Bulletin 229, Diseases of Citrus in Florida


dropping. This form of the disease is brought on by some im-
proper condition that temporarily affects the tree and, with
changed soil, fertilizer, cultural, or seasonal conditions, the dis-
ease may disappear in time without other treatment. It is only
in the chronic cases that the disease persists year after year.


Fig. 67.-Orange affected by exanthema, showing the characteristic dark
 brown eruptions of the rind.
 CAUSE OF EXANTHEMA
 Following the early work of Fowler, the investigations upon
exanthema in Florida were begun by Swingle and Webber and
much more detailed studies have been made by Floyd, Ruprecht,
and others. Despite the large amount of study given this dis-
ease, however, the exact nature of its cause still remains un-
known. It is not caused by an organism but is purely a function-
al disorder or physiological trouble resulting from a deranged
nutritional condition which, in some unknown way, interferes


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