301 



lated 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 English seed, but the question as to 

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

 nature of the sea-son, or the methods of storage, must remain over 



for a future investigation. 



Summary. 



It is shown in the present paper that the disease of potato tubers 

 known as Skin Spot is caused by a fungus belonging to the Hypho- 

 mycetes. The fungus is not Spicaria solani Harting as has been 

 previously thought, but a minute species hitherto un described, 

 which appears most suitably placed in the genus Oospora, and is 

 described as a new species 0. pustvlam, Owen and Wakefield. 

 The fungus grows readily as a saprophyte in culture, including 

 agar and gelatine media, but more freely on sterilised vegetable 

 tissues. The morphological 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, 



^ is 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 conditions 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 neighbour- 

 hood of the eyes these are weakened or killed. In spite of the fact 

 that in some cases secondary eyes are developed, seed tubers may 

 undoubtedly be very seriously injured, when the fungus is present 

 in quantity. The only preventive measure which can be recom- 

 mended at present is to avoid planting diseased tubers. If badly 

 affected tubers are planted, not only are " misses V likely to occur, 

 but the soil is likely to be seriously affected with Skin Spot 



fungus. 



Incidentally it is shown that the original description and 

 figures of Spicaria solani on which Halting first founded the 

 genus are at variance with the commonly accepted idea of the 



genus as framed by llarz. 



In conclusion I should like to record my indebtedness to 

 Mr. A. D. Cotton for his help and criticism during the course of 



this investigation. 



I am also indebted to Mr, F. T. ^ Brooks and Miss A. D. 

 Mackenzie, who commenced a similar investigation at the Food 

 Investigation Department' of the Bonrd of Agriculture, but who 

 discontinued the work on learning that it was being carried out 

 at Eew with better facilities. 



