June IS. 191S A New Disease of Picea Engelmanni 253 



All three of the foregoing species are truly alpine in habit and are not 

 usually found growing normally below an elevation of 5,000 or 6,000 feet. 

 This habit, of course, varies with the latitude. Moreover, they are very 

 common, and from the writer's experience they cause considerable 

 damage to alpine forests, particularly to the younger trees. Young 

 seedlings 4 to 8 years old have been found infected, and old trees fre- 

 quently succumb to their ravages. In some regions of the Northwest 

 on northern exposures, entire stands when composed mainly of even-aged 

 alpine fir are frequently so generally infected by H. nigra as to appear 

 ragged and bare. A sample acre taken on a north slope of Mt. Casey 

 (Selkirks, northern Idaho) at an elevation of 6,735 f^^t showed 95 per 

 cent of the alpine firs to be infected by this fungus. 



The influence of these fungi on their hosts is likewise discernible in the 

 gradual falling off of the annual increment. This causes a sharp con- 

 trast in the radial dimensions of the annual rings, even in the finely lay- 

 ered condition of the wood of alpine trees. The dense mat of brown or 

 black mycelium (PI. XXXIV, fig. 2) , often of sufficient thickness and extent 

 to completely spread over entire twigs, burying the leaves entirely in its 

 mass, is enabled to bring about certain phenomena of a very unusual 

 nature. It has been found by actual experiment that this mycelial mat 

 influences the temperature of the enveloped leaves in the same manner 

 as any dark covering acts on the bulb of an air thermometer. The fungus 

 acting as a pronounced epiphyte may thus be enabled through a slight 

 rise in temperature to incubate its own mycelia within the tissues of the 

 leaf and hence may hasten its parasitic activities. Probably the spread 

 of the epiphytic mycelium to leaves not infected internally at the time, 

 as sometimes occurs, is accompanied by reactions of a physiological 

 nature that are highly injurious and pave the way for the advance of the 

 mycelium into the tissues of the leaf. The spread of the mycelium over 

 young growth, from the time the snow disappears in the spring to early 

 fall, is fairly rapid. Branches of alpine fir 2 feet in length that were in- 

 fected at their tips in the early spring have had their entire leaf surface 

 destroyed by October of the following year. 



