1920.] Skin Spot Disease of Potato Tubers. 1247 



The starch grains in the invaded cells entirely disappear. The 

 progress of the fungus is apparently retarded by the cuti- 

 cularisation of the cell walls, but attempts are also sometimes 

 made on the part of the potato to form dehnite layers 

 of cork below the affected part. In some cases successive 

 layers of cork are formed. Early in April, when the fungus 

 appears to be most vigorous, the hyphae in the diseased cells 

 are very numerous and for the most part hyaline ; but later 

 in the season, if similar areas are examined, very few hyph^ 

 are found, and these are mostly brown, probably as the result 

 of age. Judging from the smallness of the area which results 

 from each infection, and from the fact that after a certain 

 period of time the hyph^, instead of penetrating to fresh cells, 

 gradually die, the fungus can be regarded as only a very weak 

 parasite on the potato tuber. 



Pure Cultures of the Fungous. — In the article pubHshed in the 

 Kew Bulletin there followed here an account of pure cultures 

 of the fungus in the Laboratory and a discussion as to its 

 systematic position. The fungus is named Oospora pustulans, 

 Owen and Wakefield, and a technical description is suppUed. 

 An account is also given of the inoculation experiments carried 

 out at Kew, where potato tubers were successfully inoculated 

 with portions of the fungus grown in pure culture. 



Possible Methods of Spread. — The following two sets of experi- 

 ments were conducted in order to ascertain by what means the 

 disease is carried from one season to another, and its method 

 of spread in the soil. 



The first set was carried out in pots. Clean tubers of " Mid- 

 lothian Early," " Witch Hill," " British Queen," and " May 

 Queen " were planted singly, at the end of April, in soil with 

 v/hich had been mixed peelings of diseased tubers. It was 

 ascertained a few weeks later that the fungus was growing 

 out from the inner side of the potato peeUngs, and was develop- 

 ing an abundant supply of spores. It is obvious, therefore, 

 that the soil could easily be infected in this way. 



The second series was coniucted on the experimental ground 

 at Kew, in a plot where the soil is a sticky silt. In order to 

 test the possibility of the plants contaminating the soil a number 

 of very badly infected tubers were planted in spring. The 

 results showed that very little spread appears to take place during 

 the first season, as on lifting in autumn, the crop was perfectly 

 clean and no spots developed on the tubers durhig winter. It 

 should be noted, however, that though the blotches or pimples 

 produced by this fungus are sometimes ruptured under artificial 



