Diseases
Although not as numerous as insects, the diseases which
attack the apple inflict great damage and are fully as difficult to
control. They are caused by bacteria and by fungi which may be
compared to weeds growing on or in the tree instead
of the soil. If
either of these works within the plant, as is sometimes the case, it
must be attacked before it enters. It is very necessary to be thorough
in order to control these diseases. Weather conditions influence
nearly all of them materially. Of those which attack the apple tree or
fruit we have selected three as the most serious and the most
necessary for the grower to combat, namely, (1) apple scab, (2) New
York apple tree canker, and (3) fire blight. To these should be added
in the South and middle latitudes, sooty blotch and bitter rot.
Baldwin spot is also frequently serious in some seasons and
localities.
(1) THE APPLE SCAB, commonly known among growers as "the fungus," is
the most important of our common apple diseases and is most evident on
the fruit, although it attacks the leaves as well. In some seasons the
fruit is made almost unsalable. This disease lives through the winter
on old leaves. In the spring about blossoming time the spores are
scattered by the wind and other agencies, and reaching the tender
shoots germinate and enter the tissues of the plant. Their development
is greatly dependent on the weather. In a season in which there is
little fog or continued damp or humid weather, they may not develop at
all, but where these conditions are present they frequently become
very virulent.
Spraying will be governed by the weather conditions, but the mixture
must be applied very promptly as soon as it is evident that it is
likely to be necessary and must cover every part of the tree to be
effective. The object is to prevent the spores from germinating, the
spray being entirely a preventive and in no sense a cure. The disease
most frequently first manifests itself on the tender new growth and on
the blossoms. Two mixtures have been found to control it, namely,
Bordeaux and a weak solution of lime and sulphur. One or other of
these should be applied just before the blossoms open, just before
they fall, and when necessary two and nine weeks later.
(2) NEW YORK APPLE TREE CANKER is usually found mainly on the trunks
of old trees, but it also affects the smaller branches. Practically
every old or uncared for orchard has more or less of this canker, and
where it is not checked it eventually destroys the tree. This fungus
is the cause of most of the dead wood found in old orchards. The
surface of the canker is black and rough and covered with minute black
pimples. It lives over winter and spreads from one branch or tree to
another. As it most frequently enters a branch through wounds made in
pruning, these should be promptly painted over with a heavy lead and
oil paint. All diseased parts should be cut out and removed as soon as
observed. The value of spraying for this disease is not definitely
known, but it is seldom very troublesome in well sprayed and well
cared for orchards.
(3) BLIGHT appears on apple trees in three forms, as blossom blight,
as twig blight, and as blight cankers. It is a bacterial disease
which is distributed by flies, bees, birds, etc., and cannot be
controlled by spraying. The bacteria are carried over the winter in
cankers on the main limbs and bodies of the trees, oozing out in a
sticky mass in the spring. These cankers should be cut out with a
sharp knife cutting well into the healthy bark and then washing the
wound with corrosive sublimate, one part to one thousand of water.
Cutting out and destroying are also the chief remedies to be used when
the blight appears in the twigs and blossoms. It is not usually as
serious on apples as on pears. Some varieties, like Alexander, are
more subject to it than others.
Previous: Insect Pests
Next: The Principles And Practice Of Spraying
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