Bulletin 140, Dieback of Citrus Trees


bark of mature or nearly mature terminal or subterminal stems.
 (Fig. 10.) In severe cases, they may be formed in the bark of
 larger branches, or on the trunk of young trees. They consist of
 longitudinal to more or less rounded breaks in the bark thru
 which a gummous tissue is forced. This forms a double convex
 to rounded pad of crystal-like gum on the stem.
 When the excrescences occur on the terminal stems, they are
 more commonly formed near the base of the stem. They may be
 formed in the leaf axil among the buds. They may also occur
 in the bark of the fruit spurs (fig. 11). When this happens, the
 fruit will also show markings by the disease. The excrescences
 frequently occur in breaks in the bark formed by the bark cam-
 bium which gives rise to the true bark of the stem. It is probable
 that they have their origin, at least in some cases, in this tissue.
 This symptom does not lead to the death of the branch. After
 a year or more, new bark is formed below and the diseased bark
 is sloughed off.
 Stained Terminal Branches. Staining of the terminal
 branches occurs while they are still young and somewhat suc-
 culent but are approaching maturity. It never develops anew
 upon either very young or mature branches. The stained ap-
 pearance is due to a brownish, glossy, gum-soaked condition of
 the bark in spots and irregular areas (fig. 4). The areas are
 only slightly, if at all, raised. The staining is caused by the
 closing of the epidermal and subepidermal cells with a gum-like
 substance. No corky tissue or other proliferous tissue is devel-
 oped beneath the areas. The staining is almost entirely confined
 to the bark of the terminal stems; it never occurs on the sub-
 terminal stems. It sometimes extends on to the leaves (fig. 5),
 in which case it is found mostly on the petioles and the base of
 the blades.
 In severe cases, the trees make a very promising growth of
terminal stems that later become stained and die. The repeated
growing and dying back of these stems give the trees a character-
istic appearance. This suggested the name Dieback," which
was given to the disease.
 It is doubtful, except in extreme cases, whether the staining
of a branch causes sufficient injury to kill it. It is more likely
that it so weakens the branch that secondary factors, such as the
withertip fungus, are able to attack and kill it. In the extreme
cases, however, where the entire area of the bark is largely