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.
INSECT PESTS
Mites.—Commonly called red spiders. There are two kinds of mites that do much damage in almond orchards, the brown or almond mite and the yellow or two-spotted mite. Both are common in all parts of the state and are the worst pests the almond grower must regularly face.
The brown mite (Bryobia pratensis) is the larger of the two, is dark red or brown in adult stage, has very long front legs, and a flattened back. It does not spin any web and works on the green bark of the small twigs as well as on the leaves, sucking the plant juices from beneath the bark. It causes a mottling of the leaves which eventually fall, although not as readily or in such large numbers as when attacked by the yellow mite. The injury to the tree is equally as great because of the serious drain on the vitality as a whole and because it commences work earlier in the season. This mite may spend its entire life on the tree; the very small, round, red eggs being laid largely on the under-side of the branches and in cracks and crevices in the bark and twigs. These remain on the tree throughout the winter and hatch early in the spring soon after the trees have their leaves half developed, leaving the white egg-shells in place. Most of their work is done in the spring and early summer.
Fig. 22.—Nonpareil almonds. Branch on left free from Red Spider and holding its full supply of leaves in green, healthy condition; branch on right defoliated by Yellow Mite. Note premature ripening of nuts on defoliated branch.