Bulletin 229, Diseases of Citrus in Florida


 TREATMENT WITH COPPER SULPHATE
 Soil dressing with bluestone and spraying the trees with bor-
deaux-mixture have become the most general treatments for
exanthema in Florida, both because of the cheapness of these
substances and the rapidity with which beneficial results are
obtained.
 Soil Dressing with Bluestone:-Bluestone (copper sulphate)
has been used in Florida for many years as a corrective for exan-
thema, with a high degree of success where the conditions in-
ducing the development of this trouble are not persistent. The
most commonly used method is to apply the bluestone directly
to the soil, broadcasting it in the same manner as fertilizer is
applied. The applications should be in proportion to the size of
the trees and should vary from 3 ounces, in the case of 1-year-old
trees, to 4 pounds in the case of large bearing trees. The blue-
stone may be applied at any time when symptoms of exanthema
occur. To avoid possible injury to the roots it is advisable, at
least where it is desired to make large applications, to divide
the application into two parts, applied from 4 to 6 weeks apart
in the fall, and to apply an additional amount in the spring if
conditions warrant. The splitting of the application is especial-
ly important if the pulverized form of bluestone be used, par-
ticularly on wet soils, or if heavy rains are anticipated. In order
that the bluestone may not become available too rapidly, it is
recommended that the coarse crystalline form be used. Ex-
cessive applications of bluestone may cause injury to the roots,
defoliation, and even the death of the trees. Growers often
insert small crystals of bluestone under the bark to cure exan-
thema but this practice is not recommended, since it frequently
causes the bark to split badly.
 The use of bluestone as a soil dressing is justly popular with
 Florida growers because of its beneficial effect in correcting
 exanthema and of the marked stimulating effect it exerts on the
 growth of backward trees. Many use it more or less periodically
 as a regular practice whether their trees have exanthema or not.
 The reason for the effectiveness of this copper salt in correct-
 ing exanthema and in exerting a marked stimulating effect on
 citrus trees has never been determined. However, its value as
 a stimulant of plant growth on the raw peat soils of the Florida
 Everglades has been clearly demonstrated recently by Allison,
 Bryan, and Hunter. The exact reason for copper sulphate cor-