299 



cultures from this tuber were taken in the usual way, and a 

 growth of typical Oospora pustulans was obtained. No definite 

 results were obtained from ik Secundus," nor. were any of the con- 

 trols affected. It should be noted that had it not been for the large 

 number o£^ tubers destroyed by bligdit an additional number of 

 successful inoculations would probably have been obtained. One 

 other set of experiments was conducted. Tubers of " Witch 

 Hill '* (an early variety) were, after lifting in late September, 

 sprayed with spore solution and placed in the cellar m damp 

 soil. Similarly treated, but unsprayed tubers, w r ere kept as 

 controls. At the end of three months the tubers were examined, 

 but neither then nor later in April, was there any development of 

 Skin Spot. It is desirable to repeat this experiment another 

 season as it is most important to ascertain whether potato tubers 

 may become infected after having been lifted and clamped. 

 Data which are being collected point to the greater suscep- 



of some varieties than others (among which may be 

 mentioned "King Edward/ 5 " Arran Chief," and "British 

 Queen ") and it is possible that the varieties used for inoculation 

 purposes were more resistant than many others. 



Possible Methods of Spread. 



The following two sets of experiments were conducted in order 

 to ascertain bv 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 

 which had been mixed peelings of diseased tubers. It was ascer- 

 tained a few weeks later that the fungus was growing out from the 

 inner side of the potato peelings, and was developing an abundant 

 supply of spores. On examination in the autumn the tubers 

 appeared clean, and in the following spring no spots were 

 manifest, except on one small tuber of May Queen." 



The second series was conducted 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 i ontaminating 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 during winter. It should be 

 noted, however, that though the blotches or pimples produced 

 by this fungus are sometimes ruptured under artificial 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 ail probability 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 wore potatoes 

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



