Why Does Snow Mold Form on Lawns? Causes and Conditions
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Why does snow mold form on lawns? Snow mold forms when certain fungi infect grass under snow cover or during long periods of cool, wet weather. The sources tie the disease to snow cover and cool, wet weather. Extension publications describe two main types, gray snow mold and pink snow mold, and name the conditions that favor each. This explainer explains the mechanism, lists the conditions the sources describe, and notes what the sources do not say.
Quick answer
- Snow mold is a group of turf diseases that develop under extended snow cover on ground that is not completely frozen, or during prolonged cool, wet weather.
- Gray snow mold is caused by Typhula fungi, and pink snow mold is caused by Microdochium nivale.
- Excess thatch, poor drainage, late fall fertilization and high soil moisture all favor the disease.
- Most lawns recover, although severe cases can kill crowns and delay regrowth.
What snow mold is
Iowa State University Extension and Outreach states that “Extended snow cover on ground that is not completely frozen is conducive to turf diseases called snow molds.” The same publication explains that “Two snow mold diseases, gray snow mold and pink snow mold, occur in Iowa and typically appear at this time of year.”
The fungi are named in the sources. Iowa State University Extension and Outreach says that “Gray snow mold is caused by two species of the fungus Typhula, while pink snow mold is caused by the fungus Microdochium nivale.” The University of Wyoming Extension publication by Gary D. Skoglund states that “Typhula blight (gray snow mold) and Fusarium patch (pink snow mold) are caused by the fungi Typhula incarnata and Microdochium nivale, respectively.”
Why snow cover and cool, wet weather matter
The University of Wyoming Extension publication states that “Snow mold diseases of turfgrass are typically dependent on snow cover or prolonged periods of cool, wet weather.” The University of Wisconsin-Madison Extension quick reference describes the conditions for the most damaging cases as “Cold wet weather with periods of prolonged snow cover over unfrozen ground often leading to the most severe damage.”
For gray snow mold, the Wyoming publication lists the optimum conditions: “lush grass growth prior to snow cover, prolonged snowcover over unfrozen ground, high soil moisture, and temperatures of 32 to 35 degrees Fahrenheit.” The phrase that matters most is “over unfrozen ground”.
Pink snow mold does not need snow to develop. The Wyoming publication states that the fungus “can infect and cause damage whenever temperatures are below 60 degrees Fahrenheit with high humidity for prolonged periods,” and that “The disease is not limited to snowcovered turf.” The same publication says that pink snow mold “often describes the phase of this disease when it is associated with snow melt.”
How the fungus appears on the lawn
Symptoms are usually seen after the snow goes. Iowa State University Extension and Outreach states that “Symptoms of snow molds first appear when snow melts in late winter or early spring.” It adds that “Circular, straw-colored patches appear in the lawn as the snow recedes.” The sizes differ by type: “Patches caused by gray snow mold may be a few inches to a few feet in diameter, while those caused by pink snow mold tend to be smaller, less than six inches across.”
The Wyoming publication describes the gray snow mold patches as “Patches of light colored grass matted together by grayish white threads (or hyphae) as the snow melts.” The Wisconsin quick reference uses similar wording, describing “Circular, matted, gray to straw-colored patches ranging from a few inches to a foot or more in diameter.”
Gray snow mold can leave visible structures behind. Iowa State University Extension and Outreach says that “Gray snow mold causes small, pinhead-sized round structures (sclerotia) to develop on the leaves and crowns of the grass plants.” The Wyoming publication gives a similar description, noting “Yellow to brown or black, pinhead-sized to 3/16 inch diameter irregularly round bodies (fungal sclerotia) embedded in leaves and crowns.”
Iowa State University Extension and Outreach also notes that “These patches may continue to enlarge if the grass remains cool and wet.” Grass inside a patch may be “matted and wet, with pink or gray colored fungal growth over the patch or on the edge.”
What favors the disease in the lawn
Lawn practices can make conditions worse. The Wyoming publication states that “Disease development also is favored by excessive thatch, poor drainage, late fall fertilization (before grass dormancy), and high soil moisture.” Iowa State University Extension and Outreach lists related prevention steps, including “Avoiding excessive nitrogen fertilization in the fall” and “Keeping the thatch layer to 1/2 inch or less.”
The fungus can also persist between seasons. The Wyoming publication states that “Fungal structures such as sclerotia, spores, and mycelium are produced on infected turf and survive warm, dry periods of summer dormant in the thatch layer.”
Snow itself can be a factor. Iowa State University Extension and Outreach advises “Preventing excessive snow accumulation on problem areas, by using snow fencing and spreading out large snow piles to promote melting.” The Wyoming publication says to “Minimize snow accumulation before ground is frozen” and to “Prevent snow compaction and ice formation.”
Leaves and mowing also matter. The Wisconsin quick reference lists “Remove leaf litter from lawns” as a management step. Iowa State University Extension and Outreach advises “Mowing the lawn at the recommended height throughout the fall” and “Raking up fallen leaves in the fall.”
How serious is the damage?
The sources describe most cases as limited. Iowa State University Extension and Outreach states that “Damage caused by snow mold usually is not serious, and affected areas typically green up eventually, though more slowly than the rest of the lawn.” The Wyoming publication gives the two possible outcomes: “If crowns are not killed, the turf will regrow but will be delayed.” And “Under optimal disease conditions, crowns will be killed and regrowth will not occur.”
For pink snow mold, the Wyoming publication states that “Under severe conditions, crowns can be killed.”
For related reading, see how to make garden soil better and how to keep garden soil healthy, which cover general soil conditions that affect lawn health.
Management steps named in the sources
The Wyoming publication lists cultural methods: “Avoid late fall nitrogen applications before turf dormancy.” It also says “Mow in fall until grass goes dormant so that snow does not fall on a tall canopy.” Iowa State University Extension and Outreach lists “Applying fungicide late in the fall on golf courses and other high-value turfs” as a measure for those turf types. The Wyoming publication states that “Many fungicides are listed for snow mold control.” This article does not name products or rates.
How we chose the sources
We selected university extension publications that describe the causes and conditions of snow mold on lawns. Pages that returned errors were not used. The three sources listed at the end of this article were each opened on 2026-10-27 and returned HTTP 200.
What we could not verify
The sources describe the conditions that favor snow mold, but they do not give a single temperature or snow depth that guarantees its onset. The cited pages also do not measure how much snow cover is needed for a given lawn, so this explainer does not state a threshold.
What the sources do not say
The sources do not state the exact number of days of snow cover that causes damage. They do not rank the two snow mold types by how common they are in every region, and they do not describe how the fungi behave in regions outside the ones named in the publications.
Frequently asked questions
Does snow mold only form under snow?
No. The Wyoming publication states that the disease “is not limited to snowcovered turf,” and that pink snow mold “can infect and cause damage whenever temperatures are below 60 degrees Fahrenheit with high humidity for prolonged periods.”
What is the difference between gray and pink snow mold?
Gray snow mold is caused by Typhula fungi and produces sclerotia on leaves and crowns. Pink snow mold is caused by Microdochium nivale. Iowa State University Extension and Outreach states that “Gray snow mold causes small, pinhead-sized round structures (sclerotia) to develop on the leaves and crowns of the grass plants.”
Does fertilizer timing affect snow mold?
The sources say yes. The Wyoming publication lists “late fall fertilization (before grass dormancy)” among the factors that favor disease, and it recommends avoiding “late fall nitrogen applications before turf dormancy.”
Will a lawn recover from snow mold?
Usually, according to Iowa State University Extension and Outreach, which states that “affected areas typically green up eventually, though more slowly than the rest of the lawn.” The Wyoming publication notes that under optimal conditions crowns can be killed.
Do fallen leaves need to be raked?
The sources recommend removing leaf litter. Iowa State University Extension and Outreach advises “Raking up fallen leaves in the fall,” and the Wisconsin quick reference lists “Remove leaf litter from lawns.”
Sources
- Iowa State University Extension and Outreach, Yard and Garden, “Snow Mold”: https://yardandgarden.extension.iastate.edu/encyclopedia/snow-mold (opened 2026-10-27, HTTP 200)
- University of Wyoming Extension, B-868, “Snow Molds of Turfgrass” by Gary D. Skoglund: https://www.wyoextension.org/agpubs/pubs/B868.pdf (opened 2026-10-27, HTTP 200)
- University of Wisconsin-Madison Extension, lawn disease quick reference: https://hort.extension.wisc.edu/articles/lawn-disease-quick-reference/ (opened 2026-10-27, HTTP 200)
