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 



