In America, unlike Europe, no large-scale projects to control fir broom rust have 

 been undertaken- -possibly because we manage fir stands less intensively and are 

 uncertain that broom rust causes significant damage here. 



CURRENT STUDIES 



Spruce Broom Rust: Damage Tallies in Outbreaks 



Survey strips were run through broom rust outbreaks in six National Forests in 

 Colorado to identify any easily recognized effects that the rust might have on Engelmann 

 spruce. Ten transects were examined in the San Juan, Rio Grande, Grand Mesa- 

 Uncompahgre, Gunnison, White River, and San Isabel National Forests. Sampled stands 

 were mature or overmature, and either uncut or lightly cut over. Transects were 0.5 

 chain wide, varied in length, and included a total of 1,249 spruce. Because surveys 

 were deliberately placed in rust outbreak areas, the infection levels recorded are not 

 representative for these forests. The transects began at arbitrary starting points 

 chosen before they were reached, therefore no trees in them had been seen before 

 tallying began. This resulted in two transects' being mostly in healthy stands. Percent- 

 ages of spruce infected varied from 5 to 25 in the 10 strips; the average was 17 percent. 



Rust infection appeared to be causally related to tree death in these samples. 

 Much higher percentages of dead trees than live trees were broomed in the eight tran- 

 sects that included dead trees (table 1). Furthermore, the dead trees that had been in- 

 fected bore an average of 2.3 brooms (or clusters of brooms) per tree, compared to 1.5 

 in infected live trees. Because the mortality rate was 7 percent for rust-free trees, but 

 23 percent for broomed trees, rust infection appeared to increase the probability of a 

 tree's death by a factor of more than 3. It could be argued that rust "picks on" weak or 

 dying trees and that the correlation, therefore, is not significant. But this would be con- 

 trary to facts known about infection by rust fungi, namely that the more vigorous mem- 

 bers of a plant population are the most likely to be infected by these obligate parasites 

 (Arthur 1929). 



Table 1 .- - Comparison of rust broom on live and dead spruce in eight transects 



in Colorado 



Tree size 





Live trees 



Dead trees 



All trees 



; Total 



' Broomed ' 



Total 



] Broomed] 



Total 



' Broomed 







Number 



Percent 



Number 



Percent 



Number 



Percent 



Less than 6" 



d.b.h. 



180 



5 



15 



13 



195 



6 



More than 6" 



d.b.h. 



818 



18 



93 



48 



911 



21 



All trees 





998 



16 



108 



44 



1,106 



18 



3 



