58 



Diseases of Economic Plants 



scab infections usually occur about the time the trees are 

 blossoming, and that serious damage may be caused by in- 

 fections taking place for four or five weeks after the petals 

 have fallen; that the first serious infections of apple blotch 

 may take place three or four weeks after the petals have 

 fallen, and that further infections, resulting in serious damage, 

 may occur for a period of five or six weeks; that the first 

 infections of ])itter-rot may occur about the middle of June, 



Fig. 28. — Unsprayed apple trees. After Scott and Rorer. 



the period of possible infection continuing throughout the 

 season, the probability of infection being greatest in hot, wet 

 weather. 



The choice of the fungicide and the decision as to when to 

 spray are governed by several considerations. These include 

 the local conditions, among which may be placed varietal 

 susceptibility to apple scab, and the probable local varietal 

 susceptibility to other fungi, seasonal weather conditions, 

 and varietal susceptibility to spray injury. 



Locality. — Apple scab, apple blotch, and bitter-rot are so 



