26 Research Bulletin No. g 



be, yet there were many plants grown in such soil that showed 

 no infection whatsoever. Less success in producing wilt was 

 observed as the season progressed. It remains a question whether 

 this is due to a loss in vitality or to a gain in resistance in the plants, 

 due to a change in the soil, tubers, or the organism. 



Whether the success in producing wilt with F. trichothecioides 

 and the apparent waning of this power is due to a gain or regain of 

 virulency and a subsequent loss again is also an unanswered question. 

 Sherbakoff (33), working with Fusarium spp., got uniformly nega- 

 tive results so far as producing wilt is concerned, and concluded 

 that the results were due to a loss of virulency of the cultures or to 

 some other important factor that had escaped attention. 



Summary 



It is quite apparent that some of the strains of F. oxysporum can 

 cause tuber rot; that they can destroy tubers entirely without the 

 aid of other Fusarium spp. or bacteria; that at least one Fusarium 

 of the Discolor section (F. trichothecioides) can produce wilt of 

 stem; and that the biological contrast drawn by Wollenweber 

 between the Fusarium spp. is not as sharp as one would infer 

 from his paper. It is possible that these strains of Fusarium spp. 

 are morphologically identical with those described by Wollen- 

 weber, but physiologically unlike them. That this rule, if it 

 exists, is not so rigid generally, however, is noted by Sherbakoff 

 (33), who found that no correlation exists between morphological 

 relationship and pathogenicity. 



Although F. oxysporum is not absolutely unable to attack potato 

 parenchyma, the potato tuber, in which usually only the xylem 

 elements are invaded, enjoys an effective immunity from its attacks; 

 and although F. trichothecioides can attack any subterranean part 

 of the living potato plant, generally all parts excepting the mature 

 tuber enjoy an effective immunity from its attacks. 



The data given in the second part of this paper may furnish a 

 partial explanation of these phenomena. 



