Florida Agricultural Experiment Station


with the normal growth functions of the tree. As a matter of
fact, a number of different conditions may contribute, either di-
rectly or indirectly, to its development. In the majority of cases it
appears to be caused by some condition which results in an over-
stimulation of the growth processes; however, in other cases,
mild forms of the disease occur on trees that are not making a
vigorous growth and occasionally on trees in the process of de-
cline from other diseases and soil conditions.



















Fig. 68.-Young grapefruit affected by exanthema, showing the character-
 istic dark brown eruptions of the rind, exudation of gum, and irregular-
 splitting.
 It was believed for many years in Florida that exanthema was.
caused by fertilizing the trees with excessive amounts of or-
ganic nitrogenous materials. Later, it was held that all sources
of nitrogen were favorable to the development of the disease,
but only when used in excess. However, nothing very definite
can be said even today concerning the role of nitrogenous fer-
tilizers in the stimulation of exanthema. While their use does
appear to have some bearing on the development of this trouble
under certain conditions, much of it attributed to their use ap-
pears to be assignable to other factors. Among other theories
that have not been substantiated may be mentioned the theory
advanced in 1914 by Lipman, who contended that exanthema-