Alternaria leaf spot (Alternaria brassicae)

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.

Aphids

Several species of aphids attack brassicas in East Africa:

The cabbage aphid (Brevicoryne brassicae)

The false cabbage aphid (Lipaphis erysimi)

The green peach aphid (Myzus persicae)

In particular the cabbage aphid is a major pest in the region. Cool, dry weather is most favourable for aphid development. Large numbers of aphids may kill small plants. Their feeding can distort leaves of older plants and causes leaf curl.

Often, the most serious problem associated with aphids is virus transmission. The cabbage aphid and the false cabbage aphid are vectors of virus diseases such as cabbage black ring spot, cabbage ring necrosis, and mosaic diseases of cauliflower, radish and turnip.

Bacterial soft rot (Erwinia carotovora var. carotovora)

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.

Bagrada bug (Bagrada hilaris)

It is typically shield-shaped, 5-7mm long and 3-4mm wide. The upper surface has a mixture of black, white and orange markings. Feeding by sucking by both adults and nymphs causes damage to leaves, which wilt later and dry. A heavy attack on young plants generally results in death of the plant.

Black leg

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.

Black rot (Xanthomonas campestris pv. campestris)

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.

Cabbage looper (Trichoplusia ni)

The cabbage looper is a green caterpillar with faint white stripes along the body. Caterpillars are about 3.5 to 4 cm long when fully-grown. As they move, they arch their back in a looping fashion, hence the common name looper. Larvae (caterpillars) chew holes in the leaves, and larger caterpillars consume great amounts of plant material. On cabbage they eat into the heads, reducing the marketability of the produce.

The adult is a moth about 2.5 cm in length and mottled, greyish-brown.

Cabbage moth (Crocidolomia pavonana (=binotalis))

Cabbage moth larva: when fully grown is about 1.6-1.9 cm long. Larvae pupate in the soil. The pupal stage lasts about 10 days. It is primarily a pest of brassicas and is occasionally an important pest of cabbage and kales. Caterpillars are found in groups. Young caterpillars chew off top leaf surfaces, while older caterpillars feed under a web of silk on young leaves, petioles and growing point of the plant, often damaging it entirely.

Cabbage webworm (Hellula undalis)

Caterpillars are 1.5 cm when fully-grown, creamy-white in colour with brown stripes along the body and a brown head. Caterpillars feed on leaves, petioles, growing points, and stems.

Cauliflower Mosaic Virus (CaMV)

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.

Cercospora leafspots (Cercospora brassicicola)

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.

Clubroot (Plasmodiophora brassicae)

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.

Cottony rot (Sclerotinia sclerotiorum)

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.

Cutworms (Agrotis spp)

Cutworms cut the stem of plants below the soil surface. The damaged plant wilts and withers. First instars are 0.7-1 cm, later instars are 3.5-5 cm long.

Damping-off diseases (Pythium spp., Rhizoctonia solani, Fusarium spp.)

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.

Diamondback moth (DBM) (Plutella xylostella)

It is a serious pest of brassicas and attacks all Brassica species. A full grown diamondback caterpillar is about 1 cm long.

Downy mildew on cabbage (Peronospora parasitica)

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.

Leafmining flies (Lyriomiza brassicae)

Leafmining flies are small about 1.3-1.6 mm in length. Maggots of leafmining flies cause mines while feeding within the leaf. Small, individual leafminers do not produce much damage, but when larvae occur in large numbers, entire leaves can be eaten out. Heavy attacks weaken seedlings and may result in dying off the young plants.

Powdery mildew (Erysiphe polygoni)

Powdery mildew is a fungal disease which appears as white, powdery spots that may form on both surfaces of leaves, on shoots, and sometimes on flowers and fruit. These spots gradually spread over a large area of the leaves and stems. Leaves infected with powdery mildew may gradually turn completely yellow, die, and fall off, which exposes cabbage head to sunburn.

Ring spots (Mycosphaerella brassicicola)

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.

Sawflies (Athalia spp)

Sawflies are sporadic but serious pests of brassicas. The cabbage sawfly (Athalia sjostedti) has been reported as a major pest in Tanzania. Sawfly adults are wasps with dark head and thorax, bright yellow abdomen, and two pairs of membranous wings. They are about 1 cm long. Eggs are laid singly inside the leaf. Larvae are oily, black or greenish in colour with a swollen part just behind the head, which makes them appear humped. They look very similar to caterpillars, but they have 6 to 9 pairs of prolegs (abdominal legs), whilst caterpillars have 5 pairs or less. Larvae measure up to 2 cm when fully grown. Larvae eat the blades of leaves leaving just the main veins. They drop from the plant to pupate in the soil.

Thrips (Thrips tabaci Frankliniella spp)

Thrips feeding on cabbage cause rough bronzed blisters on leaves inside the cabbage head. hey are less than 2 mm long. Thrips attacks are not common on brassicas in the eastern and southern Africa region.

Turnip mosaic virus (TuMV)

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.

White rust (Albugo candida)

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.

Whiteflies (Bemisia tabaci, Trialeurodes vaporariorum and Aleyrodes proletella)

Whiteflies feed on brassicas. Adult whiteflies are about 1 mm long.

In East Africa whitefly populations on brassicas usually do not build up to such an extent that control measures are required.