Role of ice nucleation and antifreeze activities in pathogenesis and growth of snow molds

We examined the ability of snow molds to grow at temperatures from -5 to 30 degrees C and to influence the growth of ice through assays for ice nuclea tion and antifreeze activities. Isolates of Coprinus psychromorbidus (low t emperature basidiomycete variant), Microdochium nivale, Typhula phacorrhiza , T. ishikariensis, T. incarnata, and T. canadensis all grew at -5 degrees C, whereas Sclerotinia borealis and S. homoeocarpa did not grow at temperat ures below 4 degrees C. The highest threshold Ice nucleation temperature wa s -7 degrees C. Because snow molds are most damaging to their hosts at temp eratures above this, our results imply that the pathogenesis of these. fung i is not dependent on ice nucleation activity to cause Freeze-wounding of h ost plants. All snow molds that grew at subzero temperatures also exhibited antifreeze activity in the growth medium and, in the soluble and insoluble hyphal fractions, with the exception of M. nivale and one isolate of T. ca nadensis. The lack of high ice nucleation activity combined with the presen ce of antifreeze activity in all Fungal fractions indicates that snow molds can moderate their environment to inhibit or modify intra- and extracellul ar ice formation, which helps explain their ability to grow at subzero temp eratures under snow cover.