WHITE-PINE BUSTER RUST. 75 
was reduced about 15 per cent by the eating of infected bark by mice, 
squirrels, porcupines, etc. Posey and Gra vatt 55 found that squirrels 
had eaten 17 per cent of the aecia-bearing bark in a given area at 
Kittery Point, Me., and this is substantially true for the infected 
forests of that section. The leaf-eating insects and mammals may 
so reduce the leafage of Kibes plants as to reduce the disease materially- 
in a given locality. 
OTHER FUNGOUS FACTORS. 
Other fungi are of some importance also. At Kittery Point, Me., 
Colley (20) and Posey and Gravatt 55 found that secondary fungi 
work in the pine bark infected by Cronartium ribicola in such a way 
as nearly or entirely to kill out the latter, probably b}^ killing the 
bark around the cankers so that the blister rust is starved out. This 
sort of thing is quite general where white pines are generally infected 
by Cronartium ribicola. Very often it appears that the diseased 
pines are killed finally by the secondary fungi rather than by the 
blister rust. The secia of Cronartium ribicola are sometimes attacked 
directly by other fungi (80, 168, 172). It has also been found that 
the uredinia and telia are attacked by various fungi, so that their 
efficiency is greatly reduced locally (116). Fungi parasitic upon the 
leaves of Ribes sp., causing their premature fall, may greatly reduce 
the leafage available for the blister-rust fungus to attack and thus 
reduce the quantity of teliospores to produce infection on pines. 
FACTORS IN THE HOSTS. 
There are certain factors in the hosts themselves which are impor- 
tant in the control of this disease where it has once become estab- 
lished. These are resistance by some of the hosts to the disease and 
the natural suppression of the lower branches of white pines.. 
Among the white pines the blister rust attacks Pinus strobus with 
especial virulence. It does not attack P. cembra nearly so readily. 
Experience shows that P. jlexilis 56 is decidedly susceptible to 
it. This is confirmed' by Moir's studies in Sweden. Knowledge of 
the relative susceptibility of the pines is extremely limited, because 
the disease has been in North America too short a time and has not 
yet reached any but the eastern white pine. In Europe, where the 
older outbreaks have occurred, there undoubtedly is an opportunity 
to obtain definite data on the relative susceptibility of the pines. 
It may prove feasible ultimately to plant another species of white 
pine which is not nearly so susceptible to the blister rust and which 
also is of value as a timber tree. 
» Posey, G. B., and Gravatt, G. F. Field studies on the white-pine blister rust at Kittery Point, Me. 
Seen in manuscript. 
5 6 Pennington, L. H., Snell, W. II., York, H. H., and Spaulding, P. Investigations of Cronartium 
ribicola in 1920. Seen in manuscript. Published in Phytopathology, v. 11, p. 170-172. 1921. 
