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. On one- 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 hyphse 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- 



