74 BULLETIN 957, IT. S. DEPARTMENT OF AGRICULTURE. 
restricts the spread of this stage of the fungus. Moreover, drought 
^ery largely reduces viability of the urediniospores. 54 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 54 (30, 88). It is an open question whether the 
erratic germination of the urediniospores is not due to this action of 
the sivn'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 MAX. 
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 strohus is known to have been the means of 
introduction of this disease to many parts of Europe (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 hi 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 may retard or reduce the fruiting of the ftmgus by eating the sori 
on both pines and Ribes; and others such as gipsy-moth larvae, other 
insects, snails, and squirrels may even eat the surrounding bark on 
pines, so that no more sori can form. (PL III.) In 1918, Pemiing- 
ton 54 estimated that the production of aBciospores in the Adiroridacks 
*< Pennington, L. H. Op. cit. 
