Bulletin 229, Diseases of Citrus in Florida


either oranges or grapefruit. Of the oranges, the Valencia ap-
pears to be most severely affected. The sour orange is highly
resistant, although not immune, to the disease. In many groves
as high as 15-25 percent of the orange trees and often 50 per-
cent or more of the tangerine trees are affected by psorosis. Oc-
casional instances occur where these figures run much higher.

 SYMPTOMS OF PSOROSIS
 Psorosis is a disease affecting the bark of the trunks, main
limbs, branches and,
occasionally, e v e n
large twigs. Although
psorosis is generally
classed as a gum dis-
ease, gu m exudes
only during the act-
ive growing season
of the tree. As a rule,
the exudation of gum
is very small and
often may be entirely
lacking. The frequent
slowness of the de-
velopment of the dis- 4'1
ease, its inconspicu-
ous appearance and
lack of effect upon
the foliage i n t h e
earlier stages, fre-
quently prevent it
from being noticed
by growers until it
has become well ad- Fig. 28.-Early stage of psorosis started about
vanced. an untreated pruning wound on a tangerine
 limb, showing the superficial scaling of the
 Psorosis usually be- bark in small pieces.
gins as an inconspic-
uous spot in which the bark dies and becomes infiltrated with
gum. The first conspicuous symptoms of the disease are the
cracking and scaling off of the outer portions of the bark from
around the small initial diseased spot. In this stage the inner
layer of bark next to the cambium is still alive, active and free