Pomaceous Fruits 61 



immunity may increase or decrease. (5) No variety is abso- 

 lutely immune. 



Varietal susceptibility to scab differs so markedly in 

 different localities that general lists are of little value. The 

 Ben Davis, for example, which is generally considered rela- 

 tively immune to scab, is very susceptible in Illinois. The 

 Yellow Transparent, stated to be seriously injured by scab 

 in Ohio, ''^^ is relatively immune in southern Illinois, and the 

 Grimes, which, in Ohio, is moderately injured by scab, is 

 relatively immune in Illinois. The Rome Beauty is seriously 

 injured in both localities. 



Weather Conditions. — Weather conditions play a very 

 important role in the outbreak of a disease. Wallace ^^ 

 states that ''The ideal condition for scab infection is a gentle, 

 continued rain followed by cloudy, calm weather and a 

 saturated atmosphere, in which cases the spores are kept 

 wet for a long time in one position. Ascospores have been 

 known to germinate in a period of four hours, but it is 

 probable that in order to produce abundant infection 

 the trees must be kept wet for eight or ten hours or 

 even longer." Ascospore discharge begins almost as soon 

 as rain begins, or within five minutes after the time the 

 leaves are wet and, under continuously wet conditions, 

 an interrupted discharge of spores can be expected for 

 some time.^^ 



Roberts ^^ states that the dependence of apple-blotch 

 infection upon rainy weather in orchards in the Ozark section 

 is very noticeable, but in Kansas, where the sources of infec- 

 tion — that is, the twig cankers — are much more numerous, 

 the dependence is less noticeable. 



"The conditions most favorable for the development of 

 bitter rot are (1) a period of hot weather accompanied by 

 frequent rains and heavy dews at a period when the apple 

 crop is approaching maturity, i. e., from the middle of July 

 to the end of August. (2) Numerous sources of infection, 

 i. e. cankers on the limbs and mummified fruits left hanging 

 on the trees." Blair,^^ from whom the above is quoted, sum- 



