TOMATO WILT PROBLEMS
W.C. Nesmith and J.R. Hartman
Fusarium and Verticillium Wilts
Fusarium oxysporum f. sp. lycopersici
and Verticillium albo-atrum are two fungi that cause similar wilt
diseases in tomato. Fusarium is more common in replant fields and gardens
while Verticillium has been destructive on greenhouse tomatoes and
in commerical stake tomatoes. Verticillium wilt is more of a problem at
cooler temperatures while Fusarium wilt is considered a warm temperature
disease. Both are favored by wet conditions.
The first symptom of both diseases
is a slight wilting of the plants. Fusarium wilt symptoms also include
strong downward bending of petioles, yellowing, wilting and dying of the
lower leaves, often on one side of the plant. These symptoms may appear
on successively younger leaves with one or more branches being affected
and others remaining healthy. Root necrosis is often extensive. After a
few weeks, browning of the vascular system may be seen by cutting the stem
open with a knife. This brown discoloration inside the stem may extend
from the roots of the plant to the top. Plant growth is stunted and, under
warm conditions, the plant may die.
Verticillium wilt symptoms are very
similar but often slower to develop and with much less yellowing. Early
symptoms on the leaves may also include yellowing of V-shaped areas between
the veins or leaf margins. Frequently, leaves on all sides of the plant
show symptoms. Plants may wilt during the day and recover at night. Stunting
occurs and plants may eventually die under relatively cool growing conditions.
The dark discoloration inside the stem can be found mainly near the base
of the plant.
Both fungi are common inhabitants of
Kentucky soils. These fungi attack the plant through the roots and grow
up through the water-conducting vessels (the vascular tissue). The cells
in the vascular tissue are destroyed and water movement through this tissue
is seriously impaired, causing wilting. Fusarium and Verticillium
may be introduced to soils in several ways: old crop residues, transplants,
wind, water, implement-borne soils, or mulches. These fungi become established
readily in most soils and can remain in the soil for years. When susceptible
tomatoes are planted in infested soil, their roots are also subject to
attack by these fungi. Both diseases are much more serious when accompanied
by root knot nematode.
1.Use resistant tomato varieties. Such
varieties are designated with the letter "V" or "F" in seed catalogs. Variety
names followed by the letters "VF" or "VFN" are resistant to both wilt
diseases. The letter "N" signifies resistance to root-knot nematode. If
root knot is also present, controls for nematodes are necessary for the
VF resistance to be effective.
2.Fumigate or steam sterilize greenhouse
Wilting of tomato plants may occur
when they are planted near walnut or butternut trees. Large amounts of
a toxic substance called juglone is excreted into the soil from the root
systems of these trees. Walnut wilt causes wilt and vascular discoloration
symptoms similar to those of Verticillium and Fusarium wilt.
Avoid planting tomatoes near walnut
or butternut trees, or in locations where these trees may have grown previously.
The juglone can remain in the soil several years after a tree has been
cut down since it continues to diffuse from the dead root system.
Bacterial wilt is not a serious problem
in Kentucky but some damage occasionally does occur, mainly through introduction
on southern produced transplants.
Symptoms and Spread
The bacterial wilt organism (Pseudomonas
solanacearum) survives in the soil and weedy hosts and infects susceptible
plants through wounds in the roots or stem. The bacterium initially invades
the water-conducting tissue of the vascular system. As a result, the vascular
tissue turns black and the plant rapidly wilts. Next, the pith becomes
decayed, appearing dark and water-soaked. A slimy bacterial ooze exudes
from the stem when it is pressed. As decay progresses, the stem may become
hollow. Unlike with the wilts caused by fungi, in hot weather, plants infected
with the bacterium collapse quickly and die. As the cycle is completed,
bacterial cells are released into the soil where they can survive even
in the absence of a host. The pathogen can be carried into "clean" fields
or gardens via infected transplants or through drainage water from adjacent
1.Plant disease free transplants.
2.Do not plant tomatoes in sites where
the disease has been a problem in the past. Practice long crop rotations.
3.Remove and destroy infected plants
4.Avoid hydroponic production systems
where this bacterium exist.
5.Resistant varieties are available,
but they do not grow well under Kentucky conditions.
Bacterial canker is a seedborne disease
that is increasing and has a wilt phase associated with it. The wilt phase
can be easily confused with any of the wilt diseases described above. It
is becoming more common in Kentucky with intensively managed tomato systems.
See articles on bacterial canker for more details.