66 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 
3 years to the disease. To judge from the results of the first five 
years, it is not likely that any of the pines within the 15-foot radius 
would remain alive at the end of 12 or 15 years. Pines outside the 
15-foot radius from the bush showed only scattering infections. 
This was in a location well protected from strong winds. 
In another case where Ribes nigrum was well exposed to the strong 
storm wind from the neighboring White Mountains, York found 
pine infections up to 600 yeards distant from heavily diseased Ribes 
bushes. 49 
In the outbreak at Kittery Point, Me., Posey found that a num- 
ber of Ribes nigrum bushes so located that the wind had moderate 
access to them caused infection of Pinus strobus trees up to a distance 
of about 300 yards. 
Our studies (146) of the distance of distribution of the various spore forms and of 
the distance that infection has actually occurred upon pines from known infected 
Ribes indicate that the Ribes-free zone should be, under average conditions, 200 to 
300 yards in width. It should be much more where conditions are exceptionally 
favorable for transfer of the spores from Ribes to pine, i. e., near large bodies of Ribes, 
where there is no screen of vegetation over the Ribes or between the Ribes and the 
pines, or in exceptionally humid situations. The cultivated black currant (Ribes 
nigrum) should not be allowed in an infected pine district because of the special 
danger from it. 
Studies by York 49 of the natural infections of pines show that the 
sporidia are blown along roads cut through heavy forest cover and 
that they do not reach pines located in isolated small pockets in the 
dense forest. Trapping of sporidia from Ribes located under dense 
cover of black alder yielded sporidia only up to a distance of 75 feet. 
Traps set 20 feet in the air and well above the cover, but directly 
over the Ribes bushes, caught no sporidia. 
AGENTS DISSEMINATING THE SPORIDIA. 
It is apparent that the sporidia produced, by the teliospores of 
Cronartium ribicola are largely disseminated by the wind. Observa- 
tions in various areas where white pines have become infected from 
neighboring Ribes bushes show plainly that this is the case. In such 
cases the infection is most intense nearest the Ribes bush acting as a 
center of infection. The degree of infection decreases as the distance 
from the center increases. Other conditions being equal, the distance 
of pine infections from the infection center is very short where there 
is a thick screen over and around that center, while the converse is 
true where the Ribes infection center is well out in the open. (See 
pp. 64 to 66 for data bearing on this matter.) 
Minor disseminating agents are known, and their number will 
undoubtedly be increased by future investigations. The investi- 
gations of Gravatt and Marshall (45) in the experimental greenhouse 
"York, H. H. Op. cit. 
