WHITE-PINE BLISTER RUST. 29 
the cankers on this slash (89). This overwintering of the fungus 
by means of the mycelium is favored by the slash lying in moist 
places. It was also noted that a piece of a trunk several inches in 
diameter was producing new secia after being kept in the dry air 
of the artificially heated laboratory in Washington about 6 weeks. 
These findings are significant, since they show that the cutting of 
diseased pines, if done in the late fall, winter, or early spring, must 
be accompanied by careful collection and burning of the infected 
slash, if infection of near-by Ribes is to be prevented. This difficulty 
may be obviated by cutting the pines in summer when the following 
hot, dry weather will kill the slash and end the life of the mycelium 
within it. 
SEQUENCE OF PYCNIA AND MC1A AND PROGRESS OF THE DISEASE IN PINE BARK. 
Study of many blister-rust cankers of varying ages in the bark of 
large trunks of pine has shown that the disease extends through the 
bark in a regular and well-defined manner. Cankers of several years' 
standing usually consist of four distinct zones (20) (PL II, fig. 2) : 
(1) An inner central zone of dead rough bark where secia have been borne in pre- 
ceding seasons. This area often has at its center a dead lateral branch or twig down 
which the disease has traveled from its first place of infection. This is the most com- 
mon method by which infection of large trunks and branches takes place. 
(2) An annual zone of living, swollen bark surrounding the dead area. Here is 
produced the latest crop of aecia. This zone varies in width from a fraction of an inch 
up to several inches. 
(3) A zone of discolored living bark bearing the pycnial spots, drops, or scars. 
(4) An outer zone of living bark, little or not at all swollen, showing a yellowing or 
bronzing of the normal green color of the smooth living white-pine bark. These zones 
of course move steadily outward with the progress of the mycelium. 
This sequence of zones of activity shows that we have a regular 
succession of events, as follows : 
(1) Invasion of healthy bark by the fungous mycelium, resulting in yellowing and 
bronzing of the invaded bark. 
(2) Formation of pycnia in discolored and often swollen, but still living bark. 
(3) Formation of aecia in living bark which has previously borne pyncia. 
(4) Death of the bark which has borne secia in abundance. 
Investigations by Gravatt and Posey, and Colley (20, p. 650-651) 
indicate that the bark is often killed by invading secondary fungi as 
well as by Cronartium ribicola itself. Rhoads, 7 York, 8 and Pen- 
nington 9 found that occasional patches of bark, where aecia have 
been borne the previous year, remain alive and bear a second crop 
of aecia the second season. 
^ Rhoads, A. S. Studies on the rate of growth and behavior of the blister rust on white pine in 1918. 
Seen in manuscript. Published in Phytopathology, v. 10, p. 513-527. 1920. 
a York, H. H. Op. cit. 
9 Pennington, L. H. Investigations on the white-pine blister rust in New York. Seen in manuscript. 
To be published as Tech. Bui.. N. Y. State Col. Forestry. 
I 
