1920.] Skin Spot Disease of Potato Tubers. 1249 



their production affords an explanation of a phenomenon noted 

 when badly spotted tubers were planted at Kew, namely, that 

 all the tubers developed strong plants. On account of the 

 check at the start, however, the plants would be later, and the 

 yield reduced. Though these secondary eyes are undoubtedly 

 developed fairly frequently, many eyes are killed 'completely, 

 and if it can be avoided it is highly undesirable to plant badly 

 spotted tubers. It has been suggested that the development of 

 Skin Spot is favoured by bad ventilation during storage, but 

 from observations made in England last year it is clear that 

 Skin Spot is often entirely absent in very badly ventilated 

 clamps, and that it sometimes develops in pits where the 

 ventilation is at all events quite up to the average. 



The evidence at present available ' points to the fact that 

 Scotch seed is more affected than EngHsh seed, but the question 

 as to whether this is the result of more generally affected soil, 

 the nature of the season, or the methods of storage, must remain 

 over for a future investigation. 



Summary. — The disease of potato tubers known as Skin Spot 

 is caused by a fungus belonging to the Hyphomycetes. The 

 fungus is not Spicaria solani, Harting, as has been previously 

 thought, but a minute species hitherto undescribed, which 

 appears most suitably placed in the genus Oospora, and is de- 

 scribed as a new species 0. pustulans. The fungus grows readily 

 as a saprophyte in culture, including agar and gelatine media, 

 but more freely on sterilised vegetable tissues. The morpho- 

 logical characters of the fungus as seen in artificial cultures are 

 described in detail. 



Field inoculations with spores of the fungus were successful 

 in a certain number of cases in producing the spot on the tubers, 

 and the fungus was re-isolated from the affected tubers. 

 Further work on the life history of the fungus is required, as 

 it is not certain whether the tubers are infected in the soil only 

 or also during storage. The exact type of soil and the con- 

 ditions under which Skin Spot develops with special virulence 

 are not yet clear. 



With regard to the injury caused by the disease, the fungus 

 is confined to the surface layers, and is often sharply isolated 

 from the mass of the flesh by a layer of cork cells. Tubers, 

 however, may be so severely spotted as to be greatly disfigured 

 and to be depreciated in value. In addition to this it is shown 

 that previous suppositions with regard to the injury caused by 

 the fungus to the eyes were correct. When infection takes 

 place in the neighbourhood of the eyes these are weakened or 



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