22 BULLETIN 580, U. S. DEPARTMENT OF AGRICULTURE. 
seriously damaged each year in the north half of the yellow-pine 
type. 
This poor showing of the injured trees would not be such a serious 
matter were it not for the fact that even though an injured tree does 
not die immediately, the needles, after repeated grazing, become 
dwarfed and turn yellowish, and the whole plant becomes stunted. 
If injury is continued the vitality is so weakened that a severe season 
or an attack by natural enemies is very likely to kill the tree. It is 
not uncommon to find saplings so impoverished by continued defolia- 
tion that they can readily be pulled out of the ground by hand. 
| The destruction of young trees is not such a serious matter where 
more reproduction has become established than is necessary to insure 
a complete regeneration of the stand. But where the reproduction 
is more or less scattered, the loss of a considerable percentage of young 
trees becomes decidedly important. 
SUSCEPTIBILITY TO FUNGOUS AND INSECT ATTACKS. 
It is commonly supposed that young trees injured by grazing are 
more susceptible to attack by fungi and by insects than are unin- 
jured trees. The extent to which this danger actually exists, and 
under what circumstances, is a subject which deserves careful con- 
sideration.” 
Dr. W. H. Long, forest pathologist for Arizona and New Mexico, 
who has made a special study of fungi attacking western yellow pine, 
has expressed the opinion that 
The fungi and diseases known at the present time to cause serious damage 
to yellow pine in the Southwest normally do not enter trees through ordinary 
injuries caused by grazing. The spores of certain species of rust (Perider- 
mium spp.) may enter through wounds in the living bark of pines, and it would 
be entirely possible for such spores to enter through wounds caused by grazing. 
However, the total damage due to such a rust entering through grazing wounds 
and finally girdling the trunks of yellow pine is probably very small. 
No heart-rotting fungus is known which is apt to start growth through grazing 
wounds other than in cases where the bark is removed or where the heartwood 
and Sapwood are exposed by breaking and twisting. Since injuries of this 
character form a very small part of the total amount of damage, the danger of 
fungus attack due to them does not require serious consideration. ‘ Injuries that 
favor this kind of rot are caused by lightning, fire, deep blazes, etc. 
It might be presumed that mistletoe (Razouwmofskya spp.) would be liable 
to attack grazing wounds, but a knowledge of the circumstances under which 
this parasite attacks tissue leads to the conclusion that this danger is relatively 
unimportant, for while it is true that mistletoe berries start growth where 
new tissue is exposed, such as on the Calloused edges of blazes, and while the 
wounds to tender shoots such as are caused by grazing might afford favorable 
conditions for germination of. these berries, the conditions are probably no 
more favorable than they are on the tips of uninjured stems. 
