COMMENTS ON SNOW BLIGHT 



Erik Bjorkman 



Dean, Royal School of Forestry, 

 Stockholm, Sweden 



The Phacidium-blight fungus, belonging to the family Phacidiacae 

 of the Ascomycetae, was first described in Finland by Karsten in 1886 

 under the name of Ph-acidium infestans Karst. The fungus is known 

 from the northern parts of Europe and Asia down to southern Ural. 

 It has also been reported from North America (Faull 1929). In Eu- 

 rope and Asia this fungus appears almost exclusively on Pinus species, 

 especially Pinus sylvestris, whereas in the northern parts of the U.S. 

 and in Canada, it appears commonly on a great number of conifers, 

 especially Abies and Picea species. 



From what has been found by the infection experiments hitherto 

 undertaken, it appears that the type called P. infestans Karst. in 

 America is not, at least physiologically, quite identical with the Euro- 

 pean and Asian type. As it seems chiefly to attack Abies species, it 

 is proposed that this fungus for the time being be called P. infestans 

 var. abietis, as it was once called by Weir. 



The ascospores of P. infestans are hyaline, kidney-shaped, 16 to 23/x 

 long and 6 toS/x wide. 



The spores of the fungus disseminate in the autumn and germinate 

 as soon as the needles become embedded in snow so that the fungus 

 receives the required moisture. Snow blight attacks needles in all 

 ages, consequently also living needles on old windfalls on the ground. 

 Generally the dissemination takes places within a restricted area 

 which, however, can be increased considerably under favorable con- 

 ditions. During germination a grayish- white mycelium spreads on 

 the needles. 



The living conditions of the snow-blight fungus: Among the con- 

 ditions of life decisive for the development of the snow-blight fungus, 

 temperature is the most important one. A series of growth experi- 

 ments showed that the optimum temperature for the fungus' develop- 

 ment was +15° C. At a temperature of +20° C. the mycelium grows 

 at a considerably much lower rate than at +10° C, and at a con- 

 stant temperature of +25° C. it dies after a certain length of time. 

 Growth is still good at +5° C. and even at 0° C. Growth has been 

 demonstrated at a temperature as low as —5° C. (Bjorkman 1948) . 



The depth of snoio cover and the temperature in it. A snow cover 

 of great compactness — which occurs after the action of thaw — 

 conducts the cold better than a snow cover that has never been 

 affected by temperatures above zero. In small openings and small 

 cutting areas the depth of the snow cover is generally greater than 

 in large clearings. Close to the trunks of old seed trees the depth 

 of snow is always less than between the trees. 

 _ The snow-blight fungus may, with regard to temperature condi- 

 tions only, grow in snow up to a height of 1 to 2 dm., even during 

 the coldest period of midwinter. 



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