124S Skin Spot Disease of Potato Tubers. [mar., 



conditions, this does not take place naturally, so that in nature 

 the fungus does not, perhaps, readily gain access to the soil, 

 probably not until after the tubers decay. Should the soil be 

 suitable for the fungus, in all probabihty tubers planted the 

 following season would yield an infected crop. 



Economic Importance of the Disease. — Since Skin Spot is a 

 disease which develops in store and is not usually visible 

 until the early spring, its economic importance might be 

 expected to be slight. With regard to ware potatoes this is 

 correct, for, as its name implies, the diseased areas are for 

 the most part confined to the skin, and though the flesh of the 

 tuber is penetrated to a shght extent by the fungus, it entails 

 no appreciable waste in peeling. Beyond, therefore, rendering 

 the tubers unsightly, and thus somewhat reducing their market 

 value, this form of injury is not great. 



In the case of seed potatoes, however, Skin Spot is of con- 

 siderably more importance. When a crop in spring shows much 

 infection, or the spots on the tubers are closely crowded to- 

 gether, there is a great danger of the eyes being injured. If the 

 e\'es are unaffected the shoots will grow and the plants will 

 yield a normal healthy crop. This has been proved during 

 two consecutive seasons at Kew. It should be remembered, 

 however, that if the soil be suitable for the fungus it may 

 become badly contaminated with the fungns, and future 

 potato crops will be ■ hable to suffer. On the other hand, 

 where infection has taken place in the neighbourhood of the 

 eyes, the eyes are definitely injured and even killed. Ex- 

 amination of tubers affected at the eyes shows that the fungus 

 appears to penetrate, particularly deeply, the tissues around 

 the eyes, and that in many cases relatively large areas of 

 tissue may be killed. It has been observed by means of 

 sections that the fungus at times not only kills the tissue 

 around the eye, but also that immediately below it. 



Although the primary eyes may be killed, it sometimes 

 happens that secondary eyes may develop. Tubers of " King 

 Edward," most of the eyes of which had been killed by Oospora, 

 were placed in boxes to sprout in the usual way, and it was 

 found that of the injured and kiUed eyes about 45 per cent, 

 developed secondary eyes. Sections through some of these 

 showed that the new buds had pushed their way through the 

 dead tissue. This method of development has been described 

 by Goebel* These new buds appeared to be fairly strong and 



* Einleitung in die Experimentelle Morphologic der Pflanzen, 1908, 

 pp. 221-222. 



