12 BULLETIN 360, U. S. DEPARTMENT OF AGRICULTURE. 







were killed by the parasite. The other trees in the figure show 

 various stages of suppression and an abnormal thinness of foliage, 

 The tree on the extreme right shows midway on its trunk a typica] 

 mistletoe trunk burl. 



It is often disputed that mistletoe is a cause of spiketop or that 

 it is totally unknown for some species. The first and heaviest seat oi 

 infection in nearly all trees of economic importance is in the lower 

 part of the crown (figs. 6 and 8). This is not necessarily a result oi 

 the seeds of the parasite falling first on the lower branches, but is 

 rather the result of the fact that the main shoot continues for a time 

 to grow in height, and the crown may attain its normal height be- 

 fore the effects of the parasite become dominant. The mistletoe 

 spreads upward from the lowermost branches, with the result thai 

 the more recently formed branches are continually being infected 

 That these infections may not cause a brooming of the branches ir 

 the beginning is abundantly shown by the entire absence of anj 

 brooming on young infected branches of several host species. This 

 however, is only the first stage in the hypertrophy of the branch, 

 After the lapse of several years, typical brooms are formed. With 

 the increasing hypertrophy of the lower portion of the crown, food 

 materials are more and more appropriated at this point. The result 

 is a drain on the resources of the entire tree to support the brooms, 

 Materials traveling upward from the roots are likewise utilized b} 

 the broomeo! branches, with the result that the upper portion of the 

 crown starves and in cases of severe infection finally dies (figs. 5, 6 

 7, and 8). Spiketop is an almost universal condition in heavily 

 infected larch. The tendency to form spiketop in this species, how- 

 ever, is greatly augmented by the brittleness of its branches. Douglas 

 fir probably comes next in order of frequency of dead tops resulting 

 from the growth of mistletoes. The condition is common for yellow 

 pine in all regions where observations have been made by the writer 

 and is reported to be of frequent occurrence by correspondents in 

 Utah and Wyoming. Lowland and mountain hemlocks, when heavily 

 infected, quite commonly exhibit dead tops. An unusual case oi 

 heavy infection of the former species was studied in the St. Joe 

 Xational Forest. Practically every tree in the entire stand was dead 

 in the top (fig. 9). Lodgepole pine is less affected in this manner 

 than any other conifer so far studied by the writer except spruce 

 and fir. The last-named species are so seldom infected, however, 

 that they would not enter into the discussion. 



There can be little doubt that spiketop is very often the result 

 of heavy mistletoe infection, but varies in degree for the several 

 hosts. This condition is of importance, since the proportion of 

 snags in the stand is thereby increased, which may promote injury 

 by fungi and insects; it also increases clanger from lightning fires. 



