APPLE DISEASES 9 
Cause of apple-scab. 
The apple-scab pathogene is a fungus known by the name 
Venturia inequalis. It passes the winter in the old fallen 
leaves as immature 
perithecia. With 
the advent of spring 
these sexual fruit- 
ing bodies come to 2 . = 
full maturity by te ? 
the time the blos- % , 
[ 
{ 
i 
4 
a 
# 
t 
som-buds are ready i 
to open. These H 
spring activities of 
the parasite are 
characterized 
chiefly by the for- 
mation, within the 
perithecia, of ascospores which constitute the principal in- 
oculum for the first infections. Inoculation is brought about 
through the agency of the wind. The ascospores are forcibly 
discharged into the air from the perithecia imbedded in the 
old leaves on the ground, and these spores, being extremely 
light, are carried to the opening buds. Spore discharge is con- 
ditioned by (1) the maturity of the ascospores themselves. 
They are maturing during a period of about one month begin- 
ning approximately at the time the blossom-buds are ready to 
open. (2) The occurrence of rain-periods when the ascospores 
aremature. It has been estimated that in forty-five minutes of 
wet weather at the proper time no less than eight billion asco- 
spores might be discharged from the old fallen leaves under a 
large apple-tree. These figures indicate how one may account 
for even the most abundant primary infection of young leaves 
and blossoms that has ever been recorded. Since the lower 
surfaces of the unfolding leaves and the young fruit-pedicels are 
i 
Fic. 4. — Scab on young apples. Lesions on both 
the fruit and pedicels. 
