Pethybridge — The Verticillium Disease of the Potato. 75 



tubers as it does in the stalks, and the black sclerotial form of the mycelium 

 has not been found in the tubers. It is true that affected tubers, when 

 kept unplanted till the late summer, often show, when cut across, a strong 

 blackening in the region of the vascular ring ; but microscopic examination 

 shows that this blackening is not due to the presence of mycelium, but to 

 some colouring matter produced in the cells in this region. 



From what has been said, it seems clear that the mycelium of the fungus 

 which is present in the tubers produced by affected plants— and very few of 

 the tubers produced by such plants are free from it — is located in the wood 

 vessels of the vascular tissues and is confined to them. But it is by no means 

 necessarily localized at the heel-end of the tuber, and may probably spread 

 slowly in the vessels of the tuber during winter storage. Judging from the 

 varying amount of aerial growth produced at the cut surfaces of the vessels 

 of affected tubers when suitably incubated, it seems likely that the amount of 

 mycelium present in such tubers may vary considerably, and that thus one 

 tuber may be more strongly infected than another. 



It is quite conceivable — although it has not definitely been proved — 

 that the strongly infected tubers give rise to plants which show the disease 

 early and soon die off, and that the less strongly infected ones produce plants 

 which attain more normal size, and which do not show symptoms of the 

 disease until considerably later. Finally, the fact that affected tubers 

 occasionally produce healthy plants may be explained by the original 

 infection of the tuber being so slight that the fungus was unable to reach the 

 plant developing from such a tuber before the conclusion of its growth. 



V. Production of Diseased Plants from Affected Tubers. 



According to Eeinke and Berthold, when affected tubers are planted, the 

 fungus grows over the outside of the tuber in the cork cells, and so reaches 

 the young sprouts through the cortical portions of the young roots. This idea 

 was based chiefly on the fact that Verticillium was found growing on the skin 

 of the tubers as well as in the cortical portions of roots and young stems. It 

 does not follow, however, that the Verticillium found in such situations was in 

 reality V. albo-atrum, and I am of opinion that Eeinke and Berthold were 

 misled by an assumption of this kind. I have found two apparently new 

 species of Verticillium (which will be described in detail in another paper) 

 which occur on potato tubers, and which in the conidial form might easily 

 be mistaken for V. albo-atrum, but which, when grown in pure culture, are 

 clearly quite different from it and from each other. Neither of these species is 

 capable of producing a disease of any kind in the potato ; they are both pure 

 saprophytes. 



