Nov. 1. 1915 Potato Tuber-Rots Caused by Fusarium Spp. 199 



than the parenchyma and are darker in color, sometimes almost black. 

 In this stage the condition might be mistaken, and probably has been in 

 the past, for stem-end ring disease caused by F. oxysporum or Veriicilliiim 

 albo-airum, or for one phase of net necrosis (10, p. 14), which it more closely 

 resembles. By the culture method, however, a species of Fusarium is 

 invariably obtained from such tubers at the border of diseased and healthy 

 tissues. The name "Fusarium eumariii" is proposed for this fungus. 



In the more advanced stages of rot caused by F. eumariii the end of the 

 tuber or the entire tuber is involved (PI. XVIII). According to the humid- 

 ity and other environmental conditions, the rot is (i) soft and light-brown or 

 (2) dry, corky to friable, and dark-brown to almost black. In general, the 

 rot proceeds uniformly as a sharply dififerentiated layer easily removable 

 when moist, but close-clinging when dry. In field material the bundles 

 are often discolored as above noted, in advance of the rot. Attempts to 

 isolate the organism from the tips of such bundles usually failed. In the 

 experiments the rot is preceded by a moist water-soaked zone of enzymic 

 activity, from the border of which no organism was obtained. No diffi- 

 culty was experienced in isolating F. eumariii from the border of the dis- 

 colored tissue and the watery zone. 



Considerable care is necessary to differentiate this rot from the one 

 caused by the closely related F. radicicola. Sometimes the determination 

 is to be decided only by the careful preparation and study of high cul- 

 tures. The morphological differences between F. eumariii and F. radi- 

 cicola are discussed on page 205. 



F. eumariii is chiefly a stem-end and wound invader, but under favor- 

 able conditions the lenticels become infected. The fact that F. oxysporum 

 was sometimes obtained in association with this fungus and the further 

 fact that this disease of the tubers is reported on plants described as 

 having symptoms of wilt suggest the probable relationship of F. oxy- 

 sporum to the trouble. A field study of wilt and the relation of F. 

 oxysporum to such field rots and storage rots should throw considerable 

 light on the problem. 



Attempts to isolate an organism from a type of stem-end necrosis 

 similar to mild cases of invasion with F. eumariii often failed. There 

 seems to be a sterile necrosis of the stem end, accompanied by browning 

 of the parenchyma and bundles, which is related to the disease described 

 as net necrosis (10, p. 14, pi. 2). Sometimes this type of stem-end necrosis 

 can be distinguished from slight infection with F. eumariii only by the 

 culture method ; but when the minute ramifications of the vascular ducts 

 are discolored, resulting in the characteristic phase of net necrosis, it can 

 not be confused with the new type of rot. 



This rot was obtained chiefly in Pennsylvania, the following localities 

 representing its known distribution: Tower City and Orwigsburg, 

 Schuylkill County, Pa.; East Greenville, Montgomery County, Euclid, 



