, no account required).
3. Hourly temperature readings are aggregated into daily max and min. Hourly RH readings are averaged into a daily mean.
4. Each day is classified as “favorable” if all three thresholds are met: daily high ≥ 85°F, overnight low ≥ 70°F, mean RH ≥ 90%.
5. A rolling 5-day count of favorable days is computed for each date. Today's count is the headline readout; the 7-day bar chart shows the projected count as conditions are forecast to change.
6. The model is gated off when daytime highs fall below 65°F — below this threshold, Rhizoctonia activity is negligible.

Honest caveats

• The thresholds were established for tall fescue. Fidanza et al. (1996) studied brown patch on tall fescue in Maryland. Warm-season species (bermuda, zoysia, St. Augustine, centipede) differ in susceptibility and in the microclimate of their canopy at typical mowing heights. The thresholds reliably identify weather conditions that favor Rhizoctonia; the risk levels may not translate directly to all grass species and mowing heights.
• Regional weather is not your microclimate. Open-Meteo provides model-based grid weather centered on your ZIP code's coordinates. Shade, low spots, irrigation patterns, soil drainage, proximity to hardscape, and canopy density all create meaningful local variation in the temperature and humidity your turf actually experiences — particularly overnight when the thresholds that matter most are active.
• The tool assumes the pathogen is present. Rhizoctonia solani is an extremely common soilborne organism found in virtually all managed turf environments — if your lawn has ever had brown patch, the pathogen is almost certainly present in your thatch and soil. Risk is lower in genuinely new-establishment or disease-free situations, though that is rare.
• Forecast days carry increasing uncertainty. The 5-day rolling count anchors to observed weather data; forecasted days inherit the uncertainty of the numerical weather model, which grows beyond 3–4 days. Treat the latter part of the 7-day bar chart as directional, not precise — use it to see whether conditions are trending toward or away from the risk window.
• The RH threshold is for daily mean values. Some extension guidance cites nighttime-only RH thresholds (e.g., >95% during overnight hours). A daily mean of 90% represents sustained high humidity across most of a 24-hour period, which is a reasonable proxy — but conditions where RH is very high at night and lower during the day could still be favorable even when the daily mean falls just below 90%. On borderline days, weight the overnight low temperature as the stronger signal.

Sources

Fidanza MA, Dernoeden PH, Zhang M. (1996). “Brown patch of tall fescue as influenced by air temperature, meteorological variables, and turf management factors.” Plant Disease 80(8):868–873. https://doi.org/10.1094/PD-80-0868 — Primary source; temperature thresholds (70°F overnight low, 85°F daily high) and their relationship to brown patch incidence on tall fescue.

NC State University Extension. “Brown Patch.” TurfFiles. https://turffiles.ncsu.edu/diseases/brown-patch/ — Threshold criteria, cultural management guidance, and species susceptibility.

Open-Meteo (https://open-meteo.com) — Free, keyless weather API providing hourly air temperature and relative humidity.

Zippopotam.us — Free, keyless ZIP code to latitude/longitude geocoding.

Informational only — not professional agronomic or pesticide advice. Always read and follow product label directions. Last reviewed: June 2026.