6 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 
varieties by reddish or purplish tones, which the writer has not 
observed with Fusarium wilt. 
OCCURRENCE OF THE CAUSAL FUNGUS. 
In the stem. — The lower portions of the potato stems show a brown 
discoloration, which extends throughout the underground portion 
and for several centimeters in the aboveground stems. The brown 
color is by no means as pronounced as in cotton wilt or in the Verti- 
cillium wilt of potatoes, nor does it extend upward through the whole 
stem and branches, as in the two other wilts mentioned. The 
Fusarium conidia are not formed in such abundance on dying or dead 
stems as those of other wilt diseases. 
Microscopic examination shows the presence of mycelium in most 
of these browned stems, and cultures yield for the most part a single 
species of fungus {Fusarium oxysporum), though other Fusaria occa- 
sionally develop in advanced stages of wilt and bacteria as well, as 
might be expected in such moribund tissues. These other Fusaria 
have not been found to be uniformly associated with wilt, nor are 
they inhabitants of the vascular bundles, like F. oxysporum. A sharp 
distinction may be made between this typical and widespread wilt 
and the infrequent cases where other fungi which have entered through 
wounds or cracks have so injured the hypocotyl that a wilting of the 
foliage results. For example, Jamieson and Wollenweber (1912) 
produced a decay of potato stems followed by wilting of the foliage 
through inoculations with F. tricothecioides, but these writers do not 
believe or suggest that this fungus causes wilt in nature. 
The amount of fungus in the vessels of the stem and the degree of 
discoloration varies, but not always in proportion to the effect on the 
life of the plant. It is not uncommon to find prematurely dead hills 
in infected fields which show comparatively slight vascular browning, 
while others remain living, yet when examined they prove to have 
both stems and tubers heavily infected with Fusarium oxysporum. 
This apparent resistance may be explained by the fact that such hills 
are either accidental admixtures of later varieties, or bud sports, 
called " run-out hills." In either case they are plants that remain 
in an active vegetative condition longer and thus resist the effects 
of the wilt. Still other hills are to be found which remain healthy 
till the normal time for maturity and are also free from fungus infec- 
tion, thereby supporting the hope that resistant strains may be 
developed by selection. Unfortunately, the experiments have not 
yet demonstrated that these hopes can be realized, for all of the 
numerous selections made were attacked by wilt the following year. 
This work was done at Middle River, Cal., in 1909 and 1910, principally 
with the Burbank variety. 
