Bulletin 229, Diseases of Citrus in Florida


 1. While canker is likely to be confused with scab only in the
early stages, it differs from scab in the typically round spots
produced and the fact that the spots generally penetrate through
the leaf tissue, look approximately the same on both sides and
develop a watery, translucent zone, or later a yellowish halo,
around them. It does not distort the leaves and there are no
wart-like projections as in scab. Moreover, canker may occur
on quite old bark, while scab does not.
 2. It differs from scaly bark or nail-head rust in the size of the
spots, which are much smaller than those of scaly bark. It also
differs in the spongy nature and crater-like appearance of the
spots, scaly bark spots being hard and glazed and rather smooth.
Furthermore, canker is common on grapefruit while scaly bark
is of rare occurrence.
 3. It differs from anthracnose on the Mexican or Key limes
in that it is not confined to the fruit but occurs also on leaves,
young shoots, and twigs. Canker on the fruit, however, re-
sembles anthracnose on limes more closely than any other dis-
ease but canker spots do not result in sunken places, distortion
and splitting of the fruit as is often the case with anthracnose
on limes. Moreover, canker lesions do not develop the thick,
corky, mound-like outgrowths generally characterizing anthrac-
nose on limes.
 CAUSE OF CANKER
 Citrus canker is a bacterial disease caused by Phytomonas
citri (Hasse) Com. S. A. B., a short, rod-shaped, motile organ-
ism that can be seen only with a good compound microscope.
These bacteria are formed in countless numbers in the canker
spots and are exuded in masses whenever the spots become mois-
tened by rain or dew. Drops of water contaminated with bac-
teria spread the disease and when the organism comes into con-
tact with susceptible citrus tissue, especially the tender foliage,
twigs, and young fruit, new canker spots develop readily. In
the older bark on the branches and trunks the bacteria probably
gain entrance through small wounds. The disease may be spread
some distance by wind and driving rains and any animal or in-
sect coming into contact with the moistened foliage of an infected
tree is apt to spread the disease to other trees. The chief agent
in the dissemination of canker, however, has been man, the dis-
ease having been brought to America on nursery trees and spread
through Florida on budwood and nursery trees.