10 MANUAL OF FRUIT DISEASES 
most exposed at the time of first ascospore-discharge, these 
constitute the first points of attack. It has been frequently 
observed that the leaves of the blossom-buds suffer more severely 
than those of the leaf-buds. This is explained on the grounds 
that the former buds open first; the leaf-buds, opening later, 
often escape the early inoculation to which the leaves of the 
blossom-buds are exposed. 
It has been noted that the ascospores are discharged under 
rainy conditions; and unless this condition prevails for several 
hours the ascospores cannot germinate, a process preliminary 
to penetration of the leaves and pedicels. Following a rain 
any conditions favoring the retention of moisture also favor 
spore-germination. Several factors acting in this connection 
are: dense foliage, which prevents prompt evaporation after 
rains; good air drainage favors this evaporation and accord- 
ingly those trees on the hill-tops are less liable to scab-infection 
than those in low pockets; showers followed by winds are 
unfavorable to infection since the moisture is quickly removed 
from the leaves, while showers in the evening followed by a 
calm night are highly favorable to infection. Spore-germina- 
tion occurs by the emission of a mycelial thread, called a germ- 
tube, which penetrates the leaf to a slight extent, usually not 
going deeper than the cuticle as long as the leaf hangs on the 
tree. Sometimes the first layer of leaf-cells, the epidermis, is 
invaded. Within a very few days the pathogene establishes 
a food relation with the host, the lesion becoming visible to the 
naked eye within two weeks or less. The germtube, as a result 
of extensive growth, becomes a dense system of branching 
threads — the mycelium. These threads grow radially from 
the point of invasion, partially dissolving the cuticle as they 
proceed. The undissolved cuticle at the advancing margin is 
uplifted and gradually breaks away in flakes, forming a pro- 
tective covering in the form of a papery rim about the border 
of the spot. While the pathogene undoubtedly gets most of 
