Spruce Broom Rust: Study of Effects on Growth 



In 1962 a small study was made to determine whether broom rust causes growth 

 loss in Engelmann spruce. Three. variable-size plots were chosen, one each in the 

 Deerlodge (Montana), Teton (Wyoming), and Wasatch (Utah) National Forests. In each 

 plot we marked 10 to 12 broomed spruce with an aggregate diameter of 80 inches. The 

 trees selected were those nearest an arbitrary point in each plot. Trees that were 

 forked or otherwise deformed, that were less than 3 inches d.b.h., or that had only one 

 or two small brooms far out on the branches were excluded from the sample. After 

 marking the diseased trees, which had an average of three rust brooms (or clusters of 

 brooms) each, the rust-free tree nearest each marked tree and having a d.b.h. within 

 10 percent of the latter 's diameter was identified. Average distance between members 

 of diseased-nondiseased pairs was 14 yards. The paired trees were then cut and dis- 

 sected to compare ages, diameter growth by decades (measured on a 6-inch stump), 

 heights, height growth in the last 10 years, and extent of decay. Assuming that infection 

 occurs on current -year shoots, we determined the approximate year of first infection 

 from the age of the stem section bearing the oldest broom. Diameter growth rates be- 

 fore and after the decade in which first infection occurred were compared for infected 

 and healthy trees. In almost all trees, rusted or not, annual increments increased with 

 tree age, because the trees had been partly suppressed in their early years. 



Table 3 summarizes data from the three plots. The difference in recent height 

 growth correlated with rust infection is real, by "t" test, unless a 1 -in- 100 mischance 

 has occurred in sampling. Measured diameter growth differences also reflect actual 

 effects of rust unless a l-in-17 mischance occurred in sample selection. Current 

 height and diameter growth were greater in healthy trees than in broomed trees in all 

 three plots, but the percentage differences averaged twice as large in the Teton plot as 

 in either of the others. In the Teton plot infections averaged only 1 .4 per broomed tree, 

 compared with 3.3 and 3.6 in the other plots; but the average time since infection was 

 61 years, compared with 20 and 30 in the others. Time since infection thus appears 

 more important than number of infections in evaluating damage. 



Table 3. -- Average measurements of 32 pairs of healthy and rust-broomed 



Engelmann spruce 



Item : 



Healthy 

 trees 





Broomed 

 trees 





Percentage 

 difference 



Diameter at breast height 



8.66 in. 





8.63 in. 





-0.3 



Total age 



126 yrs. 





127 yrs. 





+ 1 



Total height 



49.5 ft. 





49.2 ft. 





-1 



Height growth, last decade 



6.0 ft. 





4.5 ft. 





-25 



Diameter growth before 













decade of first rust 



1 .6 mm. 



/yr. 



1.9 mm. 



/yr. 



+19 



Diameter growth after 













decade of first rust 



2 . 9 mm. 



/yr. 



2.3 mm. 



/yr. 



-21 



5 



