Although this study did not demonstrate that branch brooms cause measurable 

 effects, one cannot argue from this that Melampsorella causes no damage in the Rocky 

 Mountains . The study specifically excluded the broomed firs most affected by the 

 parasite- -the dead, spike-topped, and small deformed trees (figs. 2 and 3). Their 

 frequency and significance have not been evaluated. 



In the fir sample, as in the spruce, decay associated with rust was of minor 

 importance. Of 300 infections in the sample, 29 appeared to extend into trunks, and 

 only 1 of these appeared to be a center of decay. 



SUGGESTED STUDIES 



These pilot studies suggest both additional questions and some changes in tech- 

 nique. It appeared that time since infection bears importantly on damage, but that 

 random samples of infected trees do not give a comprehensive sample of ages of 

 infection. To evaluate future effects of broom rust on fir growth in the Intermountain 

 Region we should find examples of outbreaks 50 to 100 years old. This will require a 

 radical departure from random sampling; as yet no outbreaks of fir broom rust more 

 than 26 years old have been encountered in the Region. It will also be possible, in a 

 larger sample, to determine the relation between intensity of brooming (percentage of 

 crown broomed) and growth decline. 



Another unknown to be studied is how broom rusts kill trees. Among the possible 

 explanations are (1) food loss to the parasite and to the hyperactive brooms, (2) inter- 

 ference with conduction, and (3) increase in secondary pests following rust infection. 

 Whether bark beetles single out rusted trees for attack, as several foresters have 

 suggested, is a particularly important problem. 



The work reported in this paper does not completely answer its starting question, 

 "How significant is broom rust damage?" Our studies show damaging effects of rust on 

 spruce, and suggest that similar effects might be expected in fir outbreaks older than 

 those sampled. Further work will be required to evaluate breakage and mortality due 

 to rust and to measure damage to young trees. 



8 



