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 down 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 spray 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. 
