APPLE DISEASES 



9 



Cause of apple-scab. 



The apple-scab pathogene is a fungus known by the name 

 Venturia in&qualis. It passes the winter in the old fallen 



leaves as immature 



perithecia. With 

 the advent of spring 

 these sexual fruit- 

 ing bodies come to 

 full maturity by 

 the time the blos- 

 som-buds are ready 

 to open. These 

 spring activities of 

 the parasite are 



ft ** 



1 i 



FIG. 4. Scab on young apples. Lesions on both 

 the fruit and pedicels. 



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 

 are mature. 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 



