Dealing with bacterial & fungal diseases in tomatoes

Black stem, early blight and powdery mildew are all significant threats to a tomato crop.

Dealing with bacterial & fungal diseases in tomatoes
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In this article, we focus on the bacterial and fungal diseases that every tomato producer should guard against.

Bacterial canker

This disease, which is spread through infected seed and seedlings, can be devastating in both field and greenhouse plants. Plants show progressive wilting; older leaves die, but remain attached to the plant. The leaf edges and internal stem tissues turn brown.

Control: Plant disease-free seedlings; use resistant cultivars; apply strict sanitation; practise crop rotation.

Bacterial wilt

Ralstonia solanacearum can be a serious problem in warm subtropical areas such as Mpumalanga and KwaZulu-Natal. Young leaves wilt, after which the plants die. There is no browning or yellowing of leaves or stems. If the lower stem of a badly wilted plant is cut off and placed in water, a milky stream of bacteria flowing out of the cut end is often visible within minutes. The internal tissues show a slight browning when the stem is cut open lengthways.

Bacterial wilt attacks a wide range of plants and is spread by infected seedlings, infected water and the movement of infected soil and groundwater to healthy production areas.

Plant disease-free seedlings; plant resistant cultivars; practise strict sanitation; rotate crops; control weeds/nematodes; avoid over-irrigating.

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Bacterial speck and black stem
These are both caused by Pseudomonas syringae. Bacterial speck occurs as small brown spots on leaves, stems and fruit.
Infected fruit is downgraded. Nursery plants often introduce an infection.

Black stem causes a blackening of stems and leaf stalks, and often strikes tomatoes under cover.

Control: Rotate crops; practise strict sanitation; avoid over-irrigating; reduce humidity in the greenhouse/tunnel; plant disease-free seedlings; apply chemical control.

Early blight
This widespread fungal disease occurs in humid, moderately hot areas, or semi-arid areas that regularly experience dew. The Alternaria solani pathogen attacks leaves, stems and fruit. Lesions start as small, brown spots on older leaves and stems, and grow rapidly into large, brown-black spots with concentric rings. The fruit is attacked at the stem end, where water accumulates, leading to leaf and fruit drop.

Control: Plant resistant cultivars; use only disease-free seedlings; apply chemical control; eradicate weeds and volunteer tomato plants; have a well-balanced fertilisation programme; avoid overhead irrigation; apply crop sanitation.

Late blight

This can be highly destructive under prolonged wet, cool conditions. Leaves initially show light-green blighted areas, which quickly turn black, with a whitish-grey fungal growth under the lesions when conditions are very humid. Stems show extensive black lesions. Leaves and stems die quickly as the disease spreads.

Infected fruit shows diffuse blackening with a greasy appearance, and deteriorates rapidly. Late blight can cause total crop loss within a week.

Control: Apply chemical control; do not use overhead irrigation; practise crop sanitation.

Powdery mildew

This is a serious problem in hotter areas, such as Mpumalanga and Limpopo. Initially, pale green or yellow spots occur on the upper leaf surface. Later, spots enlarge and develop into large, brown necrotic (having dead cells) lesions. Badly infected leaves die, but seldom drop, and fruit gets sunburnt. Plants grown under drip irrigation are more susceptible than those under overhead irrigation.

Control: Apply chemical control, weed control and crop rotation with non-susceptible crops.

Blossom end rot

With this disease, hard brown spots occur on the tomato’s blossom end.

Control: Mulch; avoid root pruning; irrigate regularly; apply calcium fertilisation (calcium nitrate, gypsum and lime); avoid high nitrogen fertilisation.

Next article: viral diseases.

Source: Production Guideline for Summer Vegetables, Agricultural Research Council.