5 9 8 



"Corky Scab" of Potatoes. [nov. 



one layer in thickness, generally hexagonal in optical section, 

 and varying from 3-5 to 4-5 p in diameter (Fig. 7). This 

 structure is a spore-ball, which on arriving at maturity is 

 liberated by the disintegration of the tissue of the host (Figs. 

 8 and 9). The spore-balls range in form from irregularly 

 spherical to oblong or ovate, and vary from 40 to 50 //, in 

 diameter. In colour they are reddish brown in the mass and 

 amber by transmitted light. The spore wall is sometimes 

 tinged an olive-green colour. 



Stained preparations of the mature cells formed from the 

 Plasmodium show a single nucleus in each cell. The contents 

 escape intact, so far as has been observed in the Kew cultures, 

 and are irregularly globose in form, with a few small projections. 

 They show a very sluggish movement for some time, after which 

 they become stationary, the nucleus then divides to form two 

 daughter nuclei, and fission of the mass follows. A repetition 

 of this process continues until a considerable quantity of amoe- 

 boid bodies are formed, which eventually coalesce to form 

 a plasmodium. The diameter of the amoeboid body after its 

 escape from the cell is about 3 /x. 



Professor Johnson considers that, in many instances at least, 

 the single nucleus originally present in ihe cell undergoes 

 division to form eight nuclei, and that eight swarm-spores 

 eventually escape from the cell. 



Geographical Distribution. — Spongospora is abundant, and 

 has caused a serious amount of damage in Great Britain, 

 Ireland, and Norway. It has been found in some quantity in 

 Germany. It is highly probable that " corky scab " may be 

 present in greater or less quantity wherever the potato is culti- 

 vated, and that it has been passed over as " scab " simply 

 without any attempt to discriminate between the various forms 

 of this disease. 



Preventive Measures. — It is certain that the parasite remains 

 in a diseased tuber in a passive state when conditions are un- 

 favourable for active vegetative growth. Diseased tubers, 

 therefore, should not be used for " sets," as the commencement 

 of activity on the part of the parasite is regulated by the 

 commencement of growth on the part of the potato, and a 

 scabbed crop will probably result, in addition to the risk of 

 infecting previously clean land, if diseased sets are employed. 



