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Bacterial wilt, caused by Clavibacter (Corynebacterium) michiganense
subsp. insidiosum, subsp. is a very destructive disease
of susceptible alfalfa varieties three years old or more. The disease
occurs wherever the crop is grown in the United States, except in
arid areas without irrigation. Bacterial wilt was first recognized
as a distinct disease in Illinois and Wisconsin in 1924. Bacterial
wilt has been reported in Canada, Mexico, Chile, Europe, the USSR,
the Near East, Japan, Australia, and New Zealand. Damage seldom
occurs during the first two years following seeding.
The bacterium that causes the disease has contaminated soils in
many parts of Illinois. Bacterial wilt is favored by cool temperatures
and abundant moisture, usually becoming most severe in low, poorly
drained areas. Plants weakened by bacterial wilt are more susceptible
to winterkill than are healthy plants. Crown injuries may also increase
disease levels.
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Figure
1. Figure 1. Bacterial wilt caused by Clavibacter michiganense
subsp. insidiosum. . Healthy (left) and diseased (right) alfalfa
plants
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Symptoms
Bacterial wilt causes a stunting and yellowing of the entire plant. Growth
is slow. Diseased plants are dwarfed, with a bunchy growth resulting from
numerous, spindly, shortened stems and small, light green to yellow leaflets
(Figure 1). Leaflets are commonly rounded at the tip and tend to curve
upward (a condition called "mouse-leaved"). Affected plants
may wilt during the heat of the day and recover temporarily during the
cool of the night. Plants may wilt and die rapidly during warm, dry weather.
At first, only the tips of the stems droop. This is followed by a more
or less complete wilting and finally by the death of the infected plant.
Stunting is most evident during regrowth following cutting. Progressively
less growth is produced after each cutting. Infected plants usually die
beginning in midsummer and into the next hay year. Severely diseased plants
rarely survive the winter. Once infection has occurred, susceptible plants
generally do not recover.
A sure sign of bacterial wilt is a yellow to dark golden brown discoloration
in the outer vascular tissue of the taproot when the bark is peeled. This
discoloration is in sharp contrast to the creamy white color of healthy
roots. If an infected taproot is cut across just below the crown, scattered
yellowish to brownish dots or a ring of discolored tissue are usually
evident (Figure 2). As the disease progresses, the entire stele becomes
discolored.
The disease symptoms and death of alfalfa are probably due to the water-conducting
vessels being plugged by the bacteria and to the production of a bacterial
toxin (a glycopeptide).
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Disease Cycle
The causal bacteria survive in living or dead alfalfa plant tissue
in the soil. The bacteria have survived in dry plant tissue or seed
for 10 years or more in the laboratory. The bacterial wilt organism
is spread in the field by surface water, tillage equipment, mower
sickles, infected hay, and animals. A long-distance spread most
likely occurs by means of seed and hay.
The infection of plants commonly occurs during cool, wet weather
in spring and early summer. The bacteria enter plants through wounds
in the roots and crowns produced by winter injury and animals in
the soil, or through the cut ends of stems as a result of mowing
or grazing. In advanced stages of the disease, bacteria multiply
rapidly in crown and stem tissues and are released into the surrounding
soil water.
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Figure
2. Cross sections of healthy root (bottom) and diseased roots
(center and top). Similar symptoms may be caused by Pseudomonas
marginalis var. alfalfae and Serratia marcescens (Courtesy F.I.
Frosheiser)
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1. The only practical control is to grow adapted, wilt-resistant varieties.
This is especially true for an alfalfa stand that is to be maintained
for three years or more. Varieties range from completely susceptible to
highly resistant. No alfalfa variety is immune to bacterial wilt. Disease
resistance may be conveniently divided into four groups: moderately susceptible,
moderately resistant, resistant, and very resistant. Moderately susceptible
varieties may survive for three years after seeding. Moderately resistant
ones are productive up to five years. Varieties vary somewhat in their
resistance, but usually remain productive for five to ten or more years.
Stands are usually reduced by factors other than bacterial wilt.
For a listing of currently recommended alfalfa varieties which are resistant
to bacterial wilt, read Illinois Agricultural Pest Management Handbook
(revised annually).
2. Do not plant alfalfa in poorly drained soils.
3. For best yield, harvest at the late-bud to first-flower stage and
every 30 to 40 days for succeeding harvests. This results in a rather
rapid loss of plants in wilt-susceptible varieties. Less intensive harvesting
schedules reduce the loss rate of susceptible plants.
4. Harvest young stands before old stands when using the same equipment.
Harvest fields showing wilt symptoms last. Clean equipment with steam
before moving from field to field, especially where wilt is present.
5. Mow only when the foliage is dry.
6. Use a program of high, balanced fertility based on a soil test to
help maintain plant vigor.
7. Grow other crops for two or three years before reseeding a field to
alfalfa.
8. Reduce injury to crowns (livestock movement, equipment, etc) which
provide entry wounds and may weaken plants.
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