Snow molds are caused by fungi that grow and attack dormant plants at low temperatures under snow cover. These psychrophilic or ‘cold-loving' organisms are opportunistic pathogens which can damage perennial plants or overwintering annual crops when plant disease resistance is lowered due to depletion of plant carbohydrate reserves. In addition, low temperatures limit the activity and number of competitors and antagonists of snow mold fungi, and allow these pathogens to monopolize the nutrient-rich but weakened plant tissues. Snow molds are common diseases of turfgrasses in northern temperate zones, particularly in areas with more than two months of continuous snow cover. They are visible during and after snow melt (Figure 1).
Typhula blight or gray snow mold (Figure 2) is caused
by two species of fungi called Typhula ishikariensis and Typhula
incarnata. Both of these require long continuous snow cover, and the patches
are visible as the snow melts. Typhula incarnata is favored by snow
cover between 2 to 3 months, while Typhula ishikariensis is favored
by snow cover over 3 months. A major difference between these two species
is the type of sclerotia they produce. Typhula ishikariensis produces
small, black, round sclerotia less than 2 mm (0.08 in) across (Figure 3-left). These can be found in large quantity in
circular patches over the white to grayish white fungal growth called mycelium
This appearance gave rise to the other common name for this disease, speckled
snow mold. The name gray snow mold is sometimes reserved just for Typhula
incarnata. This latter fungus produces reddish brown sclerotia up to
5 mm (0.04 in) across (Figure 3-right)
and is often found on lower parts of the infected plants such as crowns and
even roots. Because of their ability to survive on dead organic matter, sclerotia
of these species can be found on other types of plants, even the winged seeds
(samara) of maples (Figure 5).
The other major snow mold of turfgrasses across North America is pink
snow mold (Figure 6) caused by the fungus Microdochium
nivale (which used to be known as Fusarium nivale). This is the
same organism which causes Fusarium patch (Figure 7)
during cool wet weather usually in the spring and fall. The circular patches
of diseased grass may also be covered with mycelium, but the color is often
more pinkish (Figure
8-right). Other differences from gray snow mold are that pink snow patches
may be smaller and the grass less flattened. Also, pink snow mold may occur
with less than 2 months of snow cover, and a long snow melt period often contributes
to more pink snow mold patches. A major concern with pink snow mold is that
high levels of Fusarium patch in the fall may be lead to more abundant pink
snow mold in winter. And in turn, high levels of pink snow mold after snowmelt
may allow for abundant Fusarium patch in the late winter and early spring.
This necessity of managing for Fusarium patch after heavy pink snow mold
is one of the major reasons that the specific snow mold disease should be
identified after snowmelt. Another major reason for proper identification
of the snow mold organism is that different fungicides have different efficacies
against gray or pink snow mold. Notice that pink snow mold may appear
differently on different grasses (Figure 9).
|Figure 1: Pink and gray snow mold patches usually less than 0.5 m (20 in) across appear on turfgrass as the snow is melting.||Figure 2: Gray snow mold is cause by two species
of fungi called Typhula ishikariensis and Typhula incarnata.
Notice the abundant fluffy white mycelum usually at the outer edges of a patch.
|Figure 3: Typhula ishikariensis (left) produces small, black, round sclerotia less than 2 mm (0.08 in) across. Typhula incarnata (right) produces reddish brown sclerotia up to 5 mm (0.04 in) across.||
Figure 4: Sclerotia of Typhula ishikariensis can be found in large quantity in circular patches over the white to grayish white fungal growth called mycelium. This appearance gave rise to the other common name for this disease, speckled snow mold.
Figure 5: Because of their ability to survive on dead organic matter (saprophytes), sclerotia of these species can be found on other types of plants, even the winged seeds (samara) of maples. These are sclerotia of Typhula incarnata.
Figure 6: Pink snow mold is caused by the fungus Microdochium nivale (which used to be known as Fusarium nivale). The reddish color intensifies with exposure to sunlight and the development of spore producing bodies called sporodochia.
Figure 7: Fusarium patch is also caused by the fungus Microdochium nivale during cool wet weather usually in the spring and fall, and in the absence of snow cover. The reddish bronze fringe is the area where the fungus is active. The patch centers often recover giving a dead ring appearance.
Figure 8: Side by side comparison of gray and pink snow molds. Which one is which?
|Figure 9: Pink snow mold caused by Microdochium nivale on Kentucky bluegrass (Poa pratensis - left) compared to to creeping bentgrass (Agrostis palustris - right). Symptoms can differ on different species.|