Table 2. — Degree of root infection in western white pine trees. 





: Trees 



: Unin- 



Infected 



trees , 



grouped by- 



f ract ion 



of 





Drainage 



: exam- 



: fected 



: circumference of 



root collar killed 





Total 





ined. 



' 1" T*P P Q 

 ■ U X C C- O 



Under 1/4 



1/4 



2/4 



3/4 



4 /4 







Number 



Pet. 



Percent 



Percent 



Percent 



Percent 



Percent 



Percent 



Hoodoo 



86 



43 



29 



12 



11 



5 







100 



Clay : 



94 : 



39 : 



33 



12 



8 



6 



2 



100 



Duration of infection . Even though it is not yet known whether 

 Armillaria mellea (and other associated fungi) are causes or merely concomi- 

 tants of decadence, the ability to estimate duration of infection in individual 

 trees on the basis of the appearance of the affected roots would be of great 

 value in predicting loss and in its prevention through intelligent marking and 

 timely utilization. It was assumed that severe infection of any main lateral 

 root would retard cambial activity in the trunk immediately above it. It was 

 therefore thought that measurement of the width of recent annual growth rings 

 in the wood might show a recent decrease and that numerous measurements might 

 thus provide an indirect measure of the duration of infection sufficiently 

 severe to influence cambial activity and thus increment. Accordingly, the in- 

 crement cores, taken as described, above the most infected and the least in- 

 fected (or an uninfected) root were measured and analyzed by 5-year periods. 

 A stump section of a tree so analyzed is shown in plate 1-D. 



The results, which are given in figure 1, disclose that with to 25 

 percent of the bared surface infected no appreciable reduction in increment 

 occurred. With 26 to 50 percent infected, increment was significantly reduced 

 for 15 years prior to examination but not for the period 16 to 25 years be- 

 fore examination. Considered with the 0-25 percent class, this seems to 

 indicate that infection is responsible for reduction in increment only when 

 it reaches a certain extent — probably something over 25 percent as judged 

 from these data. Where more than 50 percent of the bared surface was in- 

 fected, increment was consistently reduced for the full period examined — 

 25 years — indicating that such trees have been infected at least that long. 

 In general, the data show that infection is of long standing and, with in- 

 creasing extent, causes increasing reduction in increment. 



2. Factors Possibly Influencing Root Diseases 



Of a number of factors that might influence the incidence of infection, 

 the data on incidence were analyzed only as related to site quality, tree 

 height, site moisture, and proximity of snags and stumps. The results of 

 tabulating incidence of infection in each of three site-quality classes in- 

 dicate that infection is more frequent on poor sites than on fair and good 

 sites, suggesting that, quite apart from the possible influence of site upon 

 the fungi, trees growing on good sites are better able to resist infection. 



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