Snow Mold

Washington State Univeristy Bulletin

SP0004 -- 1993

Diseases of Washington Crops.

Otis C. Maloy and Debra Ann Inglis

Small Grains Home

Cause

Hosts

Symptoms and Signs

Disease Cycle

Control

1997 PNW Pesticide

Recommendations

Cause
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.

Hosts
Wheat and some turf grasses are the main economic hosts, although these fungi have been reported on a wide range of cereals and grasses.

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.

Disease Cycle
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.

Control
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.

Reference
Bruehl, G.W. 1982. Developing wheats resistant to snow mold in Washington. Plant Disease 66: 1090-1095.