APPLE DISEASES 3 



SCAB 

 Caused by Venturia inaequalis (Cooke) Winter 



The scab disease of the apple is universally the best known 

 of all fungous troubles affecting this fruit. While it attacks 

 only the apple and certain closely related species, its ubiquitous 

 nature accounts for its prominent rank among the diseases of 

 its kind. This familiar trouble is commonly thought to be the 

 same as pear-scab, but these two diseases, while similar in their 

 symptomatic and causal aspects, are not identical. Nor should 

 the scab diseases of the peach, cherry, and citrus be confused 

 with the scab of apple. 



All varieties of apples are affected, some more severely than 

 others. A variety may be resistant in one year and susceptible 

 in another, under conditions which in both cases are apparently 

 favorable for scab on the average susceptible variety. For 

 example, the Baldwin is usually listed among the resistant sorts ; 

 yet in 1910 this variety showed in certain localities 98 per cent 

 scab on unsprayed trees. The Ben Davis, also said to be 

 resistant, often shows as much scab as the average variety. 

 A suggested explanation of these conflicting observations is 

 found in such factors as the color of the fruit, the relation of 

 weather conditions to the development of the fruit, and the 

 adaptation of the pathogene to new conditions. Scab lesions 

 are not so conspicuous on dark-colored fruit as on lighter 

 varieties, and thus the Baldwin bears the reputation of being 

 resistant, while the Rhode Island, which perhaps is really the 

 more resistant of the two, is regarded as very susceptible. 

 Further, in connection with the weather, in a given year the Bald- 

 win may be at its most susceptible stage at the time when 

 weather favorable to infection prevails ; that year the Baldwin 

 would appear susceptible while in another year it escapes the 

 disease and is then classed as resistant. This suggestion may 



