MARKING RUI.es FOR IDAHO AND MONTANA 669 



drifting and carrying spores will act as a screen and prevent the deep 

 penetration of a large number of the spores; also many spores will fall 

 on unsusceptible hosts. An open stand will allow freer circulation, 

 less obstruction, and therefore a greater distribution of the air-borne 

 spores. Spores liberated from elevated points are carried long dis- 

 tances by air currents, but few sporophores are produced on hosts in 

 such exposures compared to the number produced in the more moist 

 and shaded valleys and canyons. •* In the mountainous regions the 

 diurnal changes in temperature cause changes in the direction of air 

 currents. These currents, which usually flow along the earth's surface, 

 cause a distribution of the spores in opposite directions from the 

 source. 



Other factors in the chances for infection are found in the earliest 

 infection age of the trees, the number of injuries in the tree susceptible 

 to infection, and the age of the tree, since the risk of infection in- 

 creases with increased age. It has been found that certain tree species 

 have definite ages at which earliest infection is most likely to take 

 place. This age of earliest infection for western white pine infected 

 with Tramctcs piiii and Polyporus schzvcimtzii is approximately 50 

 years.' This information is important in respect to determining w^hen 

 the young stand first becomes susceptible to infection. The formation 

 of susceptible heartwood and the presence of branch stubs contribute 

 to the chances for early infection, as well as for subsequent infection. 

 Branch stubs, fire scars, frost and lightning cracks, blazes, and all other 

 injuries are the principal paths of entrance for heart-rotting fungi. 

 Some of the numerous spores carried by air currents find lodgment in 

 the crevices of a branch stub or a fire scar, and there germinate. 

 Mycelia develop and enter the heartwood of the tree wherever dead 

 tissues are found to bridge the live and resistant sap region. Some of 

 the parasitic fungi, such as Arnnllaria inellca, apparently do not require 

 the bridge of dead tissue, but attack the live tissues of young roots and 

 enter in this manner, although injuries causing openings in the trunk 

 or roots would, no doubt, assist in a more rapid and more virulent 

 attack.*^ The trees* selected to remain upon the sale area are in the 

 nature of a thinned stand. In the Whitman National Forest it was 

 found that thinning promoted the development of the larch mistletoe 

 in the crowns of the host. These infections were found to be a menace 

 to the surrounding trees of all age classes.^ It is evident that trees left 

 upon a cut-over area and receiving injuries during the process of log- 

 ging are thus made more susceptible to infection by spores of fungi, 



