The Yale expositor., 1917-06-14

|
| CBy LEONARD r. HARTII.L, Instructor
in Fruit Growing New York State School
OF Agriculture on Long Island, Farm
ingdale, N. "
OF all the diseases of fruit trees
| there is none more dreaded or more
destructive than the disease commonly
known as fire blight. This disease is
of bacterial origin and is extremely
infectious. 1t attacks the pear, the APP
pie, the ounce, the wild crab apple.
the hawthorn, the mountain ash and
other trees. Owing to its ravages the
commercial growing Of choice varieties
of pears is now limited TO a compare
lively small area Of the country.
Modes Of Infectlon.-Under favor
able conditions fire blight may attack
any part of the tree, but usually infec-
ton takes place in the blossoms and
in the actively growing tips of
branches. 1t is brought about by in
sects, usually bees. The bees visit the
old hold-over cankers of the blight and
become covered with the organisms in
their search for nectar. The organ
lsms multiply rapidly in the nectar of
the blossoms and enter the living It's
sues through the unprotected hectares.
Having entered the tissues they quick
ly blight the blossoms, pass down the
blossom stem into the fruit spur, kill
ing the tissues and cutting the leaves
off from a supply Of water, thus pro-
ducing "fruit-spur blight" the latter
occurring several weeks after blossom
Infection. In serious cases all the
fruit spurs may be blighted in this way
and consequently set no fruit. Some
times the infection ends with the blight
Ing of the spurs and does not extend
to the twig or branch, but frequently
1t extends into the bark of the branch
at the base of the blighted fruit spur,
resulting in Tie formation of typical
blight cankers. Blight cankers are
also originated either through In
lection taking place at the tips
Of young, actively growing branches,
particularly If these be water
sprouts, or through wounds of any
kind. Such cankers when on the larger
limbs or trunk may retain active germs
over the dormant season and form the
so-called hold-over cankers, which
very likely form the only source Of in
lection for blossoms in the spring.
Deseription.-If the infection begins
In flower cluster or at the end OF
shoot the leaves will wilt, and in the
case of the pear, drop. The bark Of
an infected tree becomes darkened.
The disease spreads rapidly, running
down the limbs and trunk until the
tree becomes dormant with the ap-
proach of winter. The following spring
the disease renews it's activity. The
progress Of the disease is most rapid
in the fleshy outer layers Of the bark.
At first II produces a watery appear
ance in the affected area, later develop
ing dark-colored, sunken cankers. A
sticky substance exudes from the cam
kers when they are active. This
substance contains myriads of live
germs.
Control.--The only known remedy
for fire blight is 10 use the knife and
saw thoroughly, cutting out all the in
fected wood. In cutting out blighted
limbs or twigs, cut when possible at
least a foot below the infection and
then examine the cambium below the
cut to make sure that no blighted por-
ton remains. Large limbs when in
fected should have the infected area
planed off down TO the sap wood. All
wounds made on q tree in cutting out
infected wood should be Immediately
washed with solution of corrosive
to 1000 parts of water). When the
wound is large IL should also be paint
ed over with good white lead paint.
The tools used In cutting out blighted
wood should be kept sterile by being
frequently immersed in the corrosive
sublimate solution.
During the growing season all trees
In the orchard should be kept under
constant inspection so that outbreaks
of the disease may be noted at once
and any blighted wood Immediately re-
moved. In the autumn. In an orchard
where the disease has made its appear
ance, the soil should be removed from
art and the crowns of the trees and
the trees gouged at the crowns with n
quarter-round chisel to detect the pres
ence of any blight at the bases Of the
trees. With large trees the main
limbs should also be gouged at the
forks.
The most serious development Of the
disease is obtained when IL attacks the
roots OF the tree. The usual method Of
procedure in such a case is TO expose
the roots and cut out all infected por-
lions, taking the usual precautions OF