Abstract
Winter injury of orchardgrass plants had been continuously surveyed in grassland fields at 17 locations over 4 years in Tokachi district, Hokkaido. Winter injury was caused mainly by Sclerotinia borealis BUB. & VLEUG. Severe injury by this snow mold was observed only in the locations where plants were exposed to air temperatures below -15℃ before snow cover and thereafter kept under deep snow cover more than 50 days. Consequently, it was recognized that not only duration of snow cover but also the lowest air temperatures before snow cover were the important meteorological factors responsible for this winter injury. Hence it was attempted to estimated degrees of winter injury using the data of these meteorological factors. The estimated degrees of winter injury at 17 locations in Tokachi district were well comparable with those actually observed. The degrees of winter injury at 170 locations over Hokkaido and for 22 years in Tokachi district were also estimated. The estimated degrees of winter injury accounted well for their local and seasonal variations in empirically known winter injury. These results supported the hypothesis that the low temperature stress before snow cover predisposed orchardgrass plant to severe injury by S. borealis.
