WHITE-PINE BLISTER EUST. 65 



In 1919, Pennington (cited in wSpaulding, 146) caught sporidia up 

 to 294 feet distant, but they failed to germinate. Under favorable 

 conditions, sporidia caught at a distance of 177 feet germinated, 

 but none beyond this distance. 



York,*'' working in the White Mountain region of New Hampshire, 

 in 1918, found that sporidia were quite common in spore traps 

 exposed 24 hours at a distance of 200 feet from the diseased Ribes 

 bush. York (cited in Spaulding, 146) in 1919 caught sporidia, under 

 favorable conditions, at 600 feet distance, which germinated. 



The infection of pines is said by McCubbin (88) to depend on 

 " (1) The nearness of cultivated Ribes, particularly black currants; 

 (2) the number of wild Ribes present; (3) the moistness of the situ- 

 ation." York" concluded that these factors are ''topographical 

 features, direction of the wind when sporidia are produced, humidity 

 of the air, precipitation, and the nature and density of vetegation 

 between the Ribes and pines." Pennington *^ stated that weather 

 conditions have much to do with the degree of infection that occurs 

 on pines; cool, moist situations favor infection; intervening barriers 

 of vegetation tend to limit infection; the amount of infection under 

 given conditions varies directly as the extent of Ribes leaf surface 

 and mversely as the square of the distance from Ribes. The uTiter 

 (145) said the width of the Ribes-free zone around pines is largely to 

 be governed by topographical features; direction of the wind pre- 

 vailing at the time the sporidia are produced; humidity; age of the 

 pines; exposm'e and species of Ribes; and the composition, height, 

 and density of the vegetation between the Ribes plants and the pines. 

 The experiments with the sporidia show that high humidity is neces- 

 saiy for these spores to live any length of time. It alone may very 

 largely determine whether infection can take place. 



A few specific instances show the effect of these factors in actual 

 outbreaks. On July 10, 1917, on Gerrish Island, at Kittery Point, 

 Me., Gravatt investigated the small trees of Pinus strohiis within a 

 radius of 15 feet of a bush of Rihes Mrtellum to determine the spread 

 of infection. The gooseberry was a small bush, having approximately 

 270 small leaves and there were no other Ribes near by to influence 

 the result on pines. The ages of the pines were as follows: Two 

 years, 12; 3 years, 17; and 5 years, 82; a total of 128 pines, none 

 over 5 years of age. There were 77 separate infections on 54 dis- 

 eased trees, 44 of these infections being on 2-year wood. As the 

 oldest pines were 5 years old and most of the infections which occurred 

 the year before were probably not detected, this infection of more 

 than 40 per cent resulted from mm exposure of only a little more than 



"York, H. H. Op. dt. 



■•s rcnninglon, L. II. Op. cii. 



46103°— 21— Bull. 957 5 



