Florida Agricultural Experiment Station


 SPHAEROPSIS KNOT
 Caused by Sphaeropsis tumefaciens Hedges
 The disease known as Sphaeropsis knot is found occasionally
in Florida, where it has been known to occur since 1911. It
has been reported from all sections of the citrus-growing dis-
trict but is a relatively unimportant trouble. This disease is
widely distributed in Jamaica, where it has caused considerable
damage to lime and orange trees. It occurs as a minor disease
in Cuba. It is known to occur naturally on lime, grapefruit,
sweet orange, and sour orange trees, but small knots have been
produced artificially by inoculation on lemon, tangerine, and tri-
foliate orange.
 SYMPTOMS OF SPHAEROPSIS KNOT
 Knot may occur on trees of any age and the old growth ap-
pears to be attacked as readily as the young. The knots vary in
size from three-eighths of an inch to two or three inches in di-
ameter and may occur singly or with several close together on
the branches (Fig. 45). They are usually round but variations
sometimes occur as elongated swellings more or less closely
crowded along the stems.
 The first indication of infection is a slight swelling of the
branch, which ultimately develops a well-formed knot at the
point of infection. In the early stages of growth the surface
of the knot is light-colored and rather smooth. With age the
surface becomes darker and more or less fissured and an old
knot from which the bark has disappeared may be almost black,
rough, and deeply furrowed. Frequently, in rapid growth, knots
of large size develop with the bark intact and the surface may
remain smooth and light in color. Although knot is quite viru-
lent and destructive in Jamaica, the disease is found only rarely
and in but a very mild form in Florida.

 CAUSE OF SPHAEROPSIS KNOT
 The disease is caused by the fungus Sphaeropsis tumefaciens
Hedges, which penetrates and grows within the tissues, stimu-
lating the formation of the typical galls or knots. Living parts
of the fungus may be detected in the tissue of apparently healthy
twigs several inches beyond the knots.