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 Nectria 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 



