Rot of Cabbage and Other Crucifers
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.
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.
on image for larger version
Figure 1. Cabbage plants infected with black rot (V-shaped
lesions on leaves).
on image for larger versions
Figure 2. Typical "V-shaped" lesion on cabbage.
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).
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.
on image for larger versions
Figure 3. Lesion originating from insect feeding injury.
on image for larger version
Figure 4. Vascular blacking on cabbage.
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.
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.
on image for larger version
Figure 5. Severe black rot in a cabbage field.
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.
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.
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.
For transplant growers:
Purchase only certified, pathogen-free seed.
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.
Seedbeds or greenhouses should be at least 1/4 mile from crucifer
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.
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.
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.
Be sure that the transplants are certified as disease-free.
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.
Do not work in the seedbed or fields when plants are wet. Use clean
or new harvest containers that are smooth and flexible.
Control all crucifer weeds which may serve as a source of inoculum.
Wherever feasible, clean up and burn or cleanly plow down all crop
debris immediately after harvest.
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.
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
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.
of Illinois Extension provides equal opportunities in programs and employment.