A similar appearing witches' broom is present in this region on 

 Engelmann spruce, but it is caused by another rust fungus, Peridermium 

 coloradense (Diet.) Arth. & Kern (6, 7, 8, 9j 10). It is not certain 

 yet that this rust has an alternate host (27. 



DAMAGE 



Heavily infected trees are noticeably reduced in growth rate, and 

 as the disease continues to intensify within them they are eventually 

 killed. Trees of all ages are susceptible to the parasite. The disease 

 is particularly destructive to seedlings and saplings and considerable 

 mortality has occurred on some areas. 



No systematic survey has yet been made to appraise accurately the 

 prevalence of this disease and the damage caused by it. Based on obser- 

 vations, however, heavily diseased stands of subalpine fir are known to 

 be present on three of the national forests in Region k and more lightly 

 infected stands have been noted on most of the other forests. 



In heavily diseased stands trees containing 30 to 50 or more brooms 

 are common. Many large trees have hundreds of infections. In such cases 

 almost every branch has one or more brooms. Estimates made in some of the 

 more severely diseased stands ranged from 80 percent to 90 percent of the 

 trees infected. 



EPIDEMIOLOGY 



It is inconceivable that subalpine fir would be present as a species 

 on certain of the diseased areas today if the intensity of infection by 

 this native rust had always been as great in the past as it is at the 

 present time. If the fungus remains uncontrolled and continues at its 

 present destructive rate the end result will be either greatly reduced 

 stocking of the fir or probable elimination of the tree from some, areas. 

 The great abundance of young brooms, ranging from one to several years or 

 more of age, provides adequate evidence that intensification of the dis- 

 ease has been occurring at a rapid rate. Literally dozens of such brooms 

 may be observed in many trees. 



Some factor, or factors, not yet determined has been responsible for 

 the increased intensification of the disease and infection of the firs to 

 a degree far surpassing that of any case hitherto reported in this country. 

 Moisture during the growing season is needed for infection of both the al- 

 ternate hosts and the firs. General observations indicate that the epi- 

 demic has been in progress for about 2^ years, perhaps longer in some cases. 

 Very favorable climatic conditions (a series of moist summers) during this 

 period could account for the present behavior of the pathogen. However, 

 weather records do not support this possibility. Furthermore, weather 



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