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"INTERNAL RUST SPOT" DISEASE OF THE 

 POTATO TUBER. 



{Preliminary communication.) 



By SYDNEY G. PAINE. 



{Lecturer in Plant Bacteriology in the Department of Plant Physiology and 

 Pathology of the Imperial College of Science and Technology, London.) 



Through the courtesy of Mr A. D. Cotton of the Kew Gardens Patho- 

 logical Laboratory the writer has obtained from two farms in the South 

 of England specimens of potatoes suffering from a disease which had 

 been diagnosed as of bacterial origin. From one farm at Dunstable 

 the disease was so bad as to render the crop quite unsaleable, over 

 90 per cent, of the tubers being badly affected, and from the other farm 

 at Christchurch came the report that at least 33 per cent, of the tubers 

 had to be wholly rejected, so that it is safe to assume that upwards 

 of 50 per cent, were affected to some extent. 



The symptoms of the disease were thought at first to indicate Pseudo- 

 ntonas solanacearum as the cause. On further examination of diseased 

 tubers, however, appearances were discovered which were quite different 

 from those described by Smith (6) as characteristic of the attack of 

 this organism. There is, for instance, no appearance whatever of a milky 

 slime in the affected part of the tissue and the potatoes do not rot off 

 during the winter. The tuber remains perfectly hard and appears sound 

 except for a few brown patches on the skin and a certain appearance of 

 " scabbiness." Around some of the lenticels the skin is raised and 

 cracked, and on removal of this there is revealed a darkened sunken 

 area of corky tissue resembling somewhat the appearance of an old 

 Spongospora scab. 



On cutting a tuber discoloured patches of tissue of varying size are 

 found scattered over the surface. These are typically of a brownish-red 

 colour resembUng that of iron rust or, perhaps, more nearly that of 

 ginger-bread. They are distributed promiscuously through the tuber and 

 only occasionally seem to have developed in proximity to the vascular 



