APPLE DISEASES 439 
when first observed are circular and 
about one-half inch in diameter and are 
not sunken. If one cuts into the bark be- 
neath one of these spots the tissues are 
found to be discolored and to present a 
water soaked appearance. The discolored 
area is found soon to extend to the cam- 
bium, where it spreads out and may in 
some stages of the development of the 
cankers be more extensive in that region 
than the discolored area on the surface 
would indicate. The spots develop very 
slowly, if at all, during the winter months, 
but spread quite rapidly during March 
and April. As they enlarge they gradu- 
ally become elliptical in outline, the sur- 
face becomes flat, then slowly sinks. 
Sometimes the bark of developing cank- 
ers shows bands of slightly varying color, 
giving the appearance of concentric zones. 
This is doubtless due to alternating 
periods of rapid and slow growth induced 
by variations in the weather conditions. 
In early stages of rapidly growing cank- 
ers an irregular crack may appear in the 
bark and drops of fermenting sap may 
exude. 
In the early spring, when the warm 
weather induces renewed activity in the 
tree, the cankers develop rapidly. The 
bark soon dries and sinks and this con- 
dition together with the normal growth in 
the surrounding healthy bark produces 
a tension which results in the forma- 
tion of a crack in the bark at the edge 
of the canker. The spread of the fungus 
ceases as soon as the cambium becomes 
active in the spring. 
After the cankers reach the full size, 
which occurs in early May, further ac- 
tivity is confined to changes which take 
place in the dead bark within the cank- 
Fig. 1. a, 
eanker. 
Canker of Anthracnose on Apple Branch. 
Note pustule of fungus in bark of 
b, Canker Two Years Old Showing Fruiting Bodies of the Perfect Stage. 
