WHITE-PIXE BLISTER RUST. 75 



was reduced about 15 per cent by the eating of infected bark by mice, 

 squirrels, porcupines, etc. Posey and Gravatt ^^ found that squirrels 

 had eaten 17 per cent of the secia-bearing bark m a given area at 

 Kittery Point, Me., and this is substantially true for the infected 

 forests of that section. The leaf-eatmg insects and mammals may 

 so reduce the leafage of Ribes 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 ^^ found that secondary fungi 

 work in the pine bark infected by Cronartium rihicola in such a way 

 as nearl}^ or entirely to Idll 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 Crorcartium rihicola. Very often it appears that the diseased 

 pines are killed finally by the secondary fungi rather than b}' the 

 blister rust. The tecia of Cronartium rihicola are sometimes attacked 

 directl}^ by other fungi (80, 168, 172). It has also been found that 

 the ureduiia and telia are attacked by various fungi, so that their 

 efficiency is greatly reduced locally (116). Fungi parasitic upon the 

 leaves of Rihes 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 pmes. 



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 wliite pines. 



Among the white pines the bhster rust attacks Pinus strohiLS with 

 especial virulence. It does not attack P. cemhra nearly so readily. 

 Experience shows that P. jiexilis ''^ is decidedly susceptible to 

 it. Tliis 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 h\ manuscript. 



■■8 Pennington, L. H., Snell, W. H., York, H. II., and Spaulding, r. Investigations of Cronartium 

 rihicola in 1920. Seen in manuscript. Published in Phytopathology, v. 11, p. 170-172. 1921. 



