Bulletin 229, Diseases of Citrus in Florida


often mummifying on the dead twig. In still other cases the
attack may not begin at the end of a twig or branch but at some
point well back on it, causing its girdling, as a result of which
the leaves above the girdled place quickly wither and the attacked
twig or branch dies. An internal gumming and death of the
bark occurs at the attacked point and a few drops of gum may
exude on the surface. The same organism, which invades both
the bark and the underlying wood, has also been shown by Faw-
cett to cause gummosis on the trunks and larger branches. The
role of Diplodia in causing gummosis, however, will be discussed
in connection with the section on that disease.
 CAUSE OF DIPLODIA DIEBACK
 It has been found that the types of injury described above
are caused by a species of Diplodia apparently identical with
Diplodia natalensis Evans. A number of conditions may be
mentioned as contributing to attacks of Diplodia on the trees.
The most important of these seems to be the presence of wounds,
maturity of woody tissues while still weak, and deficient or un-
favorable nutritional conditions. The sudden checking of growth
with the setting in of drought and heavy fruiting on poorly
nourished trees on light sandy soils are mentioned by Earle and
Rogers as supplying favorable conditions for attack. The wither-
tip fungus is frequently associated with Diplodia and Fawcett
believes that these fungi probably do more injury in combina-
tion than when each is working alone.
 CONTROL OF DIPLODIA DIEBACK
 Remedial measures for the control of Diplodia twig and
branch dieback comprise the prompt cutting off of attacked
branches well back into the healthy wood and the painting of
all wounds with a good antiseptic and waterproof wound dress-
ing to prevent reinfection. For the prevention of attacks of
this disease, however, reliance must be placed largely on such
cultural and fertilizing methods as will keep the trees in a
thrifty condition. Trees maintained in a good state of vigor will
be less adversely affected by unfavorable seasonal conditions
which may be beyond the grower's control and less susceptible
to attacks by Diplodia and a number of other fungi.