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(Erwinia carotovora var. carotovora)

Credits:Biovision-Infonet

Bacterial soft rot is caused by Erwinia carotovora var. carotovora. It is quite common on Chinese cabbage in the field. On cabbage, turnip and rutabaga the disease may be found in the field, but is most severe during storage. On cabbage, an initial infection occurs on the outer petiole (leafstalk) which is in contact with the soil, and then progresses to its head. An infected head is watery and often has a complete head rot. The affected area becomes soft and mushy and generally turns dark in colour. Soft rot infection on crucifers almost always emits a foul odour. Eventually the leaves, sterns and roots are entirely decayed by the bacteria. The bacterium is spread in the field by water splashes or contact with tools such as hoes or knives. If contaminated knives are used to harvest cabbages, the stored crop may also rot quickly.

What to do:

• Plant on ridges or raised beds to prevent waterlogging around the plants.
• Prevent other diseases (e.g. black rot; damping-off) or damage that may provide opportunities for soft rot to develop.
• Avoid harvesting when conditions are warm and moist. These conditions favour the development of soft rot.
• Harvest healthy cabbage heads first and store in a cool, dry, airy place.
• Wash hands and harvesting knives.
• Remove and destroy diseased crop or left over stems in the field after the crop has been harvested.
• Avoid growing brassica crops in the same field for a period of at least three seasons.

Credits:Biovision-Infonet

It is serious cabbage disease caused by the fungus Leptosphaeria maculans (Phoma lingam). Symptoms of black leg usually appear as oval, sunken, light brown cankers, often with a black or purple margin, near the base of the stem. The canker enlarges until the stem is girdled and the plant wilts and dies. Severely infected plants are stunted. Infected plants eventually wilt and then turn dull blue-red. With black leg, plants may suddenly collapse due to stem deterioration.

Host range: It includes most of crucifers, but with varying degrees of susceptibility. Most susceptible: cabbages, Swedes, kohlrabi and white mustard. Medium susceptible: cauliflower, turnip, broccoli, rape, kale and collards. Resistant: horseradish, winter-cress, shepherd’s purse and pepper grass.

Symptoms: All above parts of a plant may be affected from seedbed to harvest.

Seedlings: Cotyledon infection causes seedlings to die early. This is often overlooked in the seedbed. Stems: An elongated, light brown, sunken spot or lesion with purplish margin forms on stems near the soil line. Lesions gradually extend upward and downward and eventually girdle the stem and the stem turns black. Small, black dots (fruiting bodies of the fungus: pycnidia) appear on the affected area. Internally, the affected stem shows a brown, dry rot. Affected plants often wilt suddenly and die or topple over later as the heads in case of cabbages enlarge.

Leaves: Inconspicuous, circular, light brown to grayish spots form on leaves. The spots soon become well defined and develop ash-gray centres in which large number of minute black bodies (pycnidia) are scattered. The presence of pycnidia on cotyledons, leaves and stems distiguishes blackleg from other crucifer diseases.What to do:

• Avoid wet soil. Blackleg is most destructive in wet soil.
• Diseased leaves should not be fed to farm animals if manure is to be used on crucifer fields
• Plant tolerant/resistant varieties where available. Check with seed dealers
• Plough in diseased crop immediately after harvest
• Practise a 4-year rotation since the fungus remains alive in soil for at least 3 years
• Properly weed production fields
• Use certified disease-free seed since blackleg is seedborne (i.e. carried on and in the seed). In case of using own seed, treat the seed in hot water and then dress with a recommended fungicide (e.g. Captan; Thiram; etc)
• Seedbeds and should be established in soil that has never been planted to crucifers
• Hot water treatment of own seed to prevent seed borne diseases such as black leg is recommended where the disease has appeared before. 

(Plasmodiophora brassicae)

Credits:Biovision-Infonet

Clubroot caused by the slime mould fungus Plasmodiophora brassicae is particularly serious in Malawi. Its distinctive symptom is an abnormal enlargement of roots or even the underground stem. Its development is favoured by wet, cool, acidic soils.

What to do:

• Practise crop rotation.
• Irrigate properly.
• Adjust pH to 7.2 with hydrated lime

(Alternaria brassicae)

Credits:Biovision-Infonet

Alternaria leaf spot or black spot is caused by the fungus Alternaria brassicae. The disease can severely damage cabbage if uncontrolled. The initial symptoms are small, circular, dark spots on older leaf surfaces. As the spots enlarge, concentric rings develop within lesions surrounded by a yellow halo. The tan-coloured centres of lesions may eventually fall out, producing a hole, or under wet conditions, may become covered with masses of black spores. In storage, spots enlarge and soft-rot bacteria may enter lesions.

The pathogen can over-season on crop debris. Weeds from the family Cruciferae may also harbour the fungus. Spores of Alternaria can be spread by wind and water. The disease is most damaging under wet, warm (20-30.5 degC) conditions.

What to do:

• Cabbage should never be grown in fields where other Brassica crops have been grown in the past 3 years.
• Always remove infected plant debris or destroy it after the season.
• Use disease-free transplants.
• Crop rotation will reduce the severity of black spot disease.
• Hot water treatment of own seed to prevent seed.

(Albugo candida)

Credits:Biovision-Infonet

It affects every known cruciferous crop. However, this disease is generally less common on cabbage, Brussels sprouts, broccoli and cauliflower than on radish, horseradish, mustard and turnip. Leaves have chlorotic or necrotic spots on their upper surface. Pustules form on the lower side of the leaves, small stems and floral parts.

What to do:

• Clean up crop refuse and destroy cruciferous weeds.
• Crop rotation of at least 3 years can help reduce the disease.

(Xanthomonas campestris pv. campestris)

Credits:Biovision-Infonet

Black rot is caused by the bacterium Xanthomonas campestris pv. campestris, and is one of the most serious cabbage diseases in warm climates. The black rot bacterium can over-season on infected cabbage seeds, in weeds from the family Cruciferae (including: black mustard, field mustard, wild turnip, wild radish, shepherd’s purse, and pepperweed); or in infected plant material in the soil.

What to do:

• Disease-free transplants should be used or seeds must be treated with hot water treatment as described below.
• Hot water treatment of own seed to prevent seed-borne diseases such as black rot, black leg, black spot and ring spot is recommended where the disease has appeared before. However, the specified temperature and time interval should be strictly followed in order to maintain seed viability. Use a good thermometer or better ask for assistance from qualified personnel from your local agriculture office. Recommended temperature and time for heat treatment for cabbage: 50degC: for 30 minutes, for broccoli, cauliflower, kale, kohlrabi, turnip: 50degC: for 30 minutes.
• Establish crops in black rot-free soils that have not grown crops from the family Cruciferae for at least 3 years.
• Growing cabbage on raised beds with mulching helps eliminate conditions that induce black rot.
• When possible, remove, burn, or deep plough all crop debris immediately after harvest to reduce the ability of the bacterium to survive in the soil where there is no crop.