6 BULLETIN 64, V. S. DEPARTMENT OF AGEICULTUEE. 



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 \\dlt 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. 



IVIicroscopic examination shows the presence of myceUum in most 

 of these browned stems, and cultures yield for the most part a single 

 species of fungus (Fusarium oxysporum), though other Fusaria occar 

 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, hke 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 

 fohage results. For example, Jamieson and- Wollenweber (1912) 

 produced a decay of potato stems followed by wilting of the foHage 

 through inoculations with F. tricotTiecioides, 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 

 cUscoloration varies, but not always in proportion to the effect on the 

 life of the plant. It is not uncommon to fhid prematurely dead hills 

 in infected fields which show comparatively shght vascular browning, 

 while others remain hving, 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 liills." In either case they are plants that remain 

 in an active vegetative condition longer and thus resist the effects 

 of the wilt. StiU 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 

 5'et demonstrated that these hopes can be realized, for all of the 

 numerous selections made were attacked by wilt the following j^ear. 

 This work was done at Middle River, Cal., in 1909 and 1910, principally 

 with the Burbank variety. 



