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, ina 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 
