Bulletin 229, Diseases of Citrus in Florida


 CONTROL OF WITHERTIP
 Since the most important contributing conditions for the de-
 velopment of withertip and related troubles of the common
 varieties of Citrus, other than limes, are those which have to
 do with the weakening of the tree and the tissues, it is of primary
 importance to maintain the health and vitality of the trees by
 proper care in regard to fertilization, cultivation, and other
 grove practices.
 In the case of the withertip and anthracnose of limes, where
 infection of the young shoots, blossoms and young fruits takes
 place very rapidly during rainy weather, control of the disease
 can be accomplished only by timely and repeated applications
 of a fungicide, such as bordeaux-oil emulsion, to protect the de-
 veloping parts while they are susceptible to the disease. One
 application should be made at the time of blooming and be fol-
 lowed by one or two others at intervals of from 10 days to two
 weeks while the young fruits are forming. When the bordeaux-
 oil emulsion is used on the tender young fruit it is advisable to
 reduce the oil content to 1/2 of 1 percent. Close watch should
 be kept for evidence of increase of scale-insects, which may have
 to be combatted by the application of oil emulsion.

 DIPLODIA TWIG AND BRANCH DIEBACK
 Caused by Diplodia natalensis Evans
 Species of Diplodia have been reported on citrus trees, es-
pecially twigs and dead parts, in nearly every important citrus-
producing country. In Florida and the West Indies a species,
apparently identical with Diplodia natalensis, is responsible for
a number of citrus troubles, including a sudden dying back of
the twigs and branches, gumming, a destructive stem-end rot
of the mature fruit, and apparently a collar-rot type of girdling
of the trunks of young trees. That this fungus was active in
the death of twigs of both citrus and peach trees in Florida was
first shown by the inoculation experiments of Fawcett and Bur-
ger in 1911. In 1915, Earle and Rogers reported that Diplodia
was a serious factor in the dying of twigs and branches of citrus
trees in the Isle of Pines. This form of citrus dieback was re-
ported from Cuba by Bruner in 1917. Nowell (1923) considers
that Diplodia is important as a secondary agent in the dying
back of lime twigs in the Lesser Antilles. But relatively little