Bulletin 229, Diseases of Citrus in Florida


 In Florida, mottle-leaf occurs most frequently on the lighter
types of sandy soils and on soils closely underlaid with limestone
or other calcareous material. It occurs much more commonly
on orange and tangerine trees than on grapefruit. The amount
of injury is extremely variable. In mild cases, it may affect
merely the foliage of a few twigs or branches on occasional trees
and the condition may disappear eventually without the applica-
tion of any corrective measures. In severe cases, however, the
trouble may be quite persistent and the greater part of the foli-
age may be affected to such an extent as to retard seriously the
growth of the tree and the production of fruit. With the
exception of the groves on marly subsoils, mottle-leaf is rarely
found generally distributed throughout any one grove unless
the trees have been limed sufficiently to change markedly the
soil reaction, root-pruned by deep cultivation, or otherwise gross-
ly mismanaged. The trouble is often more or less seasonal in
its development, temporarily following certain soil or weather
conditions.
 SYMPTOMS OF MOTTLE-LEAF
 Mottle-leaf or frenching is a condition of partial chlorosis of
 the foliage in which the affected leaves develop with pallid to
 conspicuously yellowed areas of varying shape and extent be-
 tween the main veins on either side of the midrib, the remainder
 of the leaves becoming green. This results in an irregular
 mottling or variegation of the foliage (Fig. 69). The mottle-leaf
 condition makes its appearance with the development of the
 leaves and leaves that have once matured with normal green
 color do not develop the trouble. The contrast in color between
 the green and chlorotic parts of the leaves, however, becomes
 more definitely marked as the leaves expand.
 The yellowed areas on any one leaf may vary from small, ir-
 regular spots, in mild cases of the disease, to extremely pro-
 nounced instances of chlorosis in which the normal green color
 is developed only immediately along the midrib and main lateral
 veins. The worst mottled leaves occur toward the ends of the
 shoots. Frequently, the greater portion of the leaves borne on
 the shoots of a given flush of growth may be mottled, while those
 immediately preceding and following this flush may be entirely
 normal in appearance. In trees only mildly affected, the leaves
 exhibiting mottle-leaf may be normal in size and the mottle-leaf