WINTER CROWN ROT OR SNOW MOLD OF ALFALFA, CLOVERS, AND GRASSES IN ALBERTA: I. OCCURRENCE, PARASITISM, AND SPREAD OF THE PATHOGEN

Abstract

Winter crown rot or snow mold of alfalfa, clovers, and grasses, caused by an unidentified low-temperature basidiomycete, is widespread and often of major importance in the central and northern areas of Alberta and Saskatchewan. For example, during a six year period an average of 62% of the alfalfa stands examined in west-central Alberta were affected, and the average estimated damage was 10%. Severe damage also occurs in alsike and white Dutch clover, and in timothy, red top, and creeping red fescue. The disease is less prevalent in red clover, Kentucky bluegrass, and meadow fescue, and seldom occurs in brome grass, crested wheat grass, and slender wheat grass. Iris and other garden perennials, as well as dandelion, quack grass, and various wild plants are also damaged.The pathogen attacks the dormant plants beneath the snow during the first thaw in the late winter or early spring. The plants are killed or weakened in irregular patches as the result of rotting of the crown buds and tissues. These symptoms are distinct from those of true winter killing, with which the damage has been confused. The pathogen is difficult to isolate, except at a temperature near freezing from superficial mycelium or freshly infected tissues.The results of infection experiments in the field and under controlled conditions indicate that the development of the disease is associated with physiological changes in the host. Inoculated alfalfa plants brought inside at weekly intervals became susceptible at dates varying from late November to late December in different seasons. Infection was most severe under the conditions provided by a slowly melting snow cover and was also influenced by soil temperature, soil moisture, and growth of the pathogen.The pathogen appears to spread mainly by means of mycelium, since no sporulating stage has been found. The mycelium spreads both above and below ground at the time of the first spring thaw. The distance of radial spread, as measured by killing of the plants, varied in different years from 2.7 to 7.5 in. in dense alfalfa stands, and from 0. 5 to 6.0 in. in bare land.