Pathogenicity of the Chestnut Bark Disease 55 



perature of 15.5 C., and distinctly less rapidly at temperatures 

 5 degrees above or below this point. 



The ascospores are not as exacting in their temperature 

 requirements as the conidia. They germinate best at a tempera- 

 ture of about 21 C. Even at 3 C., 25 per cent of the ascospores 

 germinated in the first 24 hours, and in three days 70 per cent 

 had germinated. They germinate readily after moderate freez- 

 ing. These facts indicate that the ascospores are more impor- 

 tant than the conidia in causing infection under conditions result- 

 ing from a considerable range in temperature. 



Growth of Mycelium. 



The most rapid early growth occurs at the optimum tem- 

 perature for germination. In a nutrient solution of boiled chest- 

 nut bark, the ascospores will, in the first 24 hours at 21 C., send 

 out hyphae ten to fifteen times the spore length, forming a large 

 mass of mycelium in two days. At 3 C. the growth is much 

 slower, being only about one spore length the first day and fifteen 

 times this in five days. From field observations, also, it appears 

 that the rate of growth of the mycelium depends on the amount 

 of moisture and the temperature. 



In December, 1911, specimens were received for identifi- 

 cation in which the flocculent mycelium of the fungus, until then 

 prevalent, had disappeared. At this time many pustules on the 

 outside of the bark, which had until then been actively producing 

 threads of summer spores, dropped from the bark. Both of thes"e 

 phenomena were probably caused by the sudden severe frosts 

 which occurred at that time. The bright yellow growth of my- 

 celium so common in the autumn of 1911 had not appeared on 

 any of the specimens received up to June 1, 1912. 



Vitality and Longevity of the Fuuyns. 



A detailed examination was made in August, 1912, of 262 

 stumps of infected trees which had been cut and charred during 

 March and April, 1911, on the Pennypacker State Forest Reserve, 

 in Perry County, Pa. Of these 15 per cent showed pustules of 

 the fungus on the inside of the bark and on the charred wood. 

 It was rather remarkable that only one of these infected stumps 

 showed the presence of "spore horns". Between 50 and 60 per 

 cent of the sprouts springing from infected stumps showed evi- 

 dent cankers, while less than 1 per cent of the sprouts from un- 

 infected stumps showed evidence of the blight. In nearly every 

 case the stumps showed infection at or near the surface of the 

 ground. Over 50 per cent of the stubs of sprouts and small 



