28 BULLETIN 
downward into the large branch, and from that into the main trunk 
of a tree, finally girdling it and killing the entire tree. This is true 
of nearly all of the older trees that have been killed in North America. 
TYPES OF INFECTION ON PINES. 
There may be said to be three types of infection on white pines 
resulting from natural inoculations. These are (1) direct infection 
of the main trunk on the leader; (2) direct infection of j~oung branches 
or twigs; and (3) infection of an old trunk by spread of the mycelium 
from an infected branch. (PL II, figs. 1 and 3.) All of these are 
present in outbreak areas in North America. Direct infection ap- 
parently occurs only on growth not more than 3 years old. Infec- 
tion of large branches or trunks, so far as we can judge, is limited 
only by the thickness of the bark.' Old rough heavy bark of Pinus 
strobus was supposed to be immune to attack, but it has become in- 
fected by spread of the mycelium from infected side branches. It 
is a common method of entry of the fungus into older parts of a tree 
which were formerly supposed to be too old to become infected. 
It is very frequent in older outbreaks. This has not been mentioned 
in European literature until 1918 when Fischer (41) called attention 
to it. 
DIAGNOSIS OF BLISTER RUST IN PINE BARK BY MEANS OF THE MYCELIUM. 
In 1916, and to some extent before that date, when numerous 
specimens of diseased white pines were sent to the Office of Inves- 
tigations in Forest Pathology for quick and reliable diagnosis of the 
blister rust, many specimens were received which bore no fruiting 
bodies of the parasite. The appearance of many of these made it 
practically certain that they were infected with Cronartium ribicola. 
Colley (16, 19) studied the problem and shortly decided that the 
mycelium and the haustoria did furnish reliable evidence for 
identifying this parasite in the bark of Pinus strobus. The large 
intercellular hyphae, the large and abundant haustoria, and their 
manner of attacking the living cells, were found to be entirely differ- 
ent from the characters of any other known parasite of Pinus strobus. 
The use of these characters for four successive seasons with great 
numbers of specimens in various stages of development has indicated 
that such diagnosis of the disease is absolutely reliable. 
LONGEVITY OF THE MYCELIUM IN PINE SLASH. 
In November and December, 1916, some diseased native white 
pines were cut in outbreak areas in Ontario and in Maine, the slash 
being left lying upon the ground. Entirely independent observa- 
tions made by McCubbin in Ontario and by Posey in Maine early in 
May, 1917, showed that new secia were forming abundantly upon 
