Bulletin 229, Diseases of Citrus in Florida


bonate brings about an excess of alkalinity and this condition
results in the precipitation of the iron so that the roots fail to
get what might otherwise be available.
 It is quite evident that further study of the mottle-leaf prob-
lem is required before anything definite can be said in regard to
the exact cause of this trouble when it occurs under so many
widely differing soil conditions.
 CONTROL OF MOTTLE-LEAF
 No specific method can be recommended for the control of
mottle-leaf since its development may be induced by many wide-
ly differing factors. It is highly important to discontinue, or
modify, those conditions or practices that induce the develop-
ment of this trouble. On soils that range from weakly acid to
alkaline in reaction it is also highly important to discontinue any
practice tending to increase the alkalinity of the soil, since this
would, at the same time, decrease the availability of iron. Only
substances or practices resulting in greater acidity rather than
greater alkalinity should therefore be used on soils where mottle-
leaf occurs. For this reason, the addition of stable manure and
green manurial cover crops is very beneficial, because the pro-
cesses of fermentation and decay set up tend to increase soil
acidity and, consequently, to increase the availability of iron.
Stable manure is especially valuable as a means of renewing
and stimulating the bacterial activity in soils that have become
more or less "dead". Applications of either sulphate of ammonia
or sulphur to the soil should prove helpful in increasing soil
acidity and the availability of certain essential elements, which,
although present, may be locked up so that they cannot be util-
ized by the tree. Since it is only in subsequent growth that nor-
mal leaves can again be obtained, it is necessary that the trees
have sufficient nitrogen for the production of new growth. It
has been observed, however, that the branches on which the foli-
age has developed a chronic mottle-leaf condition and the bark
has become hardened rarely ever develop any good shoots and
seem to have been rendered useless for this purpose. In such
cases, the subsequent development of healthy growth almost
invariably takes place from points farther back on the branches.
 The application of soluble iron compounds to the soils with
 a highly calcareous subsoil has failed to produce the beneficial
 results that might be expected of such a treatment. The precip-