74 BULLETIN 95*7, U. S. DEPARTMENT OF AGRICULTURE. 



restricts the spread of this stage of the fungus. Moreover, drought 

 very largely reduces viability of the urediniospores.^* With the 

 very short-lived sporidia of the teliospores it is evident that lack of 

 moisture immediately after their production may entirely prevent 

 their infecting pines at all, and drought is known greatly to limit 

 their formation. Drought causes the premature fall of leaves of 

 Ribes bushes so as to leave practically nothing for the fungus to sub- 

 sist upon late in the season. Thus the crop of teliospores is so 

 greatly reduced in times of drought that infection of pines is largely 

 or entirely prevented. Drought kills many young Ribes seedlings 

 and many are winterkilled (23, p. 8). On the other hand, rain 

 undoubtedly beats down the spores floating in the air and washes 

 spores from the host plants, so that infection by them is prevented. 



Sunshine, by influencing the moisture of the air, may be very potent 

 in reducing the activities of the fungus. It has a direct deleterious 

 effect upon the spores ^* (30, 88) . It is an open question whether the 

 erratic germination of the urediniospores is not due to this action of 

 the sun's rays. By promoting the quick maturity and hardening of 

 the leaves of Ribes in the open, bright sunlight may greatly reduce 

 the infection which develops upon them. 



Wind is apparently the chief agent disseminating all forms of 

 spores of this fungus. Its activity greatly influences the spread of 

 the disease. 



THE AGENCY OF MAN. 



Man is a most potent agent in the dissemination of the white-pine 

 blister rust. Through his activities it has made all of its known 

 long-distance jumps. There is reason to believe that it is a native 

 of northern Asia, whence it spread to Europe. The extensive trade 

 in young trees of Pinus strohas is known to have been the means of 

 introduction of this disease to many parts of Em'ope (111, 120, 155, 

 162, 170). It certainly came to North America in young white-pine 

 stock from Europe and has attained its present wide distribution 

 here in such imported stock. See figures 2 to 12, showing the progress 

 of the disease since 1909. 



INSECTS AND OTHER ANIMAL FACTORS. 



Various animals (insects, snails, mammals, man, etc.) may aid in 

 the distribution of the disease by carrying spores on their bodies, or 

 they niaj retard or reduce the fruiting of the fungus by eating the sori 

 on both pines and Ribes; and others such as gipsy-moth larva3, other 

 insects, snails, and squirrels may even eat the surrounding bark on 

 pines, so that no more sori can form. (PI. III.) In 1918, Penning- 

 ton^* estimated that the production of seciospores in the Adirondacks 



" Pennington, L. H. Op. cit. 



