16 BULLETIN 1291, U. S. DEPARTMENT OF AGRICULTURE 
trees are characterized by a smooth, light-greenish bark all over the 
trunk, even the old branch scars. An unhealthy tree has a bumpy 
bark, and old branch scars are black and large. The larger these 
black scars are the more likely the tree is to be decayed. Fire scars 
(" cat-faces ") are, of course, almost certain indicators of decay. 
Aspen canker is a disease the causal organism of which is unknown. 
It is found plentifully in localized areas through the region covered 
by this study. It does not appear to spread very easily, and the in- 
fected areas are usually small — an acre or less. In these areas, how- 
ever, practically every tree may be badly infected. Bad attacks of 
this disease make the wood unfit for any use'but round timbers. In 
the absence of knowledge of the cause of the canker or the life 
history of the organism, no special treatment can be recommended. 
Canker-infected cutting areas should be clean-cut, whether merchant- 
able or not, and the debris burned, in order to eliminate this disease, 
if possible, from future stands. 
Another canker disease which is frequently fatal to trees in dry, 
exposed situations, and sometimes to larger thrifty specimens in 
favorable sites is caused by Cytospora chrysosperma. The disease 
is not common in Utah, and is more an enemy of the planted species 
of poplar in the valleys than of the wild aspen (8) . Occasionally, 
however, it has occurred in other places within the range of aspen to 
such an extent that entire stands have been destroyed. 
A powdery mildew, Uncinula salicis, the mycelium of which forms 
white powdery areas on both sides of the leaves, is found throughout 
the range of aspen in the Rocky Mountain region. No great dam- 
age is done by this fungus except in dense stands of sprouts where 
the most serious injury has been' observed. 
In Colorado extensive damage has been caused in certain localities 
by a leaf fungus, Sclerotium bifrons, which defoliates the trees. 
The fungus is only slightly known and special remedial measures 
can not be given now. 
The conifers associated with aspen are very much less disposed to 
diseases caused by fungi, and there is no difficulty in growing any 
of them to saw-log size in a sound condition. 
INSECTS 
Insects attack both the leaves and trunks of aspen to some ex- 
tent in the Rocky Mountains, but rarely do great damage. The 
most noticeable insect ravages are caused by defoliation by adult 
Curculionid beetles (Thricolepis inomata) and by the larva of 
some Chrysomelid beetles which are very plentiful in local regions in 
certain years and take practically every vestige of foliage from the 
trees. This is not usual, however, and since repetitions do not 
seem to occur at very frequent intervals in the same stand, the 
damage is limited to the reduction of the year's growth. 
A cerambycid borer (Sapercla calcarata) is very destructive 
to aspen in its northern Rocky Mountain range and in Colorado. 
The larva, which is nearly 2 inches long when fully grown, bores 
under the bark and through the wood, weakening the trees or caus- 
ing their death. Hofer describes in detail the aspen borer and also 
lists and describes briefly other minor insect enemies (6). 
