Blister Canker 
61 
areas were much smaller than those which appeared at the 
unwrapped wounds. 
Heavy pruning was found to be injurious rather than bene- 
ficial where the trees were heavily infected. While it induced 
more vigorous growth of the tree, it also exposed the mycelium 
of the fungus to the air wherever large wounds were made. 
Cankers almost invariably appeared at these pruning wounds 
within one or two seasons. 
Radial growth of the fungus in badly infected trees was 
observed by cutting otf large limbs at the trunk and noting 
both the amount of wood in inches and the number of annual 
rings free from the disease. It was noted that the fungus 
occupied about the same relative position in the trunk and 
larger limbs when the number of annual rings from the sur- 
face was considered. There was considerable variation in the 
amount of wood free from disease when measured in inches, 
due to the fact that some branches were much larger than 
others of the same age. 
The fungus is not always confined to tissues of the same 
age. Most limbs grow faster and have a much thicker annual 
growth on one side than on the other. Hence the fungus is 
usually much nearer the surface on one side of the branch 
than on the other, even when confined to tissues of the same 
age. It was found that if the growth of a branch was ma- 
terially checked, the fungus grew thru the greatest number 
of annual rings of tissue on the side where these rings were 
the thinnest. Usually the least deposit of tissues is made on 
or near the upper surface of the branches. Here also, as a 
rule, the largest number of injuries occur. Often a very shal- 
low wound extends entirely thru one year's growth of wood. 
This permits a more rapid advance of the fungus. These 
tissues are also much less active than on the lower side of the 
branch where the heaviest deposit of tissues usually occurs. 
There are actually fewer cell walls to penetrate on the side 
where the annual rings are the thinnest, since lack of growth 
consists of a fewer number of cells rather than lack of size of 
individual cells. It was noted that the proportion of cells pro- 
duced late in the season and having relatively thick walls was 
much less in the regions of least growth than on the opposite 
side of the branch. 
In the most susceptible varieties of trees under normal con- 
ditions the fungus was found to grow very rapidly in the 
heartwood. When active tissues were encountered the growth 
of the fungus was checked, but it still advanced slightly faster 
