Bulletin 140, Dieback of Citrus Trees


the disease due to the persistence of the causal agent. In the
chronic form, the disease often attains complete development in
which all of its symptoms are evident. If the conditions con-
tinue, the trees will be badly injured and may die. This form of
the disease usually occurs where the disease has been brought
on by some persistent, improper soil conditions, such as lack of
drainage or a hardpan too near the soil surface.
 In the acute form, the disease is only temporary. In this
 form the disease does not always attain complete development.
 Only one or more of the primary symptoms may be present. For
 example, the only evidence of the presence of the disease may be
 the gum pockets and stained terminal branches, or it may be
 the gum pockets, stained terminal branches, and bark ex-
 crescences, etc. It may happen that the fruit is the only part of
 the tree affected, and the marking of the fruit may not be ac-
 companied by any extensive splitting or dropping. This form of
 the disease occurs where it is brought on by some improper soil
 treatment, such as overfeeding the trees with organic fertilizers,
 or by excessive cultivation; or by some soil condition such as an
 irregular moisture content. In these cases, with the changed soil
 and seasonal conditions, the disease may in time disappear with-
 out treatment.
 The development of the disease is the result of the complex
 action of many factors, and is dependent not only upon the dis-
 ease stimulus but also upon stages of growth in the tree. Con-
 sequently, a complete development of the disease is attained only
 where the disease stimulus is more or less continuous so that the
 two are coincident. It is probable that the partial development
 of the disease in which only a few of the symptoms are evident-
 which so often occurs-is explained by the temporary or inter-
 mittent action or existence of the causal agent.
 In'the typical development of a case of dieback, the first indi-
cation of the disease is an exceptionally deep green color of the
foliage and a rather rank growth of the branches. The trees
have a thrifty and promising appearance. Many of the terminal
stems are long, angular, and distorted in shape. Frenching of
the foliage may or may not be present. Gum pockets are the first
of the gum symptoms to appear. With further growth of the
tree and development of the disease, staining of the terminal
branches and the fruit, and the formation of bark excrescences
and multiple buds occur. The formation of these is coincident
with the development of the parts which they affect. In later