Fusarium in Tuber Rot and Wilt of Potato 27 



II. Ecology and physiology of the organisms 



METHOD AND DATA 



i. Temperature relations. — Observations of cultures grown 

 at ordinary temperatures showed that there is a striking difference 

 in the rate of growth of the two organisms. Potato cylinder, rice, 

 liquid potato, and glucose media, and glucose and potato agar cul- 

 tures all showed that F. oxysporum makes a considerably greater 

 initial growth at temperatures above 20 C. than does F. trichotheci- 

 oides. At temperatures in the vicinity of 10-15 C, however, 

 F. trichothecioides makes the greater initial growth, although these 

 temperatures lie below its optimum. The same difference was noted 

 in cultures on neutral and acid potato agar. This point was also 

 tested with cultures on sterile slabs of potato tubers kept in Petri 

 dishes. At 25 C. F. oxysporum covered such slabs completely 

 when F. trichothecioides barely had made a start, while at 12 C. 

 the situation was reversed. 



When 1 per cent liquid glucose media were inoculated with spore 

 suspensions of F. oxysporum, visible growth was made in 16 hours; 

 when F. trichothecioides was used, 30-42 hours elapsed before visible 

 growth was made. This holds for temperatures above 20 C. The 

 optimum temperature for F. oxysporum was about 30 C, and for 

 F. trichothecioides about 20-22 C, both varying slightly with^the 

 medium used. The maximum for F. oxysporum lay between 38 and 

 40 C. The optima and maxima were higher for cultures in potato 

 extract than for glucose media cultures. The writer has not been 

 able to determine the minima accurately because of inadequate 

 apparatus. Humphrey (15) gives 4 C. as the minimum growth 

 temperature for a certain strain of F. oxysporum. 



Potato agar cultures of F. oxysporum and F. trichothecioides 

 could endure a temperature of 40 C. for 5 and for 20 hours respec- 

 tively and remain viable. Exposure at 50 C. for 5 hours killed 

 F. trichothecioides, but not F. oxysporum; while exposure for 20 

 hours killed both. Some F. oxysporum cultures survived 5 hours 

 exposure at 57° C. 



The growth relations were also checked up quantitatively. 

 In these experiments, as well as in all the following ones, the method 



