Reports on Plant Diseases |
| RPD No. 924 -
Black Rot of Cabbage and Other Crucifers |
December
1999 |
[ Symptoms
] [ Disease Cycle ] [ Control
]
|
Black rot, caused by the bacterium Xanthomonas campestris pv.
campestris, is one of the most destructive diseases of cabbage
and other crucifers. Cauliflower, cabbage, and kale are among the crucifers
most susceptible to black rot. Broccoli, Brussels sprout, Chinese cabbage,
collard, kohlrabi, mustards, rape, rutabaga, and turnip are also susceptible.
Several cruciferous weeds are also hosts of the pathogen. Radishes are
resistant to most, but not all, strains of the bacterium that causes black
rot. A closely related species of Xanthomonas infects horseradish,
radish, winter cress, and garden stock.
In warm and wet conditions black rot losses may exceed 50% due to the
rapid spread of the disease. The disease is usually most prevalent in
low areas and where plants remain wet for long periods. Conditions favoring
plant-to-plant spread of the bacterium has led to a total loss of crucifer
crops.
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Click
on image for larger version
Figure 1. Cabbage
plants
infected with black rot
(V-shaped lesions
on leaves.) |
|
Click on images for
larger versions
Figure 2. Typical
"V-shaped" lesion
on cabbage.
Figure 3. Lesion
originating from insect
feeding injury
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Symptoms
Plants may be affected with black rot at any stage of growth. Seedling
infection first appears as a blackening along the margin of the cotyledon.
Later, the cotyledon shrivels and drops off, but only after the bacteria
have passed into the young leaves and stem. Affected seedlings turn yellow
to brown, wilt, and collapse. The bacteria most frequently invade the
host plant through water pores (hydathodes) at the leaf margins. The result
is initially a small, wilted, V-shaped infected area that extends inward
from the leaf edge toward the midrib (Figures 1-2).
The pathogen can also enter the plant through insect-feeding injuries,
hail, or other mechanical wounds (Figure 3). Diseased areas enlarge and
progress toward the base of the leaf, turn yellow to brown, and dry out.
The veins of infected leaves , stems, and roots turn black as the pathogen
multiplies. On cauliflower, black rot commonly appears on the leaves as
numerous, minute brown specks. The infected lower leaves of cabbage and
cauliflower are usually stunted, turn yellow to brown, wilt, and drop
prematurely. Occasionally, diseased plants have a long bare stalk topped
with a small tuft of leaves. In extreme cases, heading may be prevented.
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| The bacteria spread through the veins of the leaf into the
stem. A typical cross-section of an infected stem or petiole shows a black
ring due to invasion of the water-conducting vessels (Figure 4). Dwarfing
and/or one-sided growth is common both in individual leaves and in entire
plants.
Affected plants may quickly rot before or after harvest due to secondary
soft-rotting organisms. Soft-rot bacteria commonly invade black-rot lesions,
move into the head and turn it into a slimy, foul-smelling mess. Late
infections of black rot may merely spot the leaves or result in smaller
heads. Infected turnip and rutabaga roots show black vascular bundles
and an internal breakdown occurs.
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Click on image for
larger version
Figure 4. Vascular
blacking on cabbage.
|
Symptoms of black rot are often confused with those of Fusarium yellows. The
Fusarium fungus also produces a dark ring inside diseased stems,
as well as darkened petioles and one-sided growth. The discoloration, however,
is dark brown rather than black. V-shaped diseased areas at the leaf margin
are not as common or distinct with the yellows disease. The presence of black
veins in yellow lesions along leaf margins is diagnostic of black rot. The first
and most striking symptom of yellows is the dull yellow to yellowish green appearance
of affected leaves. A microscopic examination may be necessary to distinguish
yellows from black rot.
Back to Top
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Disease Cycle
The causal bacterium overwinters on and in seed and crop debris left
in the field. The organism survives especially well in cabbage and Brussels
sprout refuse, in plants stored for seed production, and in numerous weeds
including black mustard, field mustard, charlock, shepherd's purse, Virginia
pepperweed, and cress. The bacteria are spread by splashing or flowing
water, blowing of detached leaves or dust particles, shipping and handling
of infected plants, and insects. The bacteria are seedborne and thus are
disseminated worldwide. As few as three infected seeds in 10,000 (0 03%)
can cause black rot epidemics in a field.
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Click
on image for larger version
Figure 5. Severe
black rot in a
cabbage field. |
In the spring, when seedlings emerge, bacteria pass from the cotyledons into
young leaves directly or through the stomata. The bacteria move intercellularly
until they reach the xylem tissue and from there spread throughout the plant.
The pathogen is spread from plant to plant by splashing rain, or in films of
water moved by people, equipment, insects, and other animals. The bacteria enter
the plant through hydathodes along the leaf margins, through insect injuries,
and in very susceptible crops, such as cauliflower, directly through the stomates.
The optimum temperature for growth of the organism is from 77° to 86 °F
(25° to 30° C), the minimum is 41° F (5° C), and the maximum
is 96° F (35° C). Free moisture in the form of dew, fog, or rain is
required for infection and disease development. Under the optimum conditions,
symptoms may appear on plants 7 to 14 days after infection. At lower temperatures,
symptoms develop more slowly.
Back to Top
Control
A. For transplant growers:
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Purchase only certified, pathogen-free seed.
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Treat all crucifer seed with hot water. Seedlots should be entirely free
of black rot bacteria before planting; this is critical. Because of the
difficulties in treating seed, most growers prefer to buy seed already treated.
Proper hot water treatment also helps to eliminate seedborne infections
of other diseases such as blackleg, Alternaria leaf spot, anthracnose, Fusarium
yellows, and downy mildew.
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Seedbeds or greenhouses should be at least 1/4 mile from crucifer production
fields.
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The soil to be used for seedbeds should have had no crucifer production
for at least three consecutive years. If rotation of the plant bed is impossible,
disinfest the soil using heat or fumigation.
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Seedbeds and greenhouses must be kept free of crucifer weeds and should
receive regular applications of pesticides to insure freedom from diseases
and insect damage.
-
When watering plant beds, avoid sprinkling the foliage. Sprinkling is
one of the most common means of disseminating black rot bacteria. Also,
do not overcrowd or plant in poorly drained soil.
-
All tools and equipment used in seedbeds or greenhouses should not be used
on other crucifer crops or should be decontaminated before bringing them
back into the seedbed area.
-
Transplants should not be "topped" to fit into shipping containers or sprayed
or dipped in water prior to transplanting.
-
Only new crates or crates not previously used for crucifers should be
used for shipping transplants.
- The transplants should be certified as disease-free by the State Department
of Agriculture inspectors before shipping.
B. For field growers:
-
Purchase your own seed and verify that the seed has been hot-water treated,
certified free of the black rot bacterium, and documents that transplants
were not trimmed and that only new packaging material was used. Information
such as seedlot number and source, dates of pulling and shipping, pest control
schedules, and transit conditions is also useful in judging the health of
plant materials and helping to identify the source of disease case problem.
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Be sure that the transplants are certified as disease-free.
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Grow plants in fields that have not been in a crucifer crop for at least
three consecutive years. Locate crucifer plantings where both air and soil
drainage are good.
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Do not work in the seedbed or fields when plants are wet. Use clean or
new harvest containers that are smooth and flexible.
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Control all crucifer weeds which may serve as a source of inoculum.
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Wherever feasible, clean up and burn or cleanly plow down all crop debris
immediately after harvest.
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Cabbage, rutabaga, turnip, kale, and black mustard varieties are available
that have varying degrees of resistance. New varieties are constantly being
developed with improved disease resistance. Some cabbage varieties with
resistance to black rot include Guardian, Defender, Hancock, Gladiator,
Bravo, Supermarket, and Blueboy. Consult current seed catalogs and trade
publications for additional varieties.
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Maintain balanced soil fertility in both seedbed and field, based on a
soil test.
- Control cabbage root maggots, cutworms, cabbage worms, and other insects
to prevent injury which can serve as a point of infection for black rot.
Back to Top
For further information concerning diseases of crucifers
and other vegetables, contact Mohammad Babadoost, Extension Specialist in Fruit
and Vegetable Diseases, Department of Crop Sciences, University of Illinois
at Urbana-Champaign.
University of Illinois Extension provides equal
opportunities in programs and employment.
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