1921.] 



The Distribution of Wart Disease. 



949 



The report of each year's trials separated varieties into three 

 groups, viz. (1) Immune Varieties; (2) Non-immune Varieties; 

 and (3) Varieties of doubtful Immunity. 



In the first group were placed all varieties on which no Wart 

 Disease had been seen, and in the second were placed all those 

 potatoes found to be affected with Wart Disease in varying de- 

 grees of intensity. Where Wart Disease was found on the haulm 

 or tuber, or both, of one or two plants, and it was not possible to 

 determine that the affected plants were " rogues," the varieties 

 were, for the time being, regarded as of doubtful immunitv. 

 The tubers from the doubtful plants were destroyed, and those 

 from disease -free plants were saved and tested in the following 

 season; e.g., in 1915, Ally and Edzell Blue were both classed as 

 of doubtful immunity. After selection and on retesting in 1916 

 both proved to be immune, showing that the plants affected 

 with disease in 1915 were " rogues," i.e., plants of some other 

 variety. 



As a result of Mr. Snell's work in testing the large number 

 of varieties and seedlings dealt with, the list of varieties proved 

 to be immune from the disease was raised from 55 in 1914 to 130 

 in 1919. 



Susceptible Varieties and Infected Land. — In order to 

 demonstrate to the many potato growers visiting the trials the 

 futility of planting susceptible varieties in infected land, Mr. Snell 

 planted plots of susceptible varieties in some of the infected land. 

 Generally, Cumberland Ideal, Arran Chief, President, Up-to-Date^ 

 King Edward, and British Queen were used. On clean land these 

 are heavy croppers, but on infected land the crops were greatly 

 reduced; e.g., in 1918, three plots were planted with Cumberland 

 Ideal, with the following result : — 





Clean Tubers 



Warted Tubers 



Total Crop 





per Acre. 



per Acre. 



per Acre. 



No. 



Tons. Cwt. 



Tons. Cwt. 



Tons. Cwt. 



1 



1 11 



3 18 



5 9 



2 



1 7 



3 15 



5 2 



3 



1 13 



3 11 



5 4 



The average crop of clean tubers was only 30 cwt. to- 

 the acre, whereas that of Great Scot or Ally in the same year 

 at the trials gave average crops, respectively, of 13 tons 17 cwt. 

 and 13 tons 12 cwt. per acre. 



