Alan
McNab
Professor of Plant Pathology |
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Wilts are caused by several agents. Four common wilts
that can be confused with eachother are Verticillium wilt,
Fusarium wilt, bacterial wilt, and walnut wilt. Information
on varietal resistance, range of crops affected, proximity
to walnut trees, soil temperature, and past cropping history
help determine which of the wilts is present. usually a laboratory
isolation is needed to verify the causal agent. Bacterial canker,
southern blight, leaf roll, root knot, and stem rot -- five
other disorders often mistaken for wilts -- are also described
here.
Verticillium WIlt
Fusarium Wilt
Walnut Wilt
Bacterial Wilt
Bacterial Canker
Southern Blight
Leaf Roll
Root Knot
Stem Rot |
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Verticillium
Wilt, caused by Verticillium spp. of fungi, affects
many plants and is common on tomatoes, potatoes, eggplants,
peppers, strawberries, and raspberries. Leaf symptoms appear
on oldest leaves first and later develop on younger leaves.
Leaves turn yellow, dry up (often without even wilting), and
drop prematurely. Shoot tips wilt slightly during the day.
As defoliation progresses, tip leaves may curl upward at the
margin but usually remain alive. Internal woody stem tissue,
particularly at the lower part of the stem, is distinctly darkened.
Disease is caused by a soil-borne fungus that can persist for
many years. Disease is more prevalent in cool than in warm
climates. |
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Fusarium
Wilt, caused by the fungus Fusarium oxysporum f.
sp. lycopersicae, affects only tomatoes. Fusarium wilt,
like Verticillium wilt, is characterized by leaf yellowing
that progresses upward form the base of the plant. Unlike Verticillium
wilt, wilting or yellowing may occur on only one side of a
plant. Yellow leaves wilt noticeably before they die. Separate
shoots, and later entire plants, finally wilt permanently and
die. Woody stem tissue often is discolored throughout the plant.
Fusarium wilt is caused by a soil-borne fungus that can persist
in soil for many years. Many tomato varieties are resistant
to a common race (race 1) of this fungus; however, these "resistant" varieties
may be susceptible to the recently detected race 2. Development
on fusarium wilt is restricted by cool climate. |
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Walnut
Wilt affects several kinds of plants when they are growing
in the root zone of walnut and butternut trees. Tomatoes are
particularly susceptible. Woody stem tissue in affected plants
turns brown and plants soon wilt and die. The wilt is caused
by a toxin produced in the roots of the walnut tree; the toxin
may persist after walnut roots are dead. |
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Bacterial
Wilt, caused by the bacterium Pseudomonas solanacearum,
affects several plant species, including tomatoes, potatoes,
eggplants, and peppers. Disease is characterized by sudden
plant wilting without leaf yellowing. Stem centers (pith) become
water-soaked, they later turn brown and sometimes become hollow.
Pith discoloration helps distinguish bacterial wilt from Verticillium
and Fusarium wilts. Woody stem tissue turns brown and roots
may start to form on the stem. The bacterium overwinters in
cold-frame and greenhouse soils in northern areas, and in field
soil in southern areas. The disease can be serious if transplants
are infected. |
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Bacterial
Canker, caused by the bacterium Corynebacterium michiganense,
is characterized by wilting of leaflets on plants of all sizes.
On older plants, leaves die from the margin inward toward the
midrib. Symptom development often is one-sided; usually the
leaflets on one side of the leaf are affected first. Eventually
the whole leaf is affected. Diseased plants may wilt and die
early but many survive, though unthrifty and wilted. Yellowish
white streaks may appear on petioles and stems and may develop
into longitudinal cankers. Cavities may develop within stems.
Spots develop on fruit. The disease-causing bacterium may survive
for 1 year in residue from diseased plants. The bacterium also
is seed-borne and therefore can infect and be spread on transplants. |
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Southern
Blight is caused by Sclerotium rolfsii, a soil-borne
fungus. The first above-ground symptom is plant wilting. The
stem at the soil line displays a brown cortical soft rot, usually
covered with whitish cottony mold embedded with tiny brown
sclerotia (fungus reproductive structures). The fungus survives
from season to season as sclerotia in soil. The fungus can
be spread in running water, in infested soil, on tools and
implements, in infected seedlings, and as sclerotia among the
seed. Diseased development is enhanced by high temperature
and high humitidy. The disease is rare in areas with cold winters. |
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Leaf
Roll is characterized by upward curling of leaflets on
older leaves. Leaf roll has been associated with varieties
having a specific gene (wilty gene); symptoms usually are seen
when plants have a heavy fruit load. Environmental factors
reported to promote symptom development include high temperature,
drought, and prolonged periods of wet soil conditions. In addition,
TMV promotes leaf roll symptoms at all stages of plant growth
in varieties with the wilty gene. |
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Root
Knot can be severe on tomatoes, cucurbits, eggplant, lettuce,
spinach, carrot, parsnip, and celery. Affected tomato plants
usually are stunted and may wilt in hot dry weather. A diagnostic
symptom is easily detected on roots of all affected plant species;
roots contain elongated and round swellings (root knots) on
both large and small roots. Root knot is caused by several
species of the root knot nematode (Meloidogyne spp.).
Only the northern species (M. hapla) is thought to overwinter
in areas with severe winters. The nematode may be introduced
on transplants and then spread within and between fields in
infested soil carried on machinery or in running water. |
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Stem
Rot is caused by Sclerotinia sclerotiorum, a fungus
that causes disease in beans, cabbage, carrots, celery, cucumbers,
lettuce, onions, peas, pumpkins, squash, and many other species.
A dry rot girdles the stem at ground level and eventually the
plant wilts and dies. A diagnostic sign for this disease is
presence of sclerotia (hard black pea-like structures) within
the girdled stem. Sclerotia, produced by the fungus act as
resistant fungus "seeds" that persist in soil for many years.
Long periods of cool wet weather promote fungus growth, fungus
spread, and disease development. |
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