(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.