24 Research Bulletin No. g 



with many small tubers, wilting down suddenly after the last 

 irrigation. The frequent occurrence of aerial tubers, the prevalence 

 of excessive numbers of small tubers, and the occurrence of few 

 abnormally large tubers on such plants is also attributable to the 

 insidious manner of attack. The organisms attack the stolons and 

 main stem as well as the roots. Stolons with tubers in all stages of 

 maturity can be found partially or completely cut off by lesions. 

 As the balance between the photosynthetic and storage centers in 

 such plants is disturbed, new stolons are developed nearer and 

 nearer the surface and the stolons that are not attacked develop 

 abnormally large tubers. Often the plant responds to this disturb- 

 ance in the assimilation-storage balance by producing swellings of 

 the aerial parts of the plant, the so-called aerial tubers. Many 

 large plants can be pulled up with ease, because lesions make separa- 

 tion of the tops from the roots or even the basal portion of the stem 

 easy. Such plants may show a comparatively sound main axis 

 (fig. 10). 



Infection carried over by the mother tuber, which is frequent, 

 rarely permits the growth of stems more than 20 cm. high, and 

 seldom allows the development of tubers. An early attack from 

 without upon the main stem leads to equally disastrous results. 



The wilts of the potato plant induced by Fusarium spp. have 

 generally been considered vascular mycoses due to a clogging of the 

 vascular elements. In fact, however, the symptoms are due to 

 killing of the root system as much as to clogging of the vascular 

 elements. It is true that members of the Elegans section, such 

 as F. oxysporum, frequent the vascular elements, spreading in these 

 rather than clogging them, but it is true also that they destroy roots 

 in numbers. Again, even though some have referred to this disease 

 as a root disease (Smith and Swingle 35), it is stated that the 

 fungus enters a root, then spreads to the stelar part, and from there 

 enters other roots and stolons. Just as much damage is done by the 

 persistent attack from without upon roots and stolons, as noted 

 by Manns (24). 



In the course of these experiments several questions were 

 raised. The soil- in these experiments surely was more severely 

 infected with the organisms than soil under field conditions can 



