[Vol. 10 



10 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



hours. During this time the official minimum was — 28° C. and 

 the maximum — 6.7° C. Soil cultures similarly inoculated were 

 incubated at 28° C. The cultures exposed out of doors were 

 placed on ice upon being brought in, in order to prevent exces- 

 sively rapid thawing. Finally transfers were made to nutrient 

 broth, and agar slants from all the cultures included in the ex- 

 periment. The organism was recovered from all the cultures. 

 The subcultures made from soil which had been frozen for 24 

 hours appeared to be as vigorous as any. 



There are few satisfactory data on varietal resistance of pota- 

 toes to the blackleg pathogen. An early contribution to this 

 phase of the problem was made by Appel ('03), who concluded 

 that the thick-skinned, late varieties which were being grown in 

 Germany were more resistant to the disease than the thin- 



nned, starch-poor, early varieties. Morse ('17) reported that 

 the Irish Cobbler and Green Mountain varieties were particu- 

 larly susceptible to the disease. The Early Ohio, an early vari- 

 ety, is notably susceptible. The Russet Burbank and the Idaho 

 Rural, varieties widely grown in the West, are commonly found 

 affected with blackleg. 



sk 



THE ETIOLOGICAL AGENT 



The writer's studies on the etiology of the potato blackle 



disease were begu 



By the close 



of the summer of 1915 extended observation led to the assump 

 tion that the blackleg disease in Montana was quite similar to 

 that occurring in some of the eastern states. In the meantime 

 some 30 isolations were made at the Montana Experiment Sta- 

 tion from potatoes affected with blackleg. The pathogenicity 

 of the strains thus isolated was thoroughly tested for the second 

 time in 1016, and a majority was found to be pathogenic and 

 capable of causing typical symptoms of the disease. Compara- 

 tive studies of the morphological, cultural, and physiological 

 features of some of the above-mentioned strains were made by 

 the writer. The results obtained led to the conclusion that all 

 were essentially alike. An extension of these preliminary in- 

 vestigations was made, and cultures of the blackleg bacillus from 

 Maine, Minnesota, and one from eastern Canada received under 

 the name B. solanisaprus Harrison, were cultivated and compared 

 with one another and with observations made previously on the 

 Montana strains. While the observations made at this time were 



