Bulletin 229, Diseases of Citrus in Florida


canals may be short, variously interrupted, or extend continu-
ously around the trunk or branch. They often alternate with
normally developed wood accretions.
 Considerable variation occurs in the severity of gummosis
 and the rapidity with which it causes the tree to decline. In
 some cases the disease may be present for a number of years
 without seriously affecting the tree, while in other cases it de-
 velops rapidly and kills large areas of bark within a short time.

 CAUSE OF GUMMOSIS
 Citrus trees, in common with stone fruit trees and a number
of others, are characterized by a strong tendency to gum. The
formation of gum may be induced by various fungi, by chem-
ical, mechanical, and insect injuries, and by certain physiologi-
cal stimulations. Although minor forms of gumming may be
induced by a number of citrus diseases, both parasitic and non-
parasitic, and by certain chemical and mechanical injuries, there
is a characteristically severe and often chronic form of gumming
of uncertain cause on large bearing trees that is commonly term-
ed gummosiss."
 The various citrus diseases characterized by the formation of
gum cannot be diagnosed by the occurrence of the gum itself,
since its nature and appearance are essentially the same regard-
less of the influence that induced its formation. The process of
gum formation is a pathological condition in which the cell walls
of the wood-forming tissue, in certain more or less localized zones
of special cell formation, degenerate either partially or entirely.
This transformation is a hydrolysis process, apparently result-
ing from enzyme action, which may change the walls of entire
cells into gum while they are still in the developmental stage.
When gum forms in sufficient quantity to rupture the bark by
the internal pressure exerted the gum exudes and runs down the
trunk. In a sense, the gum is merely a by-product or secondary
feature of an abnormal condition or disease. It will thus be
seen that it is not the exudation of gum but rather the death
of the more or less large areas of bark that precedes, accom-
panies, or follows this gum formation that is most menacing
to the life of the tree.
 Fawcett, in California, has shown recently that environment-
al conditions cannot, by themselves, initiate all the severe forms
of gummosis in Citrus earlier attributed to them, although many