Florida Agricultural Experiment Station


 If the cold injury is slightly greater in severity the leaves may
 curl to such an extent as to indicate that they are suffering from
 lack of moisture. If still more seriously injured, they may not
 only curl, but, in the course of a few days, may drop from the
 trees. The upper leaves usually fall before the lower ones, some
 of the latter frequently remaining even when the upper por-
 tion of the tree is practically defoliated. Defoliation may occur
 without serious injury to the twigs and smaller branches. When
 the frost injury is so severe as to kill the leaves and twigs out-
 right, however, they wither, blacken, and die.
 On the Twigs and Smaller Branches:-While it frequently
 happens that the cold injury is confined to the foliage and that
 the twigs may remain uninjured, in other cases, cold sufficiently
 intense to defoliate the trees may injure the young and tender
 growth, or even more mature twigs and small branches, with-
 out severe damage to the larger branches. Cold injury to the
 twigs and smaller branches commonly induces the development
 of such diseases as melanose and stem-end rot, withertip, and
 Diplodia twig and branch dieback, since the organisms causing
 these diseases live and produce an abundance of spores in the
 dead twigs and smaller branches.
 On the Trunks and Larger Branches:-The effects of severe
 cold on the trunks and larger branches may manifest themselves
 either by splitting or loosening, or by the death, of the bark on
 certain areas or certain portions of the trunk or limbs. While
 the loosening of the bark usually indicates that the cambium has
 been killed, this is not always the case. Under some conditions,
 the cambium may remain alive sufficiently to build up new
 tissue even when the bark has been split or separated from the
 wood. Certain areas, especially those adjacent to the crotches
 on the large limbs, are often particularly susceptible to cold in-
 jury. Under some conditions, the bark around the bud union is
 especially susceptible to cold. Localized injuries to patches of
 bark on the trunk and larger branches are often mistaken for
 gummosis, since these generally exude gum later.
 Even though citrus trees are frozen to the ground, the roots
seldom are killed. The tops on sour orange, rough lemon, and
sweet seedling rootstocks may be frozen back many times, yet
the vitality of these stocks is such that the roots continue to send
up sprouts for years afterward. When good-sized trees on tri-
foliate orange stocks are frozen to the ground, however, the trees
rarely come back.