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Wounding or killing of peach tissues results in the production 

 of wound gum in the surrounding areas when moisture and temperature 

 conditions are favorable. When a wound becomes infected with the 

 Yalsa fungi, however, the wound gum region is much more extensive 

 than in an uninfected wound. Such qum is usually proportional to 

 the amount of tissues killed and is known to serve as a barrier to 

 further invasion of tissues by fungi. The Yalsa fungi, however, 

 although slowed down by the wound gum barrier, are not stopped and 

 can grow through it by means of appressoria and slender penetration 

 threads . 



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In vigorous trees, the advance of the canker is stopped quickly 

 by the formation of a ring of callus tissue which heals the wound 

 and inhibits the further growth of the fungus. In trees of poor- 

 vigor, however, or trees weakened in any way, callus formation is 

 slow and the fungus usually advances before such tissue can form. 

 In weakened trees callus is formed during the periods of cessation 

 of growth of the fungus, such as during the hot months of the sum- 

 mer or during the warm spells in the winter, but even these callus 



