Damage from snow mold fungi usually becomes apparent as the snow
melts and exposes the grass in late winter. Snow mold symptoms consist
of roughly circular patches (at least 3 to 12 inches) of dead and
matted grass blades. In severe cases, these patches coalesce and
may not be recognizable as individual circles. Just after snow melt
and while the grass remains moist, it may be possible to differentiate
between the two common types of snow mold found in New England by
their color. The web-like mycelium of pink snow mold (Microdochium
nivale) may initially look white and mature to a faint pink
to salmon color. Gray snow mold (Typhula spp.) is white to
gray in color. The mycelium of both types of fungi will disappear
quickly as the grass dries. A useful identifying characteristic
of gray snow mold is the presence of tiny brown to black mycelial
masses (sclerotia) on the blades and in the leaf sheaths of infected
plants. These survival structures vary in size and color, becoming
smaller and darker as they dry. The pink snow mold fungus does not
is useful to determine whether the disease is pink or gray snow
mold because gray snow mold rarely damages more than the blades
of the grass. Lawns with gray snow mold can be expected to recover
fairly quickly even when damage appears extensive. Pink snow mold,
in contrast, may invade the crowns and roots causing more serious
injury. It is not unusual for both types of snow mold to be found
in the same area. All common lawn grasses may be infected, but Kentucky
bluegrass-fescue lawns are the least susceptible to severe damage.
|Gray snow mold on turfgrass and
close-up of mycelium
|Characteristic pink snow mold symptoms
on turfgrass and close-up of mycelium
mold fungi are active at temperatures just above freezing in moist
conditions. These conditions occur most frequently under snow cover
or anything else that covers the grass, such as fallen tree leaves.
Gray snow mold usually only occurs after prolonged snow cover. Pink
snow mold may be active in cool, wet conditions from late fall through
early spring even in the absence of snow or other covers, a factor
which greatly enhances its potential as a damaging disease.
Snow mold fungi remain inactive during the warm months when other
disease fungi are most active. They survive in thatch and on plants
as sclerotia (gray snow mold) or as mycelial threads (pink snow
mold). In the fall, Typhula species sometimes produce small,
white or pinkish, club-shaped spore-producing structures that may
be seen among grass blades in lawns that have not been recently
mowed. They grow from sclerotia that were produced in the previous
winter for survival over the summer.
cool, wet weather develops, the fungi begin to grow and infect grass
plants. Like all living organisms, these fungi require moisture
to survive. The cold, dry air of winter prevents active growth.
The shelter of leaves, snow or any other cover on the grass maintains
the necessary moisture for growth. Optimal conditions for snow mold
activity occur when snow falls suddenly and remains on ground that
has not yet frozen. In such cases, grass is often still lush, providing
an excellent food base for the fungi.
most important means of preventing or reducing snow mold problems
in lawns is the care of the grass at the end of the summer season.
As long as the grass continues to grow, it should be mowed. Fall
fertility programs should be timed so that they do not influence
the ability of the grass to become dormant for the winter season.
Fall fertilizers should be applied more than six weeks before dormancy,
or they should be applied after leaf blade growth has stopped but
while the grass is still green. Addition of nitrogen fertilizer
just before the grass becomes dormant will stimulate a late burst
of succulent green growth, making the grass prone to winter injury
caused by frost, ice or exposure and also providing the snow mold
fungi with vulnerable host plants. This condition is particularly
dangerous when an early snowfall occurs.
snow mold activity is greatest beneath covers that maintain moist
conditions, all leaves or other materials should be removed from
the lawn. In addition, it is best to avoid piling snow deeply along
sidewalks and driveways where it will form a long-lasting snow bank.
In large lawn areas, the strategic placement of snow fences and
landscape plants may prevent deep drifting of snow. In the spring,
rake away dead and matted foliage from damaged areas to allow the
new growth to begin.
applications for snow mold are not recommended for home lawns except
in extreme circumstances. The cultural practices described above
are the most effective means of reducing snow mold damage. In most
cases, the grass will resume healthy growth in the spring, even
though damage may appear widespread at snow melt. In unusual circumstances,
where cultural practices have not been sufficient, fungicide application
may be considered. Fungicides are most effective if applied just
before the first lasting snow fall. Fungicides are not effective
in spring after the damage has already occurred unless pink snow
mold is involved. Pink snow mold may renew activity if cool, moist
weather conditions prevail and may warrant treatment.
from the UMass Extension, 2000