286 
Journal of Agricultural Research 
Vol. XI, No. 6 
due almost entirely to the spreading action of the hyphae as they force 
themselves between the cells. This method of attack differs from that 
of other parasitic fungi observed on white-pine bark. In attacks made 
by species of Phoma and Fusicoccum the cells are killed, the hyphae run 
through the cells, and the bark tends to dry up, turn brown, and crack; 
whereas in the case of the blister-rust attack the cells remain alive for a 
long time, the hyphae run between the cells, the bark swells, turns a yel¬ 
lowish green color, and does not crack until the aeda are produced. 
VALUE OF MYCELIAL CHARACTERS, HAUSTORIA, AND METHOD OF 
PARASITISM IN DIAGNOSIS 
It is evident from the above discussion that the mycelium described 
must belong to the Uredinales. Furthermore, since there is only one 
rust known to attack the bark of Finns strobus , the mycelium must be 
the mycelium of Cronartium ribicola . This obvious conclusion has been 
vigorously proved by the comparison of the mycelium in suspected 
cases with the mycelium in bark on which pycnia and aeda had devel¬ 
oped. The presence in the bark of P. strobus of hyphae and haustoria 
fitting the description which has been given above is suffident evidence 
to warrant the conclusion that the pine is infected with C, ribicola . 
SUMMARY 
(1) A simple method for the use of safranin and lichtgruen in staining 
the mycelium of Cronartium ribicola and the cells of the host, Pinus 
strobus , is given in detail. 
(2) The two stains employed give permanent preparations which are 
particularly favorable for the study of the relation of host to parasite. 
(3) The mycelium of C. ribicola is distinguished from that of other 
fungi parasitic on white-pine bark by the size of the hyphae and the 
possession of haustoria. 
(4) The manner in which the parasite attacks the host cells is described 
in detail. This method of attack is characteristic of the blister-rust in 
white-pine bark. 
(5) The presence in the bark of P. strobus of mycelium such as has 
been described is sufficient evidence to warrant the conclusion that the 
pine is infected with C. ribicola . 
