WHITE-PIlSrE BLISTER EUST. 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 PYCmA AND ^CIA 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) (PI. 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 secia. 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 li\dng 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 tlie progress of the mycelium. 



This sequence of zones of activity shows th,at 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 secia in li\'ing bark which has pre\-iously borne i)>nicia. 



(4) Death of the bark which has borne ?ecia in abundance. 



Investigations by Gravatt and Posey, and CoUey (20, p. 650-651) 

 indicate that the bark is often killed by invading secondary fungi as 

 well as by Cronartium ribicola itself. Rhoads," York,^ and Pen- 

 nington " found that occasional patches of bark, whore lecia have 

 been borne the previous year, remain alive and bear a second crop 

 of fecia the second season. 



' Rhoads, A. S. Studies on the rate of growth and beliavior of the blister rust on white pine in 1918. 

 Seen in manuscript. PubUshed in Phytopatholopj', v. 10, p. 513-527. 1920. 



8 York, H. H. Op. cit. 



' Pennington, L. H. Investigations on the white-pine blister rust in New York. Seen in manuscript. 

 To be pubUshed as Tech. Bui.,. N. Y. State Col. Forestry. 



