334 MANUAL OF FRUIT DISEASES 
Lesions also occur on the fruit-pedicels. On the leaves the 
disease exhibits itself as spots very much like those on the fruit. 
Lesions on the foliage are abundant and conspicuous on the 
lower surface. This is particularly true on certain varieties, 
as, for example, Winter Nelis. Pear-scab is common on the 
twigs. In this respect the disease displays a difference from 
apple-scab; the latter affects twigs rarely in comparison to 
pear-scab. Young twigs are affected chiefly, but the lesions 
are not so conspicuous as on the fruit and leaves. The spots 
suggest the appearance of a scale insect. Onone- and two- 
year-old twigs the affected areas are velvety and enduring, 
whereas those on older twigs are soon lost to view, the bark 
being sloughed off and replaced by healthy tissue. Not infre- 
quently the blossom stalks are affected, in which case a dark- 
brown spot is produced and the young fruit falls; thus the 
young fruit fails to set. 
Cause. 
The scab spots already described are composed chiefly of 
mycelium of the fungus Venturia Pyrina. Numerous radiating 
and branching hyphe make up the velvety layer visible to the 
naked eye. Before the fungus completes its growth on the 
various organs attacked, dense erect conidiophores arise from 
the mycelium. Within a short time after their development 
numerous conidia are formed. These spores are carried to 
other susceptible organs of the pear where new spots are ini- 
tiated. Spores which by any chance fall on the apple do not 
produce the disease. Those which fall on the pear begin their 
development by germinating. Mycelium is soon formed and 
a scab spot is perceptible. If a young fruit be attacked, the 
cells beneath the lesion cease growth, as evidenced by the 
dwarfing of the fruit. This indicates strongly that there is a 
drain on the cells in the infected region. The fruit reacts 
against the fungus by forming cork, which is most evident in 
old scab spots where the fungus has ceased vigorous develop- 
