CONTRIBUTIONS FROM THE WISLEY LABORATORY. 613 



Apart from the absence of disease, the yield from diseased sets 

 was exceedingly poor. Only one-third of the sets produced tuber- 

 bearing plants. The produce from these is scheduled in Table VII. 



Discussion of Results. 



1. Planting diseased sets. 



In previous experiments carried out in the North of England, 

 the crop resulting from planting diseased tubers proved an almost 

 negligible quantity. The more favourable cultural conditions obtain- 

 ing at Wisley undoubtedly tended to make the best of the very worst- 

 conditioned tubers. Even then a large proportion of sets failed to 

 grow, but several of them were much diseased, and hence considerably 

 weakened when planted. If only slightly and moderately diseased 

 sets had been used it is probable that a greater percentage of plants 

 would have developed. 



In 1912 streak appeared in the produce from diseased sets, as in 

 previous years, and about ten per cent, of the tubers were affected. 

 But in 1913, for the first time in my experience, not a single case of 

 streak occurred. 



Messrs. Sutton, in their Report on Potato Experiments for 1906, 

 state with regard to internal disease that the " proportion of diseased 

 tubers in this year's crop does not appear to be in any way influenced by 

 the presence or absence of disease in the tubers planted." * This state- 

 ment involves the issue that a diseased crop might result notwith- 

 standing planting sound, healthy sets — the happening I experienced, 

 in 191 1, in Scotland, with streak, on land that had frequently borne 

 crops affected with streak. Again, Messrs. Sutton have pointed out 

 to me on more than one occasion that a sound crop may be obtained 

 even when diseased tubers have been used for " seed " — this is quite 

 clear to me from the present year's experience. It is certain that the 

 diseased " seed " used in Messrs. Sutton's experiments was of stronger 

 tone than that used here, and, in a favourable season, might yield 

 an excellent result. 



2. Planting streak-free sets (half- tuber) obtained from tubers 

 raised in Scotland on land upon which streak appeared in 1911. 



Out of 1018 tubers (lots B and D) only 13 were affected with streak, 

 whilst there were 45 cases among 358 tubers yielded by diseased 

 sets. Here the importance of selecting healthy tubers is emphasized. 

 In all my previous experiments, selected tubers, when planted upon 

 land not previously used for potalos, yielded very few streaked tubers 

 and upon selecting again, in the following year, streak was practically 

 eliminated. 



Sets selected from the tubers raised in 191 2 produced no cases of 

 streak in 1913, but in this season the produce from diseased sets was 

 also streak-free. 



* Sutton, Messrs., Potato Demonstration (1906), p. 29. 



