26 
Research Bulletin No. p 
infected with the organisms than soil under held conditions can 
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 of the organism 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. trichothecioidcs 
and the apparent waning of this power is due to a gain or regain of 
\ irulency and a subsequent loss again is also an unansw-ered question. 
SiiFRBAKOFF (33), workiug 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 w^ilt of 
stem; and that the biological contrast drawn by Wollenwebek 
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 AA'ollex- 
WEBEK, 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 altliough F. trichothecioides can attack any subterranean part 
of the living potato plant, generally all parts excepting the mature 
tuber enjoy an eft'ective immunity from its attacks. 
The data given in the second part of this paper may furnish a 
partial explanation of these phenomena. 
