1908.] 



"Corky Scab" of Potatoes. 



595 



pleted and the formation of the spores has commenced, the skin 

 of the raised blister is ruptured, and a dense mass of brownish 

 or snuff-coloured spore-balls is exposed. 



The further extension of the disease on a given tuber depends 

 mainly on the relative abundance of moisture present in the 

 soil, as has been proved by experiments made at Kew with 

 diseased tubers submitted for investigation. When potatoes 

 showing the disease in the stage indicated above are placed 

 in comparatively dry soil, wound-cork is quickly formed 

 immediately below the seat of injury, and the further develop- 

 ment of the parasite is arrested. On the other hand, if slightly 

 attacked tubers are placed in soil that is kept constantly wet, 

 the formation of wound-cork is arrested, and the continuous 

 development of the parasite is favoured in proportion. Large 

 cavities from half an inch to an inch deep are thus formed 

 owing to the vegetative portion of the parasite continually 

 eating into the substance of the tuber, and the cavity so pro- 

 duced becomes filled with a dense powdery mass of spore-balls. 



The presence of such masses of spore-balls suggests Spongo- 

 spora, but, as in the case of every kind of scab, microscopic 

 examination is absolutely necessary to ensure a correct deter- 

 mination of the specific nature of the disease. 



Owing to the absence of a mycelium, those portions of 

 a tuber not actually attacked by the parasite remain 

 perfectly sound, consequently partially diseased potatoes, 

 which are unsaleable, are frequently reserved for " sets." This 

 is a dangerous practice, as the parasite in the plasmodium 

 condition can remain in a passive state until the tuber 

 commences to sprout, when it may resume active growth and 

 so perpetuate the disease. 



The spore-balls produced in the cavities of the tubers repre- 

 sent the ultimate phase of development of the parasite, and may 

 be considered as the analogues of resting-spores in the fungi. 

 They become liberated by the decay of the cells of the tuber 

 in which they were originally formed, and may remain in the 

 soil in a passive condition until the following season. 



Morphology and Cytology. — If a section is taken through a 

 scab where the vegetative condition of the parasite is still in. 

 active growth, many phases of its development may be studied. 

 The earliest condition observed in a cell of the host consists of 



2 ? 2 



