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, Physalospora 
Cydonie. 
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. 
