A. S. HoHNE 195 



apparent absence of an organism and insufficient experimental evidence. 

 There are three experimental methods of attacking the problem : (1) 

 that of analysis by means of field experiments, (2) that of attempting 

 to isolate pathogenic bacteria from diseased tubers, (3) that of analysis 

 of the bacterial flora of soils from " sprained " areas. At present only 

 the first of these methods has been seriously adopted by the writer 

 with the following chief results : 



(1) In four consecutive years a certain proportion of diseased 

 tubers was obtained in the yield from diseased sets upon every occasion 

 when diseased sets were planted, in Northumberland (1909, 1910), 

 Devonshire (1909-1910), Ireland (planted by Dr Pethybridge, 1910), 

 Chelsea (1911), and Wisley (1912). 



(2) In the fifth year at Wisley (1913) no diseased tubers were 

 obtained in the yield from diseased sets. 



(3) In 1909 the writer selected disease-free tubers of the Sutton 

 Flourball variety from a sack containing a high percentage of diseased 

 potatoes given to him for experimental purposes by Messrs Sutton. 

 The issue from these selected tubers has proved healthy in each suc- 

 cessive year — 1909, Northumberland and Devon ; 1910, Northumberland, 

 Devon and Ireland ; 1911, Chelsea ; 1912, Wisley and Walton-on- 

 Thames (in sprained soil) ; 1913, Wisley. 



(4) Disease-free tubers of the susceptible variety, Duke of York 

 obtained from a locality where sprain is absent, yielded a diseased crop 

 when planted upon " sprained " land, at three different stations in 

 Scotland in 1911. 



(5) Tubers selected disease-free from the produce raised in 

 Scotland in 1911, yielded only a few diseased tubers at Wisley and none 

 at Oxshott in 1912 and none at Wisley in 1913 — but in this season 

 diseased sets produced healthy tubers. 



It is clear that " sprain " is influenced a great deal by conditions of 

 soil and weather, but it is by no means certain that it is entirely due to 

 these conditions as Frank supposed. Analyses of soils from different 

 centres where considerable harm had been done to the potato crop by 

 internal disease showed that they were exceptionally poor in phosphates, 

 potash and lime, being moorish, sandy soils, but sprain is by no means 

 confined to such soil, and moreover selected tubers of susceptible varie- 

 ties when planted experimentally on similar soils have not taken the 

 disease. It is exceedingly difficult to interpret all the evidence entirely 

 on a physiological basis. But the evidence is not at all incompatible 

 with the theory of the existence of an organism-factor — the organism 



