Bulletin 229, Diseases of Citrus in Florida


ity among investigators that rapid thawing is not in itself
injurious. Since light is known to increase permeability, it may
well be conceived to prevent the return to the cells of the water
which has been withdrawn upon freezing, thus causing injuries
to tissues which otherwise would recover their normal condition.
 It has been observed repeatedly that trees suffer from cold
more if the soil has become dry. Trees that have been sprayed
with an oil emulsion just prior to a cold spell are rendered more
susceptible to freezing. Groves in good condition as to fertiliza-
tion and cultivation invariably withstand freezing much better
than those that have been underfed or starved or that may be in
a weakened condition from attacks of diseases and insects. Trees
carrying a large crop of fruit suffer more severely than those in
the same grove from which the fruit has been removed sufficient-
ly far in advance of the freeze to enable the trees to recover
their vigor.
 SUGGESTED METHODS FOR MINIMIZING COLD INJURY
 The principal means for protecting citrus trees from cold in-
 jury are banking the trunks and heating. The first method is
 of practical value only for young trees and then serves only to
 protect the trunks. Heating, which is the only practical means
 of protecting large trees, is of little practical value in protecting
 young trees, since they offer no particular obstruction to free air
 movement and the heat is carried away rapidly. Since these
 methods of cold protection are generally understood, the present
 discussion of control measures will be confined largely to the
 treatment of cold-injured trees.
 TREATMENT OF COLD-INJURED TREES
 Citrus trees that have been injured by frost to the extent,of
 killing branches and injuring trunks should receive careful at-
 tention during the following season. No pruning should be done
 immediately after the cold spell. As soon as danger from further
 frost is past, an application of fertilizer should be made and the
 grove cultivated so as to put the trees into a good condition. No
 danger to the trees will be incurred by leaving the dead wood
 for a reasonable length of time.
 A complete fertilizer should be used for the spring application;
 however, at least half of the nitrogen should be supplied in a
 readily available form, such as nitrate of soda or sulphate of am-
 monia. Where the trees are severely frozen back it will be found