(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.