The tomato is a hugely popular crop with good demand from local markets. Because of its susceptibility to pests and diseases the tomato is considered to be a tricky crop and growers have to stay on their toes to keep their lands disease free and their plants healthy.
Tomatoes can be eaten cooked or raw; they can be processed into jam, chutney, tomato sauce, tomato paste and sundried tomatoes to name but a few uses of this crop. So it’s a good product for value adding and much of this can actually be done in a grower’s kitchen.
Try a week in the kitchen without tomatoes. You’ll be surprised at how much you miss it. Tomatoes are a good source of Vitamin A and Vitamin C and some of the essential minerals.
Before you plant tomatoes, take a soil sample and test it; keep testing every three years. Once you have the soil status you can correct your soils with fertiliser. Where soil lacks potassium (K) growers should supplement to improve fruit solidity.
If you cannot test the soil (very small-scale) use these general guidelines: A week or two before planting apply a dressing of about 1 000kg per hectare (100g/m²) of a fertiliser like 2:3:4 (30) + Zn (zinc) and mix it into the top 10cm of the soil. Otherwise use 30m3 of compost or kraal manure per hectare.
Tomato plants respond well to alternate top dressings of 50kg/ha (5g/m2 ) 1:0:1 (36) and 50kg/ha (5g/m2) LAN (28) every three weeks after transplanting until week 18, as long as the plants are healthy and growing.
Weed the crop before you fertilise and put the fertiliser 5cm to 10cm away from the base of each plant. Water well after fertilising.
How often you water tomatoes depends on the soil type and temperature. It’s not a great idea to use overhead sprinklers with tomatoes because wet leaves can encourage early or late blight. It’s critical to water straight after sowing or transplanting.
Young plants need enough water for vegetative growth (roots, shoots and leaves) before flowering. Check for wilting leaves in the late morning. If there is any sign of wilting, water the plants again in the afternoon. This will boost crop quality.
During the first four weeks, water 21mm/week. Water at 38mm/week for the following eight weeks. Water at 31mm/week for the rest of the growing season.
Tomatoes are prone to ‘cracking’ if there is too much water after fruit formation. The tomato plant needs about 500mm of water for the growing season.
Fix a stake into the ground next to each plant and tie the stem loosely to it to prevent its collapsing. You can use a trellis instead of individual stakes, but you will still need a sturdy stake, fixed into the ground every 4m along the row.
Staking stops the fruit and lower leaves from touching the ground. This limits the chances of diseases infecting fruit or leaves.
Cultivars for processing are usually not staked; strong-growing cultivars usually are. If only a few rows have been planted or space is limited, as it may be in home gardens, staking is recommended for both types of cultivar.
PESTS AND DISEASES
Root-knot nematodes, cutworms, bollworms, armyworms, leaf miners (Tuta absoluta), thrips, red spider mites and aphids are the most important tomato pests.
Caterpillars: The larvae of moths (caterpillars) are often found feeding on tomato plants and fruit. The moths usually fly in from the neighbouring area at night. Some of these would be bollworms, loopers, semi-loopers, lesser army worm, fall army worm, and Spodoptera larvae. There are a number of registered pesticides to combat pests but it is wise to think about alternative controls like pheromone traps, refugia that provide better habitat, biologically derived controls and creating a healthy crop that is disease-resistant.
If there are a few caterpillars, remove them by hand.
Cutworms: These worms are often troublesome in the seedbed and a threat to newly transplanted tomatoes. They are greyish hairless caterpillars that cut the seedling off just above the ground, severing it from its roots. Cutworms are usually found near one of the damaged plants 2cm to 5cm below the soil surface. Keeping fields free of weeds is the best way to reduce cutworm numbers before planting. Start weed control six weeks before sowing or transplanting.
Nymphs: Immature whiteflies are greenish to translucent and also found under leaves. Nymphs can be controlled with insecticides. Seedlings should be protected under mesh cover.
Tomato rust mites: Tomato rust mites are about 0,2mm long. They feed on stems and leaves. Infected leaves take on a bronze hue and curl up. Curling of lower leaves is the first sign of tomato rust. Later these leaves wither and die.
Whiteflies: These are small, white, sucking insects that can be seen flying around the plant when the leaves are disturbed. The wings and bodies of adult whiteflies are covered with a fine white powder. They gather in large numbers under leaves, sucking the plant sap. White flies can also introduce harmful viruses to the field.
Red spider mites: They are tiny (less than 1mm) red-brown and wingless. They penetrate the plant cells and suck up the chlorophyll (which gives leaves their green colour). They stay under the leaves but in a severe infestation they appear over the whole plant. Fine webbing is visible between leaves when infestations are severe, and the leaves turn light yellow. Infected plants may die and those that survive will produce very little fruit. Red spider mites prefer dry, hot conditions. There are various insecticides to control mites on tomatoes.
Leaf miners: The tomato leaf miner originates from South America. It reduces yield and fruit quality of tomatoes with losses of up to 100% reported. Leaf miner flies puncture the leaves to lay eggs and the larvae create long thin tunnels while feeding inside the leaves. The tomato leaf miner (Tuta absoluta) has already shown some resistance to chemical insecticides. Scientists recommend an integrated pest control management strategy that combines biological and chemical methods. The pheromone traps lure and contain male moths during night-time mating flights.
Aphids: These insects are generally wingless and have a colour range from green to black. They hide under leaves or gather near the growth points of a host plant. Aphids can reproduce without mating and under favourable conditions, they don’t lay eggs but bear live young. Aphids suck plant sap, but more importantly they transmit plant viruses.
Bollworms: These are the larvae of a yellow to brown night flying moth that deposits her yellowish-white eggs under the soil. Fully grown larvae are about 30mm to 40mm long. Young bollworm caterpillars have hairy bodies and their colours range from black to brown-beige. Bollworm larvae feed on flowers, leaves and the fruit of tomatoes. Later they may feed inside a hollowed-out fruit. Any insect damage may cause secondary diseases to develop. There are almost 30 insecticides registered for use against bollworm. When numbers are low, remove them by hand.
Potato tuber moths. Tiny, light-coloured caterpillars that bore into unripe and ripe tomatoes, starting at the point where the stem is attached. The point of entry is indicated by a tiny black ring and tuber moth damage causes the fruit to rot. This is not easy to spot in the early stages. Tuber moth larvae also tunnel into leaves, leaving ‘windows’. Infected fruit cannot be marketed and infected fields must be burnt after harvest. Never try to plant new tomato crops in or hear old plots. There are insecticides available for the control of tuber moths.
Nematodes. These microscopic worms live in the soil and cause the formation of galls (large bumps) on the roots. Nematode infestation will lead to low yields, stunted plants and top growth wilting. The best way to manage nematodes is to use the right cultivars and rotate with other crops. Growing marigolds (Tagetes spp) will reduce nematode populations in infested soil in one season, but a single replanting of tomatoes on the same field may build up the nematode numbers to unacceptable levels. Several nematicides are registered for nematode control on tomatoes. You could also avoid nematode-infected fields by practising fallow ploughing.
SMOKERS SHOULD ALWAYS WASH THEIR HANDS BEFORE HANDLING TOMATO PLANTS TO AVOID TRANSMITTING VIRAL DISEASES TO THE PLANTS.