38 



MANUAL OF FRUIT DISEASES 



death of the bark on the southwest or sun-exposed side of a 

 tree. The frosted bark may peel, exposing the discolored sap- 

 wood ; or sometimes it adheres closely to the wood and a sunken, 

 cankered area is thus formed. In some cases the sapwood is 

 killed and the cambium is left alive to form new wood outside 

 the dead area. The sapwood is stained by the diffusion into it 

 of some substance apparently originating in the protoplasm of 



the affected tissues. When the 

 wood is thus discolored it is 

 called black-heart. This type 

 of injury is common in the 

 northern United States. Frost- 

 killed areas in the bark are 

 commonly inhabited by sapro- 

 phytic and weakly parasitic 

 organisms, prominent among 

 which is the New York apple- 

 tree canker fungus, Pliysalospora 

 Cydonice. 



The name sun-scald owes its 

 origin to the common belief that 

 it is due to some interaction of 

 sun and cold on the sunny side 

 of the tree in late winter. It in- 

 volves the crotches and the ad- 

 joining sun-exposed bark of the 



trunk and limbs as well. The Ben Davis, Stark and Twenty 

 Ounce varieties are generally regarded as most susceptible to 

 sun-scald. Trees leaning to the northeast are most severely 

 injured. 



Twigs are killed by winter temperatures. The last season's 

 growth is killed back to a definite point. This occurs every 

 year to some extent. It depends largely on whether or not 

 the wood ripens properly in the fall. 



FIG. 9. Apple-tree showing frost- 

 cankers at crotch. 



