The broomed trees, on the whole, had more potential for growth than did the 

 healthy trees, as is shown by their faster growth before infection by rust. Therefore 

 their final diameter increments would have averaged more than the 2.9 mm. per year 

 given for healthy trees in table 3, if they had not been infected. We can assume that 

 the increase in growth rate of the broomed trees would at least have equaled the 1.3 

 mm. per year of the healthy trees; this increase would give the broomed trees an 

 expected rate of 3.2 mm. per year (1.9+1.3 = 3.2) in the broomed trees. Or we can 

 assume that the growth increase would be proportional to initial growth rates, for an 

 expected rate of 3.4 mm. per year in the broomed trees (1.6/1.9 = 2.9/3.4). On the 

 basis of these assumptions the apparent reduction in diameter growth caused by rust is 

 28 percent or 32 percent, respectively, rather than the 21 percent given in table 3. 



Decay associated with rust brooms in the sample was of minor importance. 

 There were 25 trunk infections by the rust, only 2 of which appeared to be the centers 

 of decay. The decay columns were small. 



Fir Broom Rust: Study of Effects on Growth 



To measure effects of Melampsorella on fir, we made comparisons of growth of 

 paired healthy and infected trees by the methods described above for spruce broom 

 rust. Plots were in the Sawtooth (Idaho), Cache (Idaho), and Wasatch (Utah) National 

 Forests. Average distance between members of pairs was 39 yards. The 33 broomed 

 trees of the sample bore an average of 9 rust brooms each. 



From the data of the three plots summarized in table 4, no statistically signifi- 

 cant differences in growth between healthy and diseased trees are evident. This ab- 

 sence of growth effects may result from the relative recency of infection. Of the 300 

 fir brooms examined, fewer than 10 were more than 30 years old; average age was 

 19 years. Perhaps the apparent difference in recent height growth between healthy and 

 rusted trees (table 4) represents the first damaging effect of rust. 



Table 4. --Average measurements of 33 pairs of healthy and rust-broomed 



subalpine fir 



Item : 



Healthy 

 trees 





Broomed 

 trees 





Percentage 

 difference 



Diameter at breast height 



7.61 in. 





7.58 in. 





-0.4 



Age 



100 yrs. 





100 yrs. 









Height 



43.6 ft. 





41.4 ft. 





-5 



Height growth, last decade 



6 . 5 ft . 





5.8 ft. 





-11 



Diameter growth before 













decade of first rust 



1 . 8 mm. 



/yr. 



1 . 8 mm. 



/yr. 







Diameter growth after 













decade of first rust 



2 . 6 mm . 



/yr. 



2.6 mm. 



/yr. 







6 



