Figure 2. --Limb rust fungi produce white aecia containing orange spores on twigs 

 near the zone of killed branches. This is Peridermium filamentosum. The 

 aecia of P. stalactiforme are broader and shorter. 



PATTERNS OF INFECTION 



Peridermium stalactiforme in the Sierra Nevada usually occurs in immature pines in concentrated 

 infection centers, often near lakes or streams, and often in obvious relationship to the presence of its 

 alternate hosts . Most infections to be found now are low in the tree crowns , 



Peridermium filamentosum in the Sierra Nevada and in southern Utah is found mostly in scattered 

 mature and overmature pines. Occasional concentrations of fairly high percentage infection occur without 

 apparent reference to topography. Low-, mid-, and top -crown infections are about equally common. 



Limb rust in the Rocky Mountains and the Southwest does not fall neatly into either of the foregoing 

 patterns of infection, at least in part because it is caused by mixed populations of two races of P. filamen- 

 tosum . Known outbreaks in very young pines, for instance in the Black Hills National Forest and Grand 

 Canyon National Park, are caused by the race that alternates to Indian paintbrush. Infections in older 

 ponderosa pines are caused both by this host -alternating race and a race that does not infect paintbrush. 

 In some areas, the lack of rust in young trees is due to the long interval since conditions favored infection, 

 not to resistance to the causal fungus in young trees . 



DISTRIBUTION 



Limb rust occurs in the Rocky Mountains and Sierra Madre Occidentale from western South Dakota 

 and northeastern Utah southward through Colorado, New Mexico, and Arizona to Chihuahua (5, 7, J_3, 

 15 , 16). It is found in the Sierra Nevada, southern Coast Ranges, and Sierra de San Pedro MSrtir from 

 Plumas County, CaUfomia, to northern Baja California (20, and author's reconnaissance). It has also 

 been recorded in Guatemala (9). 



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