May 29, 1916 Fieldrot of Potato Tubers 305 



ent of F. radicicola. F. oxysporum has never been found in connection 

 with the stem-end blackrot of western potatoes to the exclusion of F. 

 radicicola. 



Fully 50 per cent of all cultures made from the decayed cortical and 

 medullary tissues of tubers infected with stem-end and lenticel rot have 

 remained sterile. This may have been due to improper cultural condi- 

 tions, but it is believed that the discoloration of the tuber tissue often 

 extends some distance beyond the point actually reached by the invading 

 fungus. Stem-end blackrot-infected tubers often show a black net 

 necrosis. Isolations made from the black network of bundles, if made 

 some distance below the stem end, often fail to reveal any fungus. On 

 the other hand, many such cultures have revealed F. radicicola, and 

 occasionally both F. radicicola and F. oxysporum. That F. radicicola is 

 capable of causing the blackened net, as well as the stem-end blackrot, 

 is fully demonstrated by the results of artificial inoculations PI. (XXXVI, 

 fig. 7, and PI. XXXVII, fig. 6, 8), though the fungus may not always be 

 present throughout the entire length of the blackened bundle area. 



JELLY-END ROT 



Whenever the inoculation of Netted Gem tubers took effect at the stem 

 end, an infection typical of certain types of jelly-end rot found in the 

 commercial fields was produced. In the moist chamber under laboratory 

 conditions infections at the stem end induced by puncturing the tubers, 

 by spraying with a spore suspension, or by puncture of the stolons with 

 an inoculating needle were fairly typical of the advanced stages of jelly- 

 end rot, being soft and watery. Under field conditions, infections at the 

 stem end induced by spraying the tubers with the spore suspension, by 

 puncturing with an inoculating needle, or by the inoculation of the stolons 

 were in no case as pronounced as the infections found occurring naturally 

 in the field. Those induced by a puncture at the stem end were deeper 

 than those produced by the other methods. 



The failure of the inoculations in the field to develop as severe cases of 

 infection as those occurring in nature may be attributed to the late date 

 on which the inoculations were made and to the very dry condition of the 

 soil. Aside from the depth of the infection at the stem end, the stem-end 

 decays induced by artificial inoculation were very similar in appearance 

 to infections found occurring naturally in commercial fields of Netted 

 Gem potatoes. Wherever the attempt was made, F. radicicola was 

 recovered from the stem-end infections induced by the inoculations. It 

 is evident, therefore, that F. radicicola is capable of producing a jelly- 

 end rot of the potato tuber. However, isolations made from such rotted 

 tubers taken from the field have not always revealed F. radicicola to the 

 exclusion of other fungi. F. oxysporum is frequently obtained. 



