WHITE-PINE BLISTER RUST. 65 
In 1919, Pennington (cited in Spaulding, 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, 47 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 47 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 48 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 inversely as the square of the distance from Ribes. The writer 
(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; exposure 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- 
sary 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 Finns strohus within a 
radius of 15 feet of a bush of Ribes hirtellum 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 an exposure of only a little more than 
« York, H. H. Op. cit. 
48 Pennington, L. H. Op. cit. 
46103°— 21— Bull. 957 5 
