Wilt Diseases - Westcott's Plant Disease - page 414

Fusarium oxysporum f. sp. lycopersici -
Fusarium Wilt of tomato, general, in many sec-
tions the most damaging tomato disease in field
and greenhouse. Chief losses are in states where
air temperatures are rather high during most of
the season, susceptible varieties dying or produc-
ing little fruit. Losses go up to 30,000 t of canning
tomatoes, or 10 to 35 % of the crop in many
states.
In seedlings there is downward curvature of
the oldest leaves followed by wilting and death.
In older plants the disease is most evident as fruit
begins to mature, lower leaves turning yellow,
first on one side of the stem or leaflets on one
side of the petiole. One shoot may be killed
before the rest of the plant shows symptoms.
The fungus enters through roots and grows
into the stem, where it produces the toxic sub-
stances causing wilting and eventual death. The
vascular system in the stem shows a dark brown
discoloration. In severe infections the fungus
grows into fruit and seeds, but such fruits usually
drop, and seed is not used. Almost all original
infection comes from the soil, the Fusarium
operating best in light sandy soils and at
temperatures between 80 and 90 F, but the
disease is spread widely in transplants. It is
encouraged by low potassium and high nitrogen
nutrients.
Control Start seedlings in clean soil; do not grow
in the same land more than once in 4 years. The
use of resistant varieties is the chief means of
control. Marglobe, Pritchard and Rutgers are
moderately resistant, but infestation by nema-
todes may predispose even these to wilt. Pan
America, Southland, Homestead and Jefferson
are more highly resistant. Treating soil with nem-
aticides may reduce incidence of wilt even
though the wilt pathogen is not killed.
Fusarium oxysporum f. sp. melonis Muskmelon
Fusarium Wilt , similar to that of watermelon,
important in Minnesota, New York, New Jersey
and Maryland. Seeds rot in soil; seedlings damp-
off; vines wilt. Fungus persists in soil and is
carried internally in seed. Varieties Golden
Gopher and Iroquois are quite resistant.
Fusarium oxysporum f. sp. niveum Wa-
termelon Wilt , general on watermelon, also on

citron. The fungus is transported in and on seed
and persists in soil 15 to 18 years. It rots seeds or
seedlings, causes wilting of plant, sometimes
with cottony mycelium on surface of dying
vines. Resistant varieties include Improved
Kleckley Sweet and Klondike.
Fusarium oxysporum f. sp. pernicio-
sum Mimosa Wilt on mimosa from New Jersey
and Maryland to Florida. This extremely perni-
cious wilt started about 1930 at Tryon, North
Carolina, and mimosas have wilted and died at
a rapid rate ever since. The wilt appeared in one
city block at Morgantown, North Carolina in
1943, and by 1947 trees were dead and dying on
232 blocks.
The first external symptom is a wilting and
yellowing of leaves on some of the branches,
causing foliage to hang down, then die and
drop. Death of the tree follows from a month to
a year after first infection. The trunk has a brown
ring of discolored sapwood, usually in the current
annual ring, and the color may extend out into the
branches. The xylem is plugged with brown
gummy substances. Small branches may have
a one-sided wilting with the bark flattened over
collapsing tissue. The disease has been spreading
in Maryland since 1947, in Florida since 1952.
As with other Fusaria , this is a soil fungus
entering through the roots, and eradication of
diseased trees has no effect on spread of the
wilt. Nematodes, by their wounds, may increase
the incidence of wilt. Out of a great many seed-
lings grown from seed collected from Maryland
to Louisiana, inoculated several times with the
fungus and planted in infested soil, some have
remained mostly disease-free. These have been
propagated by the U.S. Department of Agricul-
ture. Released for commercial sale are Charlotte
and Tryon.
Fusarium oxysporum f. sp. pisi Pea Wilt ,
caused by race 1 of this pathogen and Near Wilt ,
caused by race 2. Race 1, confined to pea, pro-
duces stunted plants, pale yellow green, with
leaves curled downward, stem thickened and brit-
tle near the ground. Plants wilt and die prema-
turely. The disease may cause more or less
circular bare spots in the field, enlarging each
year if peas are planted continuously, encouraged