Let us focus on the diseases every tomato producer should guard against.
Wet weather is always difficult for tomato plants so stop contact between fruit and soil by putting grass around the plant and under the fruit. Don’t be tempted to prune because it will reduce your yield and leave you with cracked fruit.
REMOVE SOME LEAVES
Remove the leaves from the main stem when the first cluster of tomatoes starts to show colour. This is a sensible practice for strong-growing tomato cultivars, especially those under drip irrigation.
With the leaves under the cluster removed, there is greater circulation of air between plants and this helps with prevention of infection. Diseases often thrive when it’s moist and hot.
The major diseases of tomatoes are: bacterial canker, bacterial wilt, Fusarium wilt, Verticillium wilt, early blight, late blight, tobacco mosaic virus.
Bacterial canker can be devastating in both field and greenhouse plants. Plants wilt in stages and older leaves die (but stay connected to the plant). There is browning around the leaf edges and in the internal stem tissues.
Bacterial canker is spread through infected seed and seedlings.
Unfortunately systemically infected seedlings show no disease symptoms, which makes detection of the disease difficult in nurseries.
CONTROL: Plant disease-free seedlings; use resistant cultivars; apply strict sanitation in and around plantings and practise crop rotation.
Bacterial wilt is a serious problem especially in warm, sub tropical areas. There is no browning of leaf or stem.
In the first stages young leaves show wilting, then the entire plant wilts quickly and dies.
A milky white stream of bacteria flows out of the cut end of a stem from a wilted plant when it is placed in water. Stems show some internal browning when cut lengthways.
Bacterial wilt is spread by infected seedlings, infected water and the movement of infected soil and water to healthy production areas.
CONTROL: Plant disease-free seedlings; plant resistant cultivars; apply strict sanitation measures; implement crop rotation; control weeds and nematodes and don’t over-irrigate.
Black stem and Black speck are caused by a bacterial pathogen Pseudomanas syringae. Small brown spots on leaves, stems and fruit indicate the presence of Black speck. Leaves die and infected fruit is downgraded when the plant has Black speck. Nursery plants are often the primary source of infection.
Black stem causes a blackening of stems and leaf stalks, which, although it is superficial, is harmful. Black stem is a problem of tomatoes under protection and hardly ever occurs in field-grown tomatoes.
CONTROL: Practise crop rotation; implement strict sanitation measures; don’t over irrigate; reduce humidity in the greenhouse or tunnel and apply chemical control.
Early blight occurs when conditions are humid and moderately hot. But it can happen in semi-arid areas that experience dew.
The pathogen attacks leaves, stems and fruit. Lesions start as small brown spots on older leaves an dstems, and grow rapidly into large brown to black spots with concentric rings that are often visible in the lesion. Bacterial wilt attacks the fruit at the stem and where water accumulates. The outcome of the disease is leaf and fruit drop.
CONTROL: Plant resistant cultivars; use only disease-free seedlings; control with chemicals; get rid of weeds and volunteer tomato plants from previous plantings; balance your fertiliser programme; implement crop sanitation and do not use an overhead irrigation system.
Late blight is fairly common and can be very destructive when there is a prolonged wet, cool spell. Late blight attacks all parts of the plant above the ground. In the first stage, leaves show light green blighted areas that quickly turn black with a whitish-grey fungal growth under the lesions, especially in conditions of high humidity. There are extensive black lesions on the stems.
Late blight quickly kills leaves and stems as it spreads. Infected fruit shows blackening with a greasy look and deteriorates fast. Late blight can cause total crop loss within a week if the conditions are optimal for the disease.
CONTROL: Apply chemical control; practise strict sanitation measures and maintain them.
Powdery mildew likes hot conditions when humidity is lower. Initially pale green-yellow spots appear on the upper leaf surface. Later the spots enlarge and develop into large brown lesions. Badly infected leaves die, but seldom drop and fruit gets sunburnt.
CONTROL: Apply chemical control; get rid of weeds and rotate the tomatoes with non-susceptible crops.
Tobacco mosaic virus (TMV) causes malformation and stunting in leaves and a green or yellow mosaic pattern. Leaves, stems and fruit begin to brown. The fruit is small and ripens unevenly; when opened it shows browning of the fruit wall.
TMV is spread mechanically by tobacco products, the hands or clothes of workers, implements or infected water. TMV can also be spread by seed, pollen or roots.
CONTROL: Apply strict sanitation in the production area; plant resistant cultivars; control weeds and ban the use of tobacco products in and around plantings.
Always scout your fields. A farmer’s eyes are a good management tool, that will help in preventing disease or wiping it out at an early stage before too much damage is done.
This research has been gathered by agronomists from the Agricultural Research Council of South Africa. For more information go to www.arc.agric.za and look for Summer Vegetables
Seed and chemical resource in Zambia: Amitran, 9362 Mumbwa Road, Lusaka, Zambia Telephone +260 211 286751