BLIGHT 
39 
The blight first becomes apparent in the blossoms, and the young 
twigs, causing their bark and leaves to be discolored, to shrivel and then 
die. These blighted twigs look as if the entire twig or blossom had been 
scorched by fire, but the leaves and blossoms cling to the tree for some 
time, and often dead leaves are seen hanging on the bare branches during 
the winter. In the apple, the blight is usually confined to the small new 
twigs or flower clusters, and the trees usually outgrow the effects of its 
ravages in a short time. The early apple crop may sometimes be de- 
stroyed by the blight, but ordinarily it merely thins out the crop. The 
disease spreads during the period of most rapid growth of the trees, and 
attacks those trees making the most vigorous growth. It usually ceases 
rapid development during dry weather. 
Fruit Blight. The epidemic of fruit blight sometimes follows one 
of blossom blight. It will affect green fruit, but seldom directly attacks 
the fruit after it is ripe. Some of the blossoms may escape and develop 
fruits and these later be infected either by the spread of the disease 
through the twigs or from direct inoculation by curculio and other in- 
sects. The fruits which are killed in this way become mummies. A 
mummy is a partially grown fruit that has dried up, forming a hard, 
wrinkled lump. It has not rotted as fruits usually do. This is partially 
due to the fact that it hangs on the twig after it has died, remaining 
suspended in the air instead of falling to the ground. As a rule, mummies 
fall to the ground, sometimes before they have time to dry up; but in 
fruit blight they often hang on through the entire winter, and their pres- 
ence on the dead branches, along with the bunches of dry, burnt-looking 
leaves, indicates fire blight. 
Blight Canker, on the large branches and the trunk should re- 
ceive special attention, because it is in these that some of the bacteria live 
through the winter and cause infection early in the spring. They gain 
entrance into the thick bark on the branches and limbs through first 
entering the soft, sappy tip of a water-sprout, and working downward, 
and through pruning scars or injuries caused by cultivation. It is be- 
lieved that aphids and leaf hopjDers are especially active in the inoculation 
of water-sprouts and young twigs. In twig blight of old trees, only the 
current year's growth is killed. In young trees, it may go down the 
twig to the main branch as in a water-sprout. 
The bacteria may work for a while and form a small canker and 
then all of them die, or they may work actively for a while and then, after 
remaining quiet, continue to spread so the canker is outlined by several 
concentric rings. A definite boundary line between the dead and live 
bark usually indicates that the bacteria in that canker are all dead. When 
all of the bacteria die, the bark is eventually sloughed off and the tree 
tries to heal over the wound. When the canker is an active one, the ad- 
vancing margin can often be distinguished by a shght swelling at the 
outer edge and the somewhat scalded, watery appearance of the bark. 
This is not always very apparent, and must be noticed closely in order to 
be seen. 
The bacteria are alive and actively engaged in the destruction of the 
bark only at the outer margins of the canker. An enzyme which they 
secrete enables them to make an opening in the cell walls, thus passing 
from cell to cell. This poisonous enzyme kills the cells of the tree and 
at the s'dme time it drives out or kills the bacteria themselves. They do 
not work in the sap tubes, neither are they very often found in the woody^ 
parts of the tree, but generally in the soft growing layer of the bark, 
which is the most vital part of the tree. For this reason, the live bacteria 
are found only near the outer edges of the canker, which has an indefinite 
outline. There are other kinds of cankers, some of which are caused by 
fungi, and are mistaken for the "blight canker." The true blight canker 
is found most often on the body, trunk, and limbs of a young tree, usually 
eight to fourteen years old. 
