was found to furnish an only slightly reliable measure of degree of root in- 

 fection and is not a dependable criterion of absolute infection. 



Further extensive tree examinations to determine incidence of infection 

 as related to site or other factors could be made more rapidly and with less 

 injury to the examined trees if degree of root infection could be determined 

 without baring the roots as thoroughly as had been deemed necessary in the 

 1938 examinations. It was thought that there might be a direct relationship 

 between the degree of root infection in the root collar and the height of 

 infection on the stump. If this relationship were found to exist, degree 

 of root infection could then be estimated merely by determining the maximum 

 height of infection without the necessity of baring the root collar. Accord- 

 ingly, the possible degree of such relationship was investigated by correlat- 

 ing maximum height of infection (measuring from the ground line) as the 

 dependent variable, with the fraction of circumference of root collar killed 

 as the independent variable. The correlation was so poor that it was obvious 

 that height of infection can not be used to estimate degree of root infection 

 with useful accuracy. A scatter diagram plotted from these data does show, 

 however, that infection seldom reaches the ground line until more than one 

 quarter of the circumference of the root collar has been killed. This means 

 that if the objective of extensive tree examination were merely to determine 

 the number of trees with more than one quarter of the circumference of root 

 collar killed, it would be necessary merely to examine the trunk at ground 

 line. 



RELATION TO BEETLE ATTACK 



It has long been suspected that root infection may predispose trees to 

 attack by the mountain pine beetle even though it is known that the beetle 

 is capable of killing healthy trees. If this suspicion could be verified, 

 attack of infected trees by the beetle would be the immediate cause of death, 

 but the initial cause would have to be sought either in previous infection by 

 root-attacking fungi or in circumstances leading to such infection. Investiga- 

 tion of this point was made by examining the root systems of a number of 

 beetle-attacked trees to determine whether insect or fungus came first, and 

 to compare the incidence and degree of infection in beetle-attacked trees 

 with incidence and degree in line trees. 



These root examinations of beetle-attacked trees were made by the 

 Coeur d'Alene Forest Insect Laboratory, Bureau of Entomology and Plant Quaran- 

 tine, as a supplemental part of regular infestation studies made in 1938 in 

 the Deception, Yellow Dog, and Kelley Creek drainages within the Coeur d'Alene 

 National Forest, On these areas root examinations were made on as many of 

 the trees being examined by the infestation study crew as time permitted. 

 Of 92 trees examined, 58 had been attacked by the mountain pine beetle in 

 1937, and 34 in 1938. The method of root examination of the beetle-attacked 

 trees was similar to that of the trees studied in the extensive tree ex- 

 aminations. 



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