Black Rot Of Crucifer Crops
Table of Contents
- Introduction
- Symptoms
- Disease Spread
- Disease Management
- Seed Treatment
- Avoid Disease Spread
- Field Selection
- Crop Rotation
- Weed Control
- Insect Control
- Cull Pile Management
- Resistant Varieties
- Chemical Control
- Crop Nutrition
Introduction
Black rot is caused by a bacteria, Xanthomonas campestris
pv. campestris, that can infect most crucifer crops at any growth
stage. This disease is difficult for growers to manage and is considered
the most serious disease of crucifer crops worldwide (Figure
1). The disease can cause significant yield losses when warm,
humid conditions follow periods of rainy weather during early crop development.
Late infections can provide a wound for other rot organisms to enter and
cause significant damage during storage.
Figure 1.
Black rot infected cabbage.
Symptoms
Symptoms of black rot vary considerably depending on
the host, cultivar, plant age and environmental conditions. The bacteria
can enter plants through natural openings and wounds caused by mechanical
injury on roots and leaves. Seedborne bacteria infect the emerging seedlings
through pores on the margin of the cotyledons and then spread systemically
through the seedling. Infected seedlings grown in the greenhouse under
cool conditions (below 15–18°C) frequently do not show any symptoms
of the disease. When infected seedlings are transplanted to the field
and temperatures rise to 25–35°C during periods of high relative
humidity (80–100%), they become stunted with dead spots on the cotyledons
(Figure 2) and will eventually wilt, and die.
In regions with temperate climates (where temperatures remain cool), disease
symptoms on infected seedlings may not always be obvious or appear severe.
Infected seedlings grown under cool conditions may ooze bacteria from
pores and lesions, which then serve as a source of the pathogen for neighbouring
plants.
Figure 2.
Young cabbage leaf with
V-shaped lesion characteristic of black
rot symptoms.
On older plants, the disease symptoms often appear as
yellow or dead tissue at the edges of leaves, similar to tip burn, except
the lesion frequently progress into a V-shape with the base of the V usually
directed along a vein (Figure 3). Close
inspection of infected leaves and stems may reveal black veins running
through the infected tissue from which the disease gets its name (Figure
4). Lesions on leaves can expand down toward the base of the leaf
causing the leaf to wilt and die.
The bacteria produce a sticky polysaccharide called xanthan
that eventually plugs the vascular tissue inside the veins causing them
to collapse and turn black. The tissue above the plugged, collapsed xylem
eventually turns yellow, wilts and dies. During hot humid environmental
conditions, the bacteria can move from the leaf into the stem through
the xylem. Once inside the stem, the bacteria can move up or down to other
parts of the plant including the roots. Systemically infected plants may
produce chlorotic areas anywhere on the leaf. Severely infected leafy
cole crops such as kale and cauliflower tend to shed their leaves from
the bottom up leaving only a tuft of distorted leaves separated from the
root system by a scarred barren stem. Symptoms on cauliflower often appear
as black flecks or scorched leaf margins. The curds of infected cauliflower
heads often become blackened.
Figure 3. Black
rot symptoms appear as dead tissue at the tips of (a) kale, (b) cauliflower,
and (c) cabbage leaves. Note the V-shaped lesion progressing from the
tip along the vein of the black rot infected cabbage leaf.
a)
b)
c)
Figure 4. Black
rotting veins running through black rot lesion on tip of cauliflower leaf.
Foliar symptoms may not be visible on infected root crops such as rutabaga
and radish but blackened vascular tissue can appear inside the edible
root tissue rendering the plants unmarketable. Although some infected
plants may appear healthy, cutting across infected stems will reveal characteristic
blackened vascular tissue. This is a simple method of determining the
presence of the disease
(Figure 5).
Figure 5. Cross
section of the base of a black rot infected cabbage (a) stem and (b) leaf
revealing the black, collapsed xylem.
a)
b)
Some symptoms of black rot closely resemble those caused
by Fusarium yellows, which causes the vascular tissue to turn brown.
Most commercial crucifer cultivars are resistant to Fusarium (Figure
6).
Figure 6.
Leaf symptoms of Fusarium yellows sometimes appear similar to black rot
except the vascular tissue turns brown instead of black.
Disease Spread
Seed contaminated with black rot bacteria is considered
the most important source of the pathogen and significantly contributes
to the spread of this disease worldwide. As few as 3 infected seeds per
10,000 (0.03% infected seeds) can result in a black rot epidemic. Seed
should be tested and certified to be disease free with less than 1 in
30,000 infected seed.
The organism survives in infected crop tissue left on
the soil until the crop tissue rots. However, the bacteria do not survive
very long in soil as unprotected free living organisms. The black rot
bacteria can also infect and survive on many crucifer weeds. This also
contributes to the persistence and spread of the disease. It can grow
and multiply on host tissue without infecting or causing disease.
Rain splashed bacteria from contaminated plant residue
left on the soil or from neighbouring diseased plants is the primary method
of disease spread throughout a field. The bacteria enter and exit through
water-secreting glands called hydathodes located at the edges and tips
of leaves (Figure 7). Hydathodes often produce
a drop of water during periods of high humidity early in the morning.
The pathogen spreads very quickly when rain droplets contaminated with
bacteria splash onto healthy leaves and enter the hydathodes. The bacteria
move into the leaf veins through hydathodes and begin to multiply, rot
and plug the veins. Contaminated water droplets that exude out of hydathodes
of infected leaves can then be rain- splashed to other plants.
Black rot is more severe and widespread in fields that
receive frequent early morning rains, particularly in May and June. Equipment,
people, animals and overhead irrigation can further spread the disease.
Insects can also spread the bacteria; however, their contribution to the
spread of black rot is limited.
Figure 7. Hydathodes
are special glands or pores at the end of vascular tissue on leaves through
which water exudes and are a natural opening for black rot bacteria to
infect.
Disease Management
Black rot management begins with the identification of
potential disease sources and utilising an Integrated Pest Management
(IPM) strategy including host resistance, planting disease free seed,
avoiding spreading the disease and proper sanitation. Sanitation is the
main method that reduces, excludes or eliminates the initial sources of
disease. General sanitation practices include crop rotation, disinfecting
seed, rouging diseased plants, elimination of refuse piles and eradication
of alternative hosts.
Seed Treatment
Seedborne inoculum significantly contributes to the spread
of black rot bacteria. Growers should only plant tested certified seed
< 1 infected seed in 30,000 or 0.003% contamination. When the infection
level of seed is not known or disease-free seed is not available, seed
should be treated to eliminate the bacteria. Growers who purchase transplants
should request proof the seedlings were grown from disease-free or treated
seed. During transplanting, diseased seedlings should not be planted in
the field.
Seed treatments do not always eliminate 100% of the bacteria
on or in the seed, and may adversely affect seed germination and vigour.
Soaking seeds in hot water at 50°C for 25–30 min. is the most
effective treatment for seedborne blackrot control. Weak seed, seed stored
for several years and seed of certain crucifer crops; such as, cauliflower,
kohlrabi, kale, rutabaga and summer turnip, may be damaged by hot water
treatment; soak for 15 min. at 50°C only.
The effect of the hot water seed treatments on every
variety of each individual crucifer crop has not been investigated. Growers
are encouraged to treat a small portion of seed and plant in pots to determine
the effect of the seed treatment on germination and vigour, prior to treating
the entire seed lot.
Avoid Disease Spread
Use new seed trays each year to avoid contaminating this
year’s crop with residual black rot bacteria from the previous year.
If purchasing new trays each year is not economically feasible, used trays
can be sterilised with steam, boiling water or chemical disinfectants
to eliminate potential contamination. Destroy infected seed trays immediately
to prevent disease spread to other seedling trays.
Avoid soaking crates or bundles of transplant seedlings
in tubs of water before transplanting. The black rot bacteria can spread
from diseased to healthy seedlings by infecting leaf scars and wounds
on roots when soaked in water.
Black rot bacteria can contaminate the surface of clothing,
equipment, tools and water sources. Reducing seeding rates and densities
to promote good air circulation, facilitating the quick drying of plants,
timing irrigation when plants will dry quickly and restricting field activities
until later in the day when fields are dry will help reduce disease spread.
Working in diseased fields last will also avoid disease spread from infected
to non-infected fields. Wash and disinfect equipment before moving from
one field to another.
Field Selection
Field selection is very important due to the distance
the pathogen can spread. Whenever possible, select fields as far away
from fields grown to crucifer crops the previous year. Select fields that
are well drained and will not receive run-off water from areas or fields
where crucifers have been grown previously. Well drained, light soils
are best for crucifer production because they can be worked early in the
season and facilitate earlier planting of transplants. Planting early
can help avoid disease because environmental conditions are usually not
conducive for the development and spread of black rot bacteria.
Crop Rotation
Planting disease-free, treated seed or seedling transplants
does not necessarily ensure a disease free crop in the field. Crop rotation
is also an important management tool. Black rot bacteria can survive in
infected crop tissue in soil until the crop tissue breaks down and rots.
The time required for crucifer crop debris to rot varies between regions
depending on the temperature, amount of soil moisture and soil type. For
example, in the states of Georgia and Washington, which experience long,
warm summers, it has been estimated that free-living bacteria can survive
in infested soil for about 60 days, and up to 615 days in infested host
debris. The bacteria can survive longer in soil during cool, wet seasons
than during hot, dry seasons. In Ontario, a 3-year rotation is recommended.
Weed Control
Black rot bacteria can infect and survive on many crucifer
weeds including bird rape (Brassica campestris), Indian mustard
(B. juncea), black mustard (B. nigra), shepherd’s purse
(Capsella bursa-pastoris), globe-podded hoary cress (Cardaria
pubescens), pepper grass (Lepidium densiflore) and wild radish
(Raphanus raphanistrum). Disease symptoms on weeds vary from small
yellow V-shaped lesions on leaf margins to no visible symptoms. The pathogen
can spread up to 30 m from infected plants (including weed hosts) to healthy
plants. The pathogen not only infects and spreads from weeds to cruciferous
crops, it can also survive on weed seeds and can grow and multiply on
weed leaves without infecting or causing disease. Good weed control within
fields will aid disease management; however, careful attention to weed
control in ditches and along fencerows is also important.
Insect Control
The crucifer flea beetle (Phyllotreta cruciferae)
can transmit black rot bacteria from infected plants to healthy ones;
however, their importance in the spread of the disease is limited. Wounds
caused by insects provide an entry point for the disease to infect plants
during heavy dews or periods of rain. Insect control will help reduce
the spread and severity of disease.
Cull Pile Management
Infected refuse or cull piles left in the field, provides
an excellent source of the black rot bacteria. Fresh cull piles left near
fields can result in severe disease epidemics during the growing season.
Prepare cole crops for market away from fields, and immediately chop and
bury the diseased tissue cut from plants.
Resistant Varieties
The development of crop varieties with disease resistance
or tolerance to black rot has been the focus of many cole crop breeding
programs worldwide. Resistance to black rot was first identified in the
Japanese cabbage cultivar, Early Fuji. Today, many crucifer hybrids with
black rot tolerance are available for both fresh and processing commercial
production.
Chemical Control
Soil fumigation can significantly reduce black rot bacteria.
Soil fumigation is expensive and alternative methods for managing plant
pathogenic bacteria are needed. For more information on chemical control
options refer to OMAFRA Publication 363, Vegetable Production Recommendations.
Crop Nutrition
The effect of plant nutrient management on the susceptibility
of host crops to black rot infection is not fully understood. A balanced
nutrient program may reduce the susceptibility of plants to disease infection.
Excess nitrogen promotes lush vegetative growth and may increase plant
susceptibility. Micronutrients may also be involved with the disease defence
mechanisms of crucifer crops.
