Pethybridge — The Verticillium Disease of the Potato. 69 



finds its way into them, probably through the cortical portions of the young 

 roots. It was certainly to be expected that since the fungus was located 

 primarily in the vessels of the affected stalks and entered the new tubers, it 

 would pass in through the vessels, remain in them during the winter, and 

 pass out from them into the vessels of the new stalks in the spring ; but it is 

 quite clear that Eeinke and Berthold did not admit this view. 



It was particularly with a view of throwing light on this point that the 

 present investigation was started, although it was also desired to obtain 

 information on other phases of the disease and on the behaviour of the fungus 

 in pure culture. The account of the fresh investigations on the disease will, 

 therefore, now be proceeded with ; and the small amount of further literature 

 on it, published while the investigations were proceeding, will be dealt with 

 where necessary. 



IV. Localization of the Mycelium in Affected Tubers. 



It is a matter of no difficulty to trace by means of suitable sections the 

 passage of the mycelium from affected stalks through the rhizomes and into 

 the heel-ends of the new tubers. It passes exclusively through the wood vessels. 

 This has been studied, not merely by sections, but by suitable incubation of 

 portions of rhizomes as well as of tubers, proving that the fungus involved was 

 the species of Verticillium in question. 



All the tubers borne by an infected plant, however, do not necessarily 

 become infected. As a rule the fungus only reaches the older and larger 

 tubers, while the smaller and younger ones often remain free from invasion. 

 Here it may be stated that the tubers which do not become infected with the 

 mycelium of the fungus produce, when planted, absolutely healthy plants, 

 and their progeny continues to do likewise. Hence, by separating out the 

 non-infected tubers (by the method to be described subsequently) from a 

 mixed stock of seed-tubers derived from affected plants, and by employing 

 them as seed-tubers, it has been possible to raise a new stock free from 

 the disease. 



As has already been pointed out, Eeinke and Berthold maintain that when 

 the fungus reaches the heel-end of a new tuber it remains strictly localized 

 there. Spieckermann 1 also states that at the time of digging, the mycelium 

 is chiefly confined to the heel-end of the tuber, and seldom proceeds further 

 or reaches the rose-end. He believes that during the winter resting period of 

 the tuber the mycelium does not make any further progress, since even in 

 spring its growth has not advanced, and the sprouts are, therefore, all viable. 



1 Spieckermann, A., Beitrage zur Kenntniss der Bakterien- ring- und Blattrollkrank- 

 heiten der Kartoffelpflanze. Jahresber. d. Ver. f. angew. Bot., viii, 1911, p. 1. 



