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Mycologia 



had all the gross characters of M. medusae although the average- 

 sized leaves on the parent tree bore the typical form of the rust, 

 M. albertensis. These same variations may be noted when the 

 host is P. trichocarpa. In regions where the only possible aecial 

 host is Pseudotsuga taxifolia the form M. medusae will exhibit 

 the same gross variations on P. trichocarpa and P. tremuloides. 

 It is equally difficult to try to separate the teliospores of these 

 rusts microscopically. The fact that either of them will go on 

 both Pseudotsuga and Larix irrespective of source or appear- 

 ance of telia indicates that these rusts which were at first dis- 

 tinguished as M. medusae and M. albertensis are the different 

 host manifestations of the same rust and should apparently be re- 

 ferred to Melampsora medusae Thttm. 



Arthur in 1910 3 and in 191 1 4 successfully inoculated Pseu- 

 dotsuga taxifolia (P. mucronata) with the telial stage of M. 

 albertensis, in every case securing negative results on Larix spp. 

 As no statement is made regarding the age of the needles used in 

 these inoculations we venture to suggest that the needles of the 

 larches used may have been too old to become infected. This 

 would undoubtedly be the case if the host trees used developed 

 their needles at the same time that trees of similar spcies develop 

 needles in the open. Observations in this locality show that the 

 new needles of Larix occidentalis growing in the open were far 

 enough advanced on May 8, 191 7, to be susceptible to infection 

 and on the same date the leaf buds of Pseudotsuga taxifolia 

 were not even swelling. New susceptible needles of Pseudotsuga 

 were not recorded in the open until May 28 and later June 8. 

 On June 8 the aecial stage of the rust was found near Missoula, 

 Montana, on Larix occidentalis, while adjacent trees of Pseu- 

 dotsuga taxifolia were just forming new needles. The host trees, 

 used in the inoculation experiments here recorded and kept in the 

 greenhouse since the preceding two seasons, developed new 

 needles at earlier dates than those recorded for similar trees in 

 the open. In the majority of cases the newly-formed needles 

 were the only ones on the trial host which became infected. 



3 Arthur, J. C. Cultures of Uredineae in 1910. Mycologia 4: 7-33. 1912. 



4 Arthur, J. C. Cultures of Uredineae in 191 1. Mycologia 4: 49—65. 1Q12. 



