86 MANUAL OF FRUIT DISEASES 
to three years old are sources of trouble. Furthermore, diseased 
bark which falls to the ground may develop the ascospores as 
described above. And finally, the inocula (conidia and asco- 
spores) may come not only from the apple-tree but from the 
pear, on which fruit-tree the fungus also occurs. 
Control. 
In applying remedial measures it should be borne in mind 
that: (1) while the disease appears to be most serious on 
certain varieties, no variety is wholly immune; (2) the disease 
is caused by a fungus; (3) the causal fungus infects the trees 
in the fall, particularly from October to December; (4) the 
fungus can enter through wounds, lenticels or unbroken sur- 
faces; (5) cankers produced on the limbs are the only source 
of the inocula — these give rise to all the trouble; (6) these 
cankers are a dangerous source even when three years old; 
(7) fallen bark from one-year-old cankers is also a source of the 
inoculum; (8) the fungus inhabits not only the apple, but also 
the pear, which may be a source of trouble. 
It is therefore out of the question at present to rely on any 
variety to resist the attacks of the fungus. Any spraying 
which is done must be accomplished before the most important 
period of inoculation and infection. It has been shown that 
old orchards may be cleaned by proper spraying. Bordeaux 
mixture 6-6-50 is said to be better than lime-sulfur for this 
purpose. For this disease spray as follows: (1) as soon as 
possible after the fruit is picked; (2) again within two or three 
weeks. Where the trouble is serious an application should be 
made at least once before the fruit is picked; this may be made 
about September 15, or before the first autumn rains. Then 
spray twice as directed above. These applications should 
thoroughly coat the limbs to protect them from the attacks of 
the fungus. Cankers on the more valuable limbs of younger 
trees may be removed, in which case the débris should be 
destroyed and the wound coated with coal-tar. The removal 
