76 Journal of Agricultural Research voi. xxii. No. 2 



At 30 per cent humidity the rotting took place in the same manner 

 as at 10 per cent, except that the cavity was larger and the rot extended 

 through about 50 per cent of the tuber (PI. 11, B). 



At 100 per cent humidity there was a total rot of all tubers, most of 

 the surface being covered with a white mycelial growth (PI. 11, C). 



The results of this experiment show very clearly that relative humidity 

 plays a very important part in determining the amount of rot produced 

 by Fusarium oxysporum. The only rot appearing at the low tempera- 

 ture of 12.5° C. was in an atmosphere of 100 per cent humidity. Even 

 at the high temperature of 25° complete rotting did not take place 

 at the lower humidities. A gradual increase in the amount of rot 

 corresponding to the increase in humidity was present in every case. 

 The fungus can live and sporulate at the lower temperatures and lower 

 humidities used in this experiment but apparently is not capable of 

 penetrating the tubers under these conditions. It can be safely con- 

 cluded that F. oxysporum under good storage conditions is not capable 

 of producing a tuber-rot of great importance. 



CONCLUSIONS 



(i) Fusarium oxysporum, F. trichochecioides , and F. radicicola are all 

 capable of producing a rot of the potato tuber. 



(2) In pure culture the amount of growth of all three species is nearly 

 equal at 25° C, Fusarium oxysporum and F. radicicola increasing in 

 growth up to 30°, where they produce their maximum growth. The 

 growth of F. trichochecioides decreases above 25°, until at 30° very little 

 or no growth takes place. It is more tolerant of the lower temperatures 

 than the other two species. 



(3) Preliminary tests with different liquid media would indicate that 

 the cardinal points for growth of these Fusaria vary to some extent 

 with the medium used. 



(4) Experimental infection of tubers was produced with all three 

 organisms under various conditions of temperature and relative humidit}'. 



(5) Preliminary tests with Fusariivm oxysporum indicated that the 

 relative hmnidity plays a very important part in determining the amount 

 of rotting. 



(6) In comparative tests with new and old tubers there is a distinct 

 difference in the amount of rotting under the same conditions. The 

 rotting was much more rapid and progressed much further in the old 

 than in the new tubers. 



(7) Comparative tests with all three species at controlled relative 

 humidities from i to 100 per cent and at controlled temperatures from 

 5° to 25° C. proved conclusively that — 



(a) A temperature of 25° C. is favorable for the production of a tuber 

 rot by Fusarium oxysportim, F. radicicola, and F. trichothecioides. 



