DIEBACK, OR EXANTHEMA OF CITRUS TREES
 By B. F. FLOYD

 Dieback, or exanthema, is one of the most common and most
 important of the diseases of citrus trees. It affects all kinds
 and varieties, and occurs on trees planted on all types of soil.
 A large percentage of the groves planted on the sandy lands of
 peninsular Florida have at some time in their history been af-
 fected by it to some extent.
 The disease is important on account of its relation to the
 organic matter in the soil. Any attempt to feed the trees or to
 build up the soil by the addition of any large amount of organic
 material is likely to lead to a rank growth in the trees and to a
 development of the disease. It is not known whether the organic
 matter induces the disease directly, or merely develops a type of
 growth that is susceptible to the attack of some obscure organism
 which causes the disease. The amount of damage done by the
 disease in any one community varies considerably, and is usually
 small, but the total in the State as a whole amounts to many thou-
 sands of dollars.
 Dieback has been studied in Florida and in California by
a number of investigators. Swingle and Webber studied it and
other diseases in Florida from 1892 to 1895. They gave a com-
plete description of the disease, with a discussion of its cause
and methods of treatment, in bulletin No. 8, The Principal
Diseases of Citrous Fruits in Florida," published by the Division
of Vegetable Physiology and Pathology of the United States De-
partment of Agriculture.
 Grossenbacher, in a paper entitled Some Bark Diseasea of
Citrus Trees in Florida (Phytopath., Vol. 6, p. 29, 1916), dis-
cussed dieback and classed it as a bark disease that begins with
injuries which occur in the cambium and phloem of the bark of
certain of the plant parts. He does not suggest a possible cause
for the injuries. Dr. C. B. Lipman studied dieback in California.
He suggests (Science, Vol. 39, p. 728, 1914) the hypothesis that
it is caused either by a lack of nitrogen or by ammonia poisoning.
 The purpose of this bulletin is to give a popular summary of
the information which is extant at the present time concerning
the disease, its cause and treatment. A technical discussion of
this subject is avoided wherever possible to do so. Our informa-