28 BULLETIN 957, U. S. DEPARTMENT OF AGPJCULTURE. 



downward into the large branch, and from that into the main trmik 

 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 young branches 

 or twigs ; and (3) infection of an old trunk by spread of the mycelium 

 from an infected branch. (PI. 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 rihicola. 

 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 hyphse, 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 strohus. 

 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 £ecia were forming abundantly upon 



