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Snow Molds Blight Minnesota Lawns - For Now

M. Grabowski, UMN Extension

Photo 1: Pink and gray snow mold on a Minnesota lawn

Michelle Grabowski, UMN Extension Educator

Persistent cold weather and late snow fall mixed with freezing rain have created ideal conditions for the growth of snow molds this year. Snow molds are caused by fungal pathogens that thrive in cold temperatures (just above freezing to about 60F) and high humidity.

M.Grabowski, UMN Extension

Photo 2: Small round patch of pink snow mold with a light center and dark border

Two types of snow mold are common in Minnesota. Pink snow mold, caused by the fungus Microdochium nivale, results in round dead patches in lawns that can be an inch or two across or as large as a dinner plate. These dead patches are often pale tan in color, matted down and may have a dark brown border. In sunlight pink snow mold patches turn pink to salmon colored as the fungus produces spores of these colors.

M.Grabowski, UMN Extension

Photo 3: Pink matted grass in a pink snow mold patch

Gray snow mold, caused by several species of Typhula, also cause pale gray to tan patches of matted down brittle grass that can spread up to 3 feet across. If the matted down grass of gray snow mold is examined closely, gardeners can see small black dots, about the size of a pen tip. These black dots are sclerotia, resting structures produced by the fungus that allow it to survive through harsh conditions. It is not uncommon to find both pink and gray snow mold in the same lawn.

M.Grabowski, UMN Extension

Photo 3: Large irregular matted patch of gray snow mold

M.Grabowski, UMN Extension

Photo 5: Small black sclerotia of gray snow mold

The good news is that although snow molds thrive in cold wet conditions, they go dormant as the weather becomes warm and dry. In many cases the snow mold fungi blight the grass leaves but do not kill the crown. In these situations, the grass will recover as the fungal pathogen goes dormant. In severe cases of snow mold, the grass plant may die and gardeners will need to reseed the infected area.

For now there are a few things that gardeners can do to speed the fungal pathogen into dormancy. Remove or spread out any remaining snow piles, so the snow melts quickly and the grass below is exposed to sun and air. Rake the lawn to remove any leaf debris and to fluff up matted down patches of turf. This will help improve air movement around the grass plants so leaves dry quickly after dew or rain. A light application of fertilizer in the infected patch can help grass recover. If no recovery is seen, reseed the area, taking care to rake away matted down grass so that seed has good contact with the soil below. Fungicides are not effective in controlling snow mold once the disease is established and should not be used at this time of year.