INVESTIGATIONS ON THE CONTROL OF DISEASE IN PLANTS. 32 1 



Biffen's experience lead him to suppose that there was any physiological 

 relationship between such defects and the power to resist diseases ? 



Mr. Arthur Sutton said he was very glad of the opportunity 

 of saying a few words, more especially with regard to the diseases of 

 the potato, which the lecturer had so ably dealt with. 



Referring to the question whether the spores of Phytophthora 

 affected the potato plant directly through the tubers or through 

 the leaves, Mr. Sutton called attention to the well-known fact that 

 it is quite easy to grow a healthy crop of potatos from seed-tubers 

 which were themselves very badly affected with Phytophtlwra, and 

 in fact so long as there were some remaining eyes in a seed-tuber 

 which were sound there was nothing to prevent a healthy crop being 

 grown, provided the climatic conditions in the ensuing season did 

 not favour the spread of Phytophthora. 



Mr. Sutton also mentioned that healthy crops of potatos were 

 frequently grown on land which had produced a very badly diseased 

 crop of potatos in the previous season, and the conclusion to be drawn 

 seemed to be that the spores of Phytophthora only became a source 

 of danger when the climatic conditions were favourable to their 

 growth. In some parts of Ayrshire remarkably heavy crops of 

 early potatos were grown on land where potatos had been grown 

 successively for forty years — sometimes the crops being affected with 

 disease and sometimes not, according to the character of the season. 



Referring to the question dealt with by the lecturer, as to whether 

 varieties of potatos which were immune to the attacks of Phytophthora 

 could be considered in any respect " inferior " to others which 

 succumbed to the disease, Mr. Sutton stated that in his experience 

 many thousands of seedling potatos were discarded because in some 

 character or other they were " inferior " to others already in commerce. 

 It frequently happened that seedlings which for several years showed 

 immunity to Phytophthora were far less productive than others, 

 or were in other respects " inferior/' possibly in quality, or in shape, 

 or in colour. 



The general conclusion which cultivators seemed to draw was 

 that it paid better to grow a very productive variety of good or fair 

 quality, taking the uncertain risk of Phytophthora, rather than to grow 

 sorts which would probably be immune but which would certainly 

 yield a relatively small crop. The most immune variety of potato 

 Mr. Sutton has ever known was one which never yielded more than 

 half the crop of the well-known ' Up-to-Date.' On the other hand, 

 the old favourite, ' Magnum Bonum/ was not only relatively immune 

 for a great number of years, but was also very productive and of fair 

 table quality. 



The lecturer had not referred to the Wart Disease (Synchytrium 

 endobioticum) . This, up to the present, has only appeared in certain 

 districts of the country, but the number of outbreaks is increasing 

 year by year, and when the disease does appear it is far more destructive 

 than Phytophthora ; not only are the tubers rendered absolutely 



