ENVIRONMENT AND WHITE PINE BLISTER RUST INFECTION 



481 



The variation is tremendous between the 29 stands prior to eradica- 

 tion (18 of these stands are shown in Fig. 1) . The pre-eradication 

 incidence of rust ranged from no cankers per 100 trees to the greatest 

 infection in a given year of 118 cankers per 100 trees. This variation 

 can be ascribed to environmental differences since all stands had abun- 

 dant ribes present. The number of repeatedly observed sample trees was 

 adequate. The median number was about 250 trees per stand, with the range 

 in individual stands from 75 to 1,500 trees. After eradication, nearly 

 all of the variation was removed since all plots had less than 20 cankers 

 per 100 trees and all but two had less than 5 cankers per 100 trees. 



Incidentally, the total results of 5,174 cankers in the four years 



prior to eradication (67 per 100 trees) compared to 103 cankers in the 



four years after eradication (1.34 per 100 trees) was significant at the 



1 in 1000 level (Van Arsdel, 1968). 



Almost any series of infection plots will show variation in the 

 infection level. The question is, can we systematize this variation so 

 we can predict where the heavy, light., and lack of infection will occur? 

 The answer is: we can. 



Figure 1. Differences in the amount of blister rust infection 

 occurring on 18 of 29 study areas. 



