Nov. 1. 1915 Potato Tuber-Rots Caused by Fusarium Spp. 185 



author is convinced that in many cases the only sure way to determine 

 the cause is by cultural studies. In general, specimens of the types of 

 rot developed spontaneously in the field or storage are more character- 

 istic than those produced by inoculation and developed under uniform 

 conditions. 



The powdery dry-rot with pink-mycelium-lined cavities caused by F. 

 trichothecioides is quite characteristic and not easily confused with the 

 others; the same is true of the rot produced by F. discolor, var. sulphu- 

 reum,\with its ocherous yellow mycelium, but the rot caused by F. coeruleum, 

 in its typical form with external dark-blue mycelium masses and internal 

 blue coloration of the tissues, may be easily confused with some of those 

 herein described unless mature spores are found on the specimen or high 

 cultures are obtained. On some tubers more than one of the wound- 

 parasitic types of Fusarium are present; in others, the diagnosis is com- 

 plicated by the secondary action of bacterial and fungous saprophytes. 

 While the author can in typical cases determine the cause of Fusarium 

 rot without the preparation of cultures, the latter is not infrequently the 

 safer method. Our inability to differentiate surely the various rots ma- 

 croscopically complicates the attempt to differentiate them as types 

 caused by specific organisms. 



METHOD OF TESTING PARASITISM 



The method employed to demonstrate the wound-parasitic nature of 

 species of Fusarium will be outlined in detail before proceeding with the 

 discussion of the several types of tuber-rot and the inoculations with the 

 causal organisms. 



Sound tubers as free from skin diseases as possible were selected from 

 the following varieties of potatoes: Burbank, Netted Gem, Early Rose, 

 Idaho Rural, Jersey Peachblow, People's, and Pearl grown at Jerome, 

 Idaho, in 191 3 and 191 4 and each year kept in cold storage at Washing- 

 ton, D. C, until needed; Irish Cobbler grown in Maine in 1913 and kept 

 in storage through the winter; Green Mountain grown at Arlington, Va., 

 in 1 91 4 and used soon after harvesting. 



The selected tubers were washed and disinfected in a solution of 0.5 

 per cent of formalin, in the majority of the experiments for half an hour, 

 and rinsed in distilled water. Some tubers taken at random were 

 wounded with a large platinum needle, dipped in distilled water, imme- 

 diately wrapped in waxed paper, and placed in disinfected Altmann incu- 

 bators. Other tubers were similarly wounded, dipped in distilled-water 

 spore suspensions of the organism to be tested, wrapped, and placed 

 with the controls. 



By this method there are chances for secondary invaders, but the used 

 organism is primarily the predominating orfe. In addition to the con- 

 trol tubei'S, in every case reisolation, identification in pure culture, and 



