128 MANUAL OF FRUIT DISEASES 
continues to extend. The prevalence of this type of canker 
on erect branches is explained on the basis that such limbs 
lack the necessary food supply to overcome the progress of the 
pathogene. Here the pathogene appears to have the advantage 
as evidenced by the expansion of the lesion and the failure of 
the host to check the spread. In the closed cankers the host 
has the advantage. Along the margins of the older cankers, 
brilliant red fruiting bodies of the pathogene develop. These 
are easily seen with the naked eye, even though they are never 
larger than a pin-head. 
The lesions center about a wound, a bud, or the fork of two 
branches. Such wounds as those caused by hail, insects, prun- 
ing and frost are common seats of the injury. Very commonly 
a stub or twig may be found at the center of the canker. 
Cause. 
The European canker is caused by the fungus Necetria galli- 
gena. As previously intimated the mycelium lives over from 
year to year in the diseased bark. In the spring and early sum- 
mer red perithecia develop in the wound and under favorable 
conditions discharge their ascospores. Conidial tufts are 
developed at this time of year also, so that there are two kinds of 
spores for initiating primary infections. It has been shown 
that insects are highly important as agents of inoculation; the 
woolly aphis, for example, is very active in carrying the spores 
of the fungus. In Europe an outbreak of canker is said to follow 
closely an unusual prevalence of this insect. It has been 
estimated that in a single canker 300,000 ascospores are avail- 
able for dissemination. The spores germinate in a few hours 
and the germtubes enter the bark through wounds or lenticels. 
Within a week the effects of the fungus are visible. The 
mycelium, developing from the germtube, permeates the bark, 
the wood and the pith. The attacks are confined chiefly, 
however, to the bark, where the cortical cells are killed by the 
fungus. As a result of death, the affected portion of the bark 
