Incidence and presumed cause of infection . Table 1 shows that 59 

 percent of all trees examined were infected in the root collar and in the 



Table 1. — Incidence and presumed cause of root infection in 

 western white pine trees. 









■ Presumed cause 



of infection 1/ 



Drainage 



: Trees 

 : examined 



: Trees 

 : infected 



■ Armi 1 1 aria 

 me Ilea 



Fomes 

 annosus 



Unknown 

 species 





Number 



No . Pc't . 



Percent 



Percent 



Percent 



Hoodoo 



86 



49 57 



84 



14 



10 



Clay : 



94 : 



57 61 



86 



12 



12 



Total or : 

 average : 



180 : 



106 59 



85 



13 



11 



1/ These figures total more than 100 percent because certain 

 trees were infected by more than one species of fungus. 



proximally bared portions of the main lateral roots. It is noteworthy that 

 85 percent of the infected trees showed evidence of infection by Armillaria 

 mellea and only 13 percent by Fomes annosus . In interpretation, it must 

 be borne in mind that the estimate of incidence of infection is conservative 

 because only those trees which showed infection in the bared portions of the 

 root system were recorded as infected, whereas the preliminary intensive 

 tree examinations had demonstrated that infection may begin in other parts of 

 the root system as well. On the other hand, these figures do not disclose 

 the relative degree of infection in the trees recorded as infected and hence 

 may lead to an exaggerated impression of the importance of root infection in 

 the drainages sampled. It is also noteworthy that there is close agreement 

 between the two drainages both in incidence of infection and in presumed cause 

 of infection, but this is perhaps not as remarkable as it might appear be- 

 cause both are southerly drainages and both were known in advance of examina- 

 tion to contain a large proportion of infected trees. 



Degree of infection . Since the data shown in table 1 do not disclose 

 the degree of infection, the infected trees were grouped in classes based 

 on the fraction of the circumference of root collar killed. This fraction 

 was estimated in the field for each tree. Table 2 gives results of this 

 grouping. Although 59 percent of the trees showed root infection in the por- 

 tions examined, only 16 percent of the trees had half or more of the circum- 

 ference of the root collar killed and might therefore be regarded as doomed 

 to death in the near future. Again it is noteworthy that the percent of trees 

 in each infection class in the two drainages is close. 



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