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 ease 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.'*^ 



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 ■^^ 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 rihicola 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. 



