196 Potato Diseases 



being of bacterial nature. The apparently conflicting evidence, such 

 as is production of a healthy crop from susceptible varieties upon 

 " sprained " land and the occurrence of healthy issue from diseased 

 tubers, is paralleled among the organism-caused diseases. Thus the 

 potato crop including that obtained from the susceptible varieties may 

 not be appreciably affected by the canker organism although this parasite 

 is actually present in the soil, whilst the production of healthy plants 

 and potatoes from sets affected with Phytophthora is, as I have already 

 stated, of common occurrence. The failure to obtain sprain in the 

 vegetative generations proceeding from tubers originally selected as 

 disease-free from diseased stock of susceptible varieties when planted 

 in soils of several kinds in different localities seems opposed to a purely 

 physiological explanation. 





Fig. 5. Section of a tuV*er showing sprain or streak. 

 Fig. (i. Another section of the same tuber. 



Pending attempts to isolate pathogenic bacteria, additional evidence 

 might be gained by attempting to infect potatoes with sprain through 

 the agency of soil derived from lands liable to it. 



In the meanwhile the best means of preventing loss through sprain 

 will be those which have certainly occurred to skilled potato-growers ; 

 firstly, to avoid as far as possible the use of " sprained " tubers for seed, 

 and secondly, to cultivate varieties which are the least susceptible to the 

 disease. The most thorough method of procedure would be to obtain an 

 accurate record of the distribution in Britain of the lands liable to sprain, 

 and to carry out potato trials in different localities where the disease is 

 most troublesome, with the object of discovering the most suitable and 

 relatively sprain-immune varieties for the district about each experi- 

 mental area. 



