APPLE DISEASES 
new wood, followed by further killing 
of the living tissue. In early stages of 
development, cankers show a region of 
sunken discolored bark and it is only in 
later stages that the bark breaks away. 
Cankers have been described as caused 
py frost, sun scald, fungi, and bacteria. 
A considerable number of different fungi 
have been reported as causing canker of 
apple trees in different parts of the 
United States. These vary greatly in 
the amount of damage which they do in 
different regions. In some cases, a 
fungus which causes a great amount of 
injury to the trees of one region occurs 
rarely or not at all in another region. 
Frost Canker 
Much of the disease of apple trees 
which orchardists have been calling 
canker has its origin in severe winters. 
Some injuries so resulting might be 
properly classified under the term ‘“‘frost 
canker.” On the other hand, when whole 
trees are so badly injured that they die 
either that year or the year following, 
the injury is too widespread and acts 
too quickly to be regarded as canker. 
There are a number of forms of winter 
injury and the frost canker is only one 
of them. The frost canker is a local 
injury which tends to heal over under 
favorable conditions for growth unless 
the new growth is killed by another 
period of low temperature before it has 
become hardened. In this way the frost 
canker may spread, or in other cases 
the injured bark may serve as a place 
for the entrance of a parasite which may 
then spread in the bark and outer layers 
of wood and kill a rather large area in 
a single year. 
So far as they have been investigated 
it has been found that the organisms 
which cause canker of fruit trees are, 
in a large measure, wound parasites. 
They are unable, as a rule, to penetrate 
the uninjured bark but must enter 
through wounds. In this sense, the 
places injured by freezing serve the 
same end as wounds of any other kind. 
However, it may be pointed out that 
cankers caused by fungi do not spread 
455 
so rapidly as to kill trees in the short 
time which has been observed in the 
case of winter-killed trees. In the case 
of young trees the fungus may in some 
cases girdle the tree in a few weeks and 
thereby cause its death. The same holds 
true of small branches of old trees, but 
in the case of large branches the fungus 
usually spreads but a few inches each 
year forming true cankers, and the 
rough, blackened areas that are fre- 
quently seen on large branches often 
represent a development of several years. 
The living tissues attempt to heal over 
the wound by the formation of callus 
and in some cases with considerable 
success. Often the parasite ceases to 
spread in the bark when the dry season 
of summer comes on and a crack forms 
between the healthy and diseased bark. 
The following year the diseased area 
may continue to spread or the callus 
may check it considerably. Often other 
fungi, some of them saprophytes, grow 
upon the dead bark. 
There are many other wounds than 
those caused by freezing through which 
parasitic fungi may enter. By this it is 
not meant that every wound that is 
made in the bark will necessarily be- 
come infected and develop into a dis- 
eased area. In many cases, however, 
the spores of parasitic fungi are carried 
to wounds. This is especially liable to 
be the case when diseased branches are 
allowed to remain on the trees, or old 
neglected trees in the neighborhood pro- 
duce abundant crops of fungus spores 
from year to year. Some of the ways 
in which wounds are made are: Bark- 
ing of trunk and branches by machinery 
in cultivating and caring for the or- 
chard; injuries by ladders and by men 
in picking fruit; branches are some- 
times injured by props used to support 
a heavy load of fruit especially when 
they are carelessly placed in position; in 
some cases hailstones split the bark of 
small branches. Care should be taken 
to avoid any injury which is within the 
eontrol of the orchardist. Wounds are 
sometimes kept from healing over by 
the woolly aphis which forms little cot- 
