298 



some 



ica 



the general failure of this experiment was probably partly due to 

 the high temperature of the incubator. The other set of experi- 

 ments in the laboratory was made at ordinary room temperature. 

 A few tubers inoculated in a similar way to the last and at the 

 same time, were placed in a dark cupboard immediately after 

 inoculation. On examining again after 

 tubers was observed to be covered with a low-growing white 

 mould. The mould proved to be Oospora pustulans. Later 

 examination of the same tubers showed that the white growth had 

 disappeared and its only effect on the tuber was a large brown 

 patch on the skin. The discoloration did not penetrate through 

 the skin to the flesh, and there were no signs of the development 

 of the normal " Skin Spot." This es^ 

 under very damp and warm conditions the fungus might possibly 

 spread in the clamp without necessarily infecting the tubers, and 

 that certain obscure markings on the skin of potato tubers might 

 be due to injuries caused by Oospora pustulans. 



Experiments were also carried out on living plants during the 

 growing season, June-August. Inoculations were made on young 

 tubers of potato plants growing in the experimental ground at 

 Kew. In the first set "Midlothian Early" was selected, and 

 the inoculations were made at the beginning of June. The soil 

 was drawn away from the plants until a number of young tubers 

 of different sizes were exposed, and the tubers were sprayed with 

 a concentrated solution of spores of Oospora pustulans in sterile 

 water. The soil was then replaced and the plants marked, and 

 a similar number of other plants selected as controls. On 

 lifting the plants in late summer no sign of Skin Spot was present, 

 and examination during the winter gave a similar result. It 

 may be noted that there are as yet no records of early varieties 

 being attacked by this disease, though this may be partly 

 accounted for by their earlier consumption. 



The remaining cases inoculated were the late varieties 

 "Provost," "The Ally," and " Secundus." The inoculations 

 were made at the end of July, but that date appeared to be some- 

 what early, as few tubers had developed. About 4-8 young tubers 

 were inoculated on about 6-8 plants of each variety, and a similar 

 number of unmoculated plants were regarded as controls. The 

 weather at that time was hot and dry and 

 after inoculation the plants were watered. The dry weather, 

 however, continued for some time and it may perhaps account for 

 the small amount of infection that resulted. At the time of 

 lifting no trace of Skin Spot in any of the three varieties was 

 evident. Much blight was present, and a large number of the 

 tubers had to be discarded. The remainder were stored in a 

 cellar and examined at intervals. No very definite spots 

 developed during the winter. Early in April a few positive 

 results were obtained in the case of both " The Ally" and 



Provost.' 5 The number of tubers that developed, the typical 

 Skin Spot was in each case small, and the infection slight except 

 in the case of one tuber of the "Provost." In this case the 

 spots were crowded together into a circular area as is often the 

 case in normal infection. To confirm the identity of the parasite, 



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