2 
FRUIT TREE CANKERS AND THEIR CONTROL. 
DAMAGE FROM CANKERS 
Cankers may cause general unthriftiness of the trees ; or the 
death of the twig, branches, or even of the whole tree, depending 
on location, size, and kind. In addition to the damage caused 
by the cankers themselves, they are dangerous because they 
often allow the entrance of wood-rotting fungi which may be 
unable to get into a healthy, living part of the tree, since many 
can grow only on dead wood. When the canker has killed the 
bark, however, the natural protection of the tree is destroyed. 
The wood-rot fungi can then easily get into the heart wood and 
rot it, thus weakening the tree considerably, or may get into the 
sap wood (living wood) and kill it. (See Figure I.) 
KINDS OF CANKERS 
There are several kinds of cankers commonly found in Min- 
nesota. Among them the most important are those often re- 
ferred to under the general term of winter injury, sun scald, or 
crown rot ; fire blight cankers. Figure II ; black rot cankers and 
numerous others. 
CAUSES OF CANKERS 
Cankers may be caused by the weather. The exact reason 
for this injury is not known. The wood may be winter killed by 
drying or the injury may occur as a result of alternate freezing 
and thawing. 
Mechanical injury may often cause cankers. Branches may 
rub together and wounds result. (See Figure III.) When a tree 
is poorly shaped in pruning, the limbs may be so long that the 
swaying of the branches caused by the wind may injure the 
crotch and cankers and wood rots follow. 
Wounds caused by careless pruning or careless cultivation 
may serve as points of entrance for canker-producing fungi. 
(See Figure IV.) Animals, particularly mice and rabbits, very 
often cause cankers on trees. If the injury is not too great, these 
may heal over rapidly. If it is quite extensive, however, wood- 
rotting fungi may enter the dead areas. 
Some of the worst cankers are caused by fungi or bacteria. 
The brown rot of plums and cherries, which causes the fruit rot, 
may also cause cankers on the twigs or even on the larger 
