Bulletin 229, Diseases of Citrus in Florida


may be brought under conditions unfavorable for nitrification
and excessive ammonification.
 Exanthema occurs in citrus trees under such a diverse array
of soil and soil moisture conditions and cultural practices that
it is difficult to attribute the cause to any particular condition
or practice. While growers have popularly believed that a great
number of conditions and practices were responsible, there are
but very few of these beliefs that appear to have any justifiable
basis. Excessive cultivation, especially that of sufficient depth
to cut many of the roots, inadequate drainage, deficiency of soil
moisture, and lack of organic matter appear to be the chief con-
ditions and practices inducing the development of this trouble.
Certain soil types or conditions, often of purely local occurrence,
such as "sand-soak" areas, "hardpan" soils, and other poorly
drained soils subject to fluctuating moisture conditions, are
strongly conducive to the development of exanthema. In the
growth of heavy leguminous cover crops it appears to be the
deep plowing often resorted to in turning this material that in-
duces the development of the disease rather than the addition
of nitrogen through the cover crop. In the growth of truck
crops between the tree rows of young groves it is more likely
that the exanthema that develops is due to the deep plowing and
repeated cultivation than to the kind or additional amount of
fertilizer applied to the growing crop, although this may help
somewhat. Trees growing near stables, outhouses, cesspools and
poultry yards, or those that have had dead animals, garbage, or
other organic offal buried near them have been repeatedly ob-
served to develop exanthema.
 CONTROL OF EXANTHEMA
 Care should be taken in selecting citrus lands to avoid, as far
as possible, those conditions known to favor the development of
exanthema. It is far easier to prevent the occurrence of the
disease than it is to correct the diseased condition after it has
developed. On soils known to be favorable to its development,
cultivation, fertilization and cover-crops should be carefully
studied and handled in regard to their relationship to this dis-
ease.
 Methods for the prevention and treatment of exanthema are,
 for the most part, based on experience and observation rather