20 BULLETIN 684, U. S. DEPARTMENT OF AGRICULTURE. 



thermore, since the agency of birds or insects would be required for 

 the infection of apples from such sources, only a few isolated fruits 

 would probably be infected. Thus, while plants other than the apple 

 would not be very important sources of infection in so far as the cur- 

 rent year's fruit is concerned, provided the grower exercises ordinary 

 watchfulness, they are of importance in that they may be a means 

 of introducing the disease into an orchard which previously had been 

 free from it and in which it may increase and eventually become very 

 destructive. 



In addition to those mentioned above, the fungus also occurs on 

 the pear, apricot, tomato, sweet pea, and other plants, according to 

 Halstead (8), Chester (5), and Sheldon (16). 



INFECTED FRUITS OF THE CURRENT YEAR. 



From the primary sources of infection previously discussed, espe- 

 cially mummies and cankers, the fruits of the current year become in- 

 fected. These in turn become of the greatest importance as secondary 

 sources of infection just as soon as acervuli begin to be produced in 

 the diseased spots. 



While the sources of infection, mummies, cankers, etc., which 

 carry the disease over from season to season are of prime importance 

 in that they initiate the disease, nevertheless, except in occasional 

 orchards in which they are so prevalent as to be able to infect nearly 

 every apple at the outset, the infected fruits of the current season 

 are most important in the subsequent spread of the disease. 



When the infected spot becomes a few millimeters in diameter, 

 acervuli begin to form, and in a few days, if weather conditions are 

 favorable, spores by the thousand may be washed clown upon the 

 sound fruits below. Thus a few infected apples may soon infect the 

 entire crop of a tree. 



When one considers that each infected apple becomes after a few 

 days a new source of infection, and that the fungus grows and fruits 

 very rapidly, he will have little difficulty in understanding why the 

 disease can destroy an apple crop so quickly and will comprehend 

 why it is so important that the application of spraj^ for the control 

 of the disease should be made before the first appearance of the dis- 

 ease. After the disease has become well established, attempts to 

 control it by spraying not supplemented by the removal of infected 

 fruits are usually ineffective if subsequent weather conditions are 

 at all favorable to the development of the bitter-rot. 



Often only a very few trees in an orchard will possess cankers and 

 mummies in which the fungus has survived the winter. The fruit 

 of these trees, then, is the first to become infected. From these 

 infected apples the disease may be transmitted to the fruit of sur- 

 rounding trees. 



