Pink snow mold is caused by Microdochium (synonym Fusarium)
nivale, a fungus with stout, sickle-shaped conidia that are
1- to 3-septate. The sexual stage is Monographella (synonym
Calonectria) nivalis. Perithecia of this ascomycete
are found in mycelial mats and contain asci with 1- to 3-septate,
spindle-shaped ascospores. The fungus attacks plant parts during
wet, cool weather, but is not dependent upon snow.
Speckled snow mold is caused by Typhula idahoensis
and the similar T. ishikariensis, two basidiomycete fungi
that produce small (0.5-1.5 millimeter), black sclerotia. Both
species are restricted to areas with deep snow. Less serious
is T. incarnata, which has a wider geographical range
and is found even in areas without prolonged snow cover. It forms
slightly larger sclerotia, similar to radish seeds in size, shape,
and color. All three Typhula species form clubshaped basidiocarps
about 1 centimeter long.
Wheat and some turf grasses are the main economic hosts, although
these fungi have been reported on a wide range of cereals and
Symptoms and Signs
Snow molds are most apparent in early spring when the snow first
melts. Pink snow mold produces pinkish mycelium and conidia that
cover dry yellow or dead leaves (Photo 12). Dark colored fruiting
bodies may be embedded within lower leaf sheaths
In the case of speckled snow mold, leaves appear scalded or
bleached-white or tan in color and have a tendency to crumble.
Enzyme action of the pathogen on leaves under the snow releases
chlorophyll and produces "green snow." Plants dug out
of the snow will reveal dissolution of leaf tissue. Numerous
scattered dark sclerotia that give diseased plants a speckled
appearance are a key diagnostic feature. Plant vigor may be markedly
reduced, and in severe cases, the crowns are killed. Surviving
plants recover slowly and are sensitive to additional stresses.
Fusarium usually survives as conidia or mycelium
on living plants, and it can maintain itself as a crown and root
rotting fungus. Typhula survives as a parasite or as sclerotia
in plant debris or soil. The sclerotia germinate to form basidiocarps
(Photo 13), which produce basidiospores. When Typhula
or Fusarium spores germinate, they invade plant tissue.
Older leaves in contact with soil under snow are attacked first.
Crowns may be invaded later. The fungi continue to develop under
the snow and eventually produce conidia or sclerotia. The snow
mold pathogens are most aggressive at low temperatures, that
is, slightly above freezing. Early snowfall and deep (about 1
foot), prolonged (about 100 days) snow cover on unfrozen ground
favor the disease. Deep snow maintains the surface of unfrozen
soil at about 41F.
Chemical. In the past, applying mercury fungicides
to plants in early winter before snowfall gave some protection.
These fungicides are no longer available, and no replacements
have been developed.
Cultural. Crop rotation to legumes or spring
grains is effective in reducing the number of Typhula
sclerotia, but because snow molds are most severe in summer fallow
areas, crop rotations are not always economically feasible. Early-seeded
plants, because they are robust, may recover to produce an acceptable
crop, but these have increased risk to dryland foot rot and stripe
rust. Attempts have been made to hasten snow melt by using blackening
agents, but cost, difficulties of application, and unpredictability
of post-application snowfall make this method unreliable.
Resistance. The cultivar, 'Sprague,' developed
for the snow mold area has good snow mold resistance. 'Luke'
has some tolerance. 'John' is resistant but has a more restricted
area of adaptation. Infection does not progress deeply into the
crowns of resistant wheats. Such plants can produce vigorous
re-growth even though the leaves may have been destroyed.
Bruehl, G.W. 1982. Developing wheats resistant to snow mold in
Washington. Plant Disease 66: 1090-1095.
Diseases * Barley
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Edited and reviewed by Ed Adams, WSU Extension Plant Pathologist
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