Bulletin 229, Diseases of Citrus in Florida


fluence of the effect upon the nutrition of citrus trees of the phe-
nomenon of chemical base exchange in the soil.
 Deep and unnecessarily frequent cultivation has long been
recognized as an important factor contributing to the develop-
ment of mottle-leaf in Florida. It is also well established that
liberal applications of ground limestone or other forms of lime
in Florida, at least on the lighter types of sandy soils deficient
in organic matter, commonly bring on a severe outbreak of mot-
tle-leaf, from which it may take years for the trees to recover,
if indeed they do.* Mottle-leaf also develops commonly after
periods of drought, hurricanes and freezes, or dehorning trees.
 In addition to certain of these contributing factors, the con-
tinuous use of nitrate of soda on soils of low organic content, ac-
cumulation of alkali salts, an excess of magnesia or an improper
relation between magnesia and lime in the soil, and infestation
of the roots by nematodes have been correlated with the occur-
rence of mottle-leaf in California. However, from the various
investigations that have been conducted recently, it is evident
that probably none of the diverse array of contributing factors,
in themselves, would necessarily result in the development of
mottle-leaf, but that they may do so when they influence the nu-
trition of the tree in such a way as to bring about a deficit of
some mineral element that is essential to the growth processes,
or the excess of one that may be toxic to them. Haas succeeded
in artificially producing a typical mottle-leaf of citrus foliage in
California by the use of boron and lithium, which are toxic to
growth processes. It is of interest in this connection to note

 *A characteristic form of decline of citrus trees on rough lemon stock
 has become especially prevalent in the central part of the peninsula during
 the past three years. In the intermediate to late stages of decline, such
 trees develop an acute case of mottle-leaf which closely resembles that de-
 veloping on limed trees on light sandy soils. This trouble occurs on trees
 that were previously in a good state of health and productivity, in un-
 limed groves on soils of good drainage and depth. Thus far, the growers
 have not been successful in restoring to health and productivity the trees
 that develop this form of decline. The trees often die out in local spots in
 the groves, the trouble developing later on the adjacent trees after one,
 or occasionally more, trees have declined to the point where they were
 removed. In other cases, however, the majority of the trees in a grove
 may be affected.
 Considerable similar decline of trees has also resulted, apparently, from
 allowing heavy crops of fruit to mature and be held unusually late on
 trees defoliated and otherwise injured by the hurricane of September, 1928.
 It is possible, however, that there may be a general decline of trees in
 various groves from several causes and that the latter type of decline may
 have no bearing upon the former one. In view of the lack of definite in-
 formation that exists in regard to this loosely defined form of decline, the
 writers have deemed it best not to attempt any further discussion of it.