Bulletin 140, Dieback of Citrus Trees


indicative of a plentiful to an over-plentiful supply of available
nitrogen. Since he was able to produce dieback in trees growing
under controlled conditions in the greenhouse by feeding them
with stable manure and with cottonseed meal (Florida Agr. Exp.
Sta. Rep. 1912, p. cii) he has adopted the working hypothesis:
First, that the disease is due to the injurious action of some
organic chemical compound or compounds obtained from the soil,
upon the growing cells in certain of the plant parts; second, that
these chemicals do not kill the cells but induce abnormal processes
in the living matter whereby a particular type of gum formation
results; third, that this gum with the secondary disturbances
incident to its formation are the manifestations which character-
ize dieback; fourth, that the organic chemical or chemicals are
formed by the decomposition of the organic matter, either added
to or residual in the soil; and fifth, that they are formed under
certain limited conditions which are common to the different
locations where dieback occurs.
 It has been shown by Schreiner and co-workers (U. S. D. A.,
Bureau of Soils, Buls. 83 and 87) that wheat plants can absorb
and make use of certain organic nitrogenous compounds in the
absence of sufficient nitrates. Lipman found lack of nitrification
and increased ammoniafication in the soil about dieback trees in
California. According to Schreiner, these are the conditions that
would be favorable to the absorption of the organic compounds
from the soil.
 The writer recognizes the possibility that dieback may be
due to the attack of some obscure organism and that the action
of the organic material in the soil merely develops a type of
growth in the tree that is susceptible to attack. But no such
organism has been found thus far that could be considered a
causal agent.
THE CONTROL OF CITRUS DIEBACK UNDER VARIOUS CONDITIONS
 In general, there are two methods used in Florida for the
control of citrus dieback. They are the preventive and the cura-
tive methods. Both are based more upon experience and observa-
tion than upon a definite knowledge of the cause of the disease.
 PREVENTIVE METHODS
 The use of preventive methods for treatment of dieback re-
quires the exercise of judgment and a knowledge of the condi-
tions under which the affected trees are growing. In this method
the soil conditions under which the disease has developed are