Pethvbeidge — The Verticillium Disease of the Potato. 71 



which the old sets had not rotted, but were fairly well preserved, the fungus 

 was found to be present in their now strongly browned woody tissues. 



From this experiment it seems safe to conclude that the fungus was 

 present in the autumn in the original tuber, not merely localized at the heel- 

 end, but also bA/ a distance from it at least greater than one-half of the 

 length of the tuber. 



The question of the exact location of the fungus within the tuber is an 

 important one from the practical point of view. For, if it is strictly confined 

 to a small area near the heel-end of the tuber in the autumn, and does not 

 progress further during the winter, it should be possible to get rid of the 

 disease by merely cutting off the comparatively small portions of the heel- 

 ends of affected tubers before using them for seed. This point was, therefore, 

 gone into in further detail. 



A preliminary experiment was carried out in 1910 with tubers derived 

 from affected plants. Small portions of the heel-ends were first cut off, in 

 order to see whether the vascular ring was browned or not. The tubers were 

 then divided into two groups : (a) those with no browning, hence apparently 

 healthy ; and (5) those showing browning, hence presumably diseased. The 

 tubers in group (b) were then further subdivided. From the heel-ends of 

 one half of them the tissues were cut away until the browning of the vascular 

 ring was no longer visible to the naked eye, while to those of the other 

 half nothing further was done. By this means the attempt was made 

 to divide the tubers into the following three classes : — (1) healthy ; 

 (2) primarily diseased, but rendered healthy by cutting away the diseased 

 tissue ; (3) diseased. 



The tubers were planted under field conditions at a time when it was 

 unfortunately impossible, owing to the pressure of other work, to devote as 

 much attention to the behaviour of the resulting plants individually as was 

 desirable. Further, a severe attack of blight, combined with another of the 

 sclerotium disease, seriously interfered with the success of the experiment. 

 Nevertheless, taking as a criterion the amount of rolling in the leaflets 

 exhibited before these diseases gained the upper hand, it was possible to see 

 that the plants derived from the tubers of Class 3 (diseased) were decidedly 

 the worst, while there was not any clear difference between those derived 

 from the tubers of Classes 1 and 2. Plants with Verticillium in the vessels of 

 their stems were found, however, in all three classes, but such plants did not 

 appear to predominate in one class more than in the others. 



The experiment, although leaving much to be desired, showed at least 

 that no reliance could be placed on naked-eye examination of the cut ends of 

 tubers as a means of discriminating between those containing the fungus and 



SOIENT. PBOC. K.D.S., VOL. XV., NO. VII. M 



