Aecial hosts of the parasite are the true or balsam firs. In the 

 Intermountain Region the rust is known on white fir ( Abies concolor 

 (Gord. & Glend.) Lindl.)j grand fir (A. grandis (Dougl . ) Lindl . ) , and 

 subalpine fir. The latter is by far the most common host of the three. 



LIPE HISTORI OF THE RUST 



The fungus is systemic and perennial not only on firs, but also on 

 those alternate host plants that are perennial species (?_, 10.). 



Spores (aeciospores) are produced during the smnmer and early fall 

 in small yellowish-orange sacs (aecia) on the needles that are borne on 

 the witches' brooms. The aeciospores are wind disseminated and infect 

 the leaves of chickweeds (3). Small orange-red pustules (uredia) soon 

 develop and release urediospores which serve only to infect other chick- 

 weeds, thus intensifying the fungus on that host. The uredial stage is 

 followed by the formation of teliospores. These spores form within the 

 leaf cells where they overwinter (lO) . Teliospore gemination and infec- • 

 tion of firs occurs the following spring {3)' 



DESCRIPTION OF THE DISEASE ON FIRS 



The outstanding characteristic of this disease is the witches' broom 

 caused by it. The brooms are particularly conspicuous from midsummer to 

 late fall, for it is then that their yellowish-orange color is at the peak 

 of intensity and stands out in striking contrast to the normal dark green 

 foliage. Aeciospore production, which is then in progress on the diseased 

 needles, contributes to this color. Witches' brooms are upright, typical- 

 ly compact with a dense growth of many small and shortened branches, and 

 rarely exceed a diameter of 3 feet. The diseased needles are greatly 

 shortened and thickened. 



In winter the brooms appear to be dead because the infected needles 

 shrivel and become dark in color. Shortly prior to the advent of spring 

 these needles drop leaving the brooms bare until new growth starts. The 

 new needles are a yellowish-green color until midsummer. 



Witches' brooms occur on trunks and branches, but are most common on 

 the latter. Swellings commonly develop on both branches and trunks in 

 association with the brooms. 



Twenty years ago Garrett (U) reported that in Utah the yellow 

 witches' brooms are often mistaken for some kind of "mistletoe." Even 

 though no mistletoes have ever been reported on subalpine fir in Region 

 hj this mistaken identity as to the cause of these brooms still exists 

 to a large extent. Mistletoes are seed-producing plants; Melampsorella 

 caryophyllacearum is a fungus and therefore reproduces by means of 

 spores. 



-2- 



