_Crown Gall._—Also commonly known as root-knot. This disease is one
of the most serious with which the grower has to contend. It is found
practically everywhere almonds are grown and either greatly reduces the
vitality of or kills the trees affected, depending upon the seriousness
of the attack.
The disease is caused by a bacterial organism, _Bacterium tumefaciens_,
that seems to be native to most California soils. It is characterized by
large swellings on the root crown or main roots just below the surface of
the ground, though lesser infections may sometimes be found also on the
smaller roots. When cut open, these knots appear spongy as if the bark
and wood were all mixed together in one mass. They are most serious when
spread over a large surface, either partially or completely girdling the
root or crown of the tree.
Control methods are of three kinds:
(1) Plant nothing but clean, healthy nursery trees, free from all trace
of galls. In planting these trees be careful to trim off all broken or
injured roots, leaving nothing but smooth clean cuts at the ends of the
roots which will heal over readily with the minimum opportunity for
infection.
(2) Galls on orchard trees may be cut out to clean, healthy wood with a
sharp knife or gouge chisel. The wound should be thoroughly disinfected
with a strong copper-sulphate or corrosive-sublimate solution, and
painted with a protective covering such as paint or melted asphaltum,
or it may be covered directly with Bordeaux paste and then the earth
returned to its place over the roots.
(3) A method used with apparent success is to bore a one-inch hole about
two-third of the way through each gall, as soon as the trees have become
dormant in the fall. Then fill each hole with a concentrated solution of
copper sulphate and plug the opening. By spring, when growth is ready
to start, the gall may be knocked off with a hammer. In most cases the
gall is so thoroughly permeated by the solution that the infection is
completely killed and further gall growth ceases in that place, unless
later infection occurs.
The use of resistant stocks has been suggested as a means of avoiding
infection, but no such stock suitable for the almond has yet been proved
to be sufficiently resistant under average conditions to be safely
recommended. The greatest hopes for future success in combating this
disease, however, lie along this line.
_Oak Fungus._—This is one of the most difficult diseases to control
because it works and spreads beneath the surface of the ground in the
roots of trees. In some sections of the state it is very serious in many
orchards.
The disease is often known as root-rot, being caused by a fungus
_Armillaria mellea_, commonly called “toadstool” fungus. It is known as
Oak fungus because the disease is most commonly found in spots where old
oak trees have stood. Where orchards have been planted on such land,
spots appear in which the trees gradually die, the disease spreading from
tree to tree, in ever-widening circles, involving ordinarily about one
row of trees each year. During the winter, clusters of toadstools may be
seen at the base of the affected trees. The fungus lives over in the old
oak roots for many years and, as the orchard becomes well established,
the fungus spreads to the almond roots. If not checked the spot will
eventually involve the entire orchard and prevent further growth of
almonds on such land for many years.
Control is very difficult but may be secured by digging a deep trench
around the affected area and preventing the infection from passing beyond
through the roots. The spread of the disease may sometimes be held in
check by grubbing out a row or two of healthy trees outside the affected
area and taking care that all of the large roots are removed to a depth
of several feet. Carbon bisulphide has been suggested for killing the
fungus, but the cost is prohibitive except in small spots just starting.
There are no resistant stocks known at present upon which the almond can
be worked. The fig, pear or black walnut might safely replace the almond
in such spots.
_Shot-hole Fungus._—There are three different fungi that produce the
shot-hole effect on the leaves of the almond, thus giving rise to the
name.
(1) _Coryneum beyerinikii_, or peach blight, is the most common form. It
is not as serious on the wood of the almond as it is on the peach, but
in seasons of damp spring weather it does much damage to the blossoms,
fruits and leaves. Affected blossoms are killed outright, the entire
blossoms turning brown and dropping much as if killed by frost. The young
fruit becomes spotted by the fungus and this causes malformation, gumming
and shriveling of the nuts, varying considerably with the severity of
the attack. On the leaves many small dead spots appear, the dead tissue
soon falling out and giving the shot-hole effect. Where the twigs are
affected, small dead spots appear during the winter, most often at the
buds. This causes the death of the buds and often the ends of the twigs.
During the spring, after growth starts, considerable gumming occurs from
these spots.
Effective control can only be secured by two sprays—Bordeaux mixture in
the fall, as soon as the tree becomes dormant, and either Bordeaux or
lime-sulphur solution (winter strength) just before the buds open in the
spring. Both fall and spring sprays must be thoroughly applied to be
effective.
(2) _Cercospora circumscissa_ is another fungus causing much the same
effect as the Coryneum. It is difficult for an untrained person to
distinguish between them. The same sprays used for Coryneum are effective
in controlling this, though if this form alone is present the Bordeaux
mixture or lime-sulphur spray in the spring should be sufficient.
(3) _Gloeosporium amygdalinum_, while apparently uncommon in this state,
has been found to exist in some places. Further work must be done on this
to determine its behavior and the most satisfactory methods of control,
but it is believed that the control measures mentioned for the other
forms of “shot-hole” will also be applicable to this.
_Prune Rust_ (_Puccinia Pruni_).—This fungus is worst in the southern
coast sections where almonds are not extensively grown. It is not
serious on thrifty trees well supplied with moisture. The disease is
characterized by reddish pustules on the under-sides of the leaves,
appearing generally about July or August and causing a premature
yellowing and dropping of the leaves.
Ordinarily the only treatment needed is to supply the necessary moisture
in the soil to keep the trees healthy and vigorous.
_Heart Rot._—This is one of the most insidious of tree diseases, for it
works inside beneath an apparently healthy exterior until the decay has
progressed so far that the tree commences to break down, and then it is
too late for remedial measures. The almond is not as susceptible to this
as most other kinds of orchard trees, but where large wounds have been
exposed to the weather, infection may take place readily, and after it is
once well started it continues at a comparatively rapid rate. Decay is
caused in most cases by one or more of about a dozen different fungi, of
which the oyster-shell fungus is by far the most common.
Control consists in taking care to leave no open wounds exposed to the
air to dry and crack, thus permitting the entrance of decay organisms.
Much of this can be avoided by care in pruning the young tree so that the
removal of the large limbs will not be necessary later on. Where such
wounds must be made, measures should be taken to prevent infection. This
can best be done by making smooth, clean cuts close to the part from
which the branch to be removed emanates, leaving no stub. Stubs dry out
and crack more quickly and require very much longer to heal over, if this
is possible at all. Further, all such wounds which will not heal over
the first season should be covered with some good disinfectant, such as
corrosive sublimate, one part to one thousand parts of water, and then
painted over with some elastic coating, such as “Flotine” or asphaltum,
grade D, applied with a brush. The entire wound must be covered or the
work is largely wasted.
_Die-back._—This is serious in many orchards where moisture is
insufficient to carry the trees through the growing season, and the
trees show considerable dying-back of the branches. Unfavorable soil
conditions, such as hardpan, gravel or sand may be the direct cause of
such moisture shortage. Lack of soil fertility is also a common cause.
Control measures consist in remedying the defective conditions and where
this cannot be done economically it is better to abandon further attempts
at almond culture on such land.
_Sour-sap._—This is one of the so-called “physiological diseases” and is
quite common with the almond. It is most frequently found where trees
are planted in heavy or poorly drained soils. The inability of the
almond to endure standing water around its roots for any length of time
and particularly so after growth commences in the spring, renders it
especially liable to sour-sap when planted in soils where excess water
from the late winter and spring rains cannot be readily drained away. The
direct cause of the trouble is sudden changes in weather from warm to
cold after growth commences, which checks the flow of sap very suddenly,
causing stagnation, cracking of the bark and then fermentation. With an
unusually strong flow of sap in trees in wet soils, such climatic changes
cause unusually severe disturbances in the normal functioning of the
trees.
The affected trees ordinarily show the disease first in the spring when
gum may be seen oozing from the bark of the trunk or main branches, and
sometimes even from the smaller branches. Small or large branches may
die, and in severe cases the tree may die soon after having commenced to
leaf out strongly. On cutting through the bark to the wood and peeling
back, a strong sour odor is noticeable. The cambium layer appears
brownish or reddish in color and often masses of gum may be found between
the bark and the wood. Mild cases may not be serious enough to show on
the outside of the tree and only portions of the cambium layer may die.
The sudden dropping of the blossoms or young fruit may in some instances
be attributable to sour-sap.
All affected parts on smaller branches should be cut back to healthy
wood, while on the main branches or trunk, where only a small portion or
one side is affected, it is best to clean out the dead bark and paint the
bared wood with a protective covering until new bark can cover the spot.
At the same time every effort should be made to remedy the soil-moisture
conditions which were largely responsible for the trouble in the first
place.
_Fruit-drop._—The same conditions which cause sour-sap may cause fruit
drop. It may be caused by lack of pollination due to improper mixing of
varieties or to rain during blossoming. Frost may also produce the same
thing by killing the germ in the young fruit. In such cases, the fruit
may remain on the tree for one or two weeks after the injury occurs
before falling, and in some cases, may even appear to continue its
development for a short while.