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(Sclerotinia sclerotiorum)

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Sclerotinia sclerotiorum is a fungus which can survive on infected tissues, in the soil, and on living plants. It causes white mold on cabbage, soybeans, green beans, sunflowers, canola, and peanuts. S. sclerotiorum affects young seedlings, mature plants, and fruit in the field or in storage. Cottony rot begins as tan, water-soaked, circular areas, which become covered by white, cottony fungal growth. As the disease progresses, the host tissue becomes soft and watery. The fungus eventually colonizes the entire cabbage head and produces large, black, seed like structures called sclerotia on the diseased tissue. Plants with root infection often wilt and die prematurely.

What to do:

• Plant cabbage in well-drained soil
• Rows should be planted in the direction of the prevailing winds to enhance aeration. Also plant cabbages in wider rows
• Practice crop rotation for at least 3 years with cereal crops such as maize
• Soil mulching with transparent polyethylene film can raise soil temperatures
• Soil amended with chicken manure

(Peronospora parasitica)

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Downy mildew is caused by a fungus which mainly attacks foliage. It is responsible for substantial losses in seedling production, especially during the cooler months. Young cabbage plants are more susceptible than older plants. The powdery white spores are the major means of dispersal of the fungus, especially once it is established within seedlings crops. Spores are produced overnight and released the following morning as the air dries out. They are dispersed by wind and rain splash. Downy mildew disease development is favored by cool, moist conditions.

Downy mildew disease is first seen as a fluffy or powdery-white mass of spores on the undersurface of brassica seed leaves (cotyledons). This is followed by a black speckling and puckering of the upper surface. Leaves prematurely yellow and fall from the plants.

Symptoms on mature plants in the field are generally confined to the leaves closest to the ground. The fungus produces spores on the under surface, and brown to black, sunken, angular, pepper-like lesions on the upper surface of leaves. The fungus can cause black lesions on cabbage heads and breakdown of cauliflower curds in storage.

What to do:

• Use seed treated with hot water or seedlings raised from such treatment.
• Plant disease-free seedlings.
• Do not water seedlings in the morning when spores are released.
• Keep seedlings as dry as possible. One heavy watering is preferable to a long light watering.
• Maintain a well-ventilated environment – a lower relative humidity minimises spore production. This may mean fewer plants and trays per square meter.
• Maintain a balanced program of nutrition – deficiency of potash will increase the susceptibility of seedlings to downy mildew.
• Remove heavily infected seedlings, old infected seedlings, and weedy crucifer weeds.
• Plough in field crop debris immediately after harvest and use a crop rotation of two to three years if possible.

(Mycosphaerella brassicicola)

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Ring spots are small, purple spots surrounded by a ring of water-soaked tissue on leaves which mature to brown spots with olive green borders 1-2 cm wide. The spots may develop numerous fruiting bodies which give them a black appearance or develop a concentric pattern. Heavily infected leaves may dry up and curl inwards. Ring spot requires cool, moist conditions to develop.

What to do:

• Avoid planting cabbage in previously infected areas
• Rotate crop with non-brassica crops
• Sanitize farm tools and equipment regularly

Credits:Biovision-Infonet

The first symptoms to develop when Brassica seedlings are inoculated with TuMV are chlorotic spots on inoculated leaves, mottling followed by systemic vein clearing, mosaic and/or necrosis, leaf distortion and often stunting. What to do:

• Remove TuMV-infected plant debris and eradicate infected plants around fields as this can help to reduce virus inoculum and hence spread.

(Pythium spp., Rhizoctonia solani, Fusarium spp.)

Credits:Biovison-Infonet

In crucifers, several fungi (e.g. Pythium spp., Rhizoctonia solani, Fusarium spp.) can cause damping-off diseases.

Characteristic is wirestem of seedlings caused by Rhizoctonia solani in the seedbed, bottom rot and head rot in the field, and storage and root rot of horseradish, radish, rutabaga and turnip.

Damping-off diseases are favoured by cool, wet soil conditions.

What to do:

• Use certified disease-free seeds. If using own seed, treat seeds with hot-water.
• Practise proper irrigation, avoid planting in wet, cold soils
• Plant on raised beds to reduce moisture content in the root zone and provide the appropriate drainage in the field to prevent waterlogged conditions
• Practise crop rotation: seedbeds and production fields should not have had crucifers for at least 3 years. All seedlings with wirestem symptoms should be discarded. During cultivation, take care to avoid throwing soil into plant heads.

Credits:Biovision-Infonet

It shows systemic symptoms such as a clearing along the leaf veins (vein clearing). This often is seen first at the base of a leaf. Later symptoms appear as dark green areas along veins (vein banding) and necrotic spotting of the leaf. Chinese cabbage is particularly susceptible to CaMV. In addition to vein clearing a striking mosaic may develop with light and dark green areas seen on the leaves. Plants can be stunted. The main source of CaMV are plants of an infected brassica crop or cruciferous weeds on which a vector has over-seasoned. The virus is transmitted by many species of aphids, such as the cabbage aphid.

What to do:

• Adequate weed control and sanitation should be practised.
• Plough-down previous crops rapidly.
• Transplant beds should be isolated from commercial crop fields and overwintering cruciferous weed hosts.

(Cercospora brassicicola)

Credits:Biovision-Infonet

Cercospora leaf spots is caused by the fungus Cercospora brassicicola. Leafspots vary in colour from pale green to white and generally are bordered by brown tissue. They may be circular or appear angular. Severely affected plants may become defoliated. This disease is not economically important in the East African region.

What to do:

• Use certified disease-free seed and, in case of using own seed, hot water treat the seed.
• Practise proper weed management, particularly, cruciferous weeds such as mustard.
• Remove crop residues from the field after harvest.