Tomatoes (Lycopersicon esculentum) belong to the Solanaceae family, which includes potatoes, capsicums (peppers), eggplant, cape gooseberry and tamarillo, as well as the nightshade weeds. Tomatoes are warm season plants since their optimum temperature range is 21-24°C and they are very susceptible to damage by frost. In temperate climates, tomatoes are an annual crop that is sown as seeds in the spring and harvested in summer and autumn. In warm climates, tomatoes can be perennial plants and flower regardless of daylength. Botanically, tomatoes are classified as a fruit because the edible portion is a giant berry.
Tomatoes originated from South America, most likely the Peru region, where they were cultivated as early as 700 AD. It is thought that Spanish explorers brought them to Europe, with the first mention of tomatoes in literature being in 1544. This described a pomi d'oro (golden apple) that was eaten in Italy with oil, salt and pepper. Thus, it is presumed that the first European tomatoes were a yellow variety and it was many years until red ones were introduced. The Spanish name for the early tomato was pome dei Moro (Moor's apple), while the French initially called it pomme d'amour (love apple).
The English regarded the tomato with suspicion for many years, considering the bright red colour to be a danger signal. This theory had some merit, as deadly nightshade is also a member of the Solanaceae family and tomato leaves do contain some poisonous compounds. The Latin genus name Lycopersicon literally means ‘wolf peach’, and is thought to have originated from the toxic properties of tomato’s close relatives, but the species name esculentum indicates that the plant is edible. Initially tomatoes were used as ornamental plants in England, but by the 1700s they were being used as food, particularly for flavouring. Many reports indicate that tomatoes were not popular with the American settlers in the early 1800s, but by the middle of that century over 23 tomato cultivars were available from a well-known seed catalogue, indicating a huge rise in the popularity of tomatoes.
One wonders whether the early settlers in New Zealand were as doubtful about tomatoes as the Americans! However, tomatoes were grown in home gardens from the late 1800s and have since become an important part of the New Zealand diet. Tomatoes were grown for processing at Nelson in the 1920s and possibly even earlier in Auckland. During the 1930s, commercial glasshouse production of tomatoes began, often as family businesses passed between generations. New Zealand tomato production is mainly for the local market, with few imports or exports due to the high costs of shipping this type of product. Processing tomatoes are mainly grown on the east coast of the North Island as this is where weather conditions for harvest are most reliable.
In the mid-1990s over 600 varieties of tomatoes were commercially available in the US and Canada, and 2500 varieties were kept as seed in a gene bank in Gatersleben, Germany.
Tomatoes, being warm season plants, should not be put into the vegetable garden until there is little danger of frost, usually between October and mid November. In warm areas tomatoes can be planted in December but tomatoes planted at this time in southern New Zealand may not reach maturity before the autumn frosts. Seeds need high temperatures to germinate so start them indoors about 6 weeks beforehand. Before transplanting into the garden allow the plants some time to acclimatise to the cooler outside temperatures.
Tomatoes need a warm, sunny site in well-drained soil. Like most vegetables, the soil pH should be 5.5-6.5 so lime may be required. Add well-rotted compost or manure and cultivate the soil deeply to ensure it is free draining. While tomatoes need good air circulation to minimise fungal diseases, protect them from strong winds that may break the stems. Tomatoes need lots of warmth to ripen so ensure that the site is not shaded during the day.
Tomato plants should be spaced 450 mm apart with 1 m between rows. Stakes should be put in the ground before planting to avoid damaging the plant roots later. Dwarf tomatoes that are not staked are planted about 1.2 m apart. Mulch around the plants, e.g. straw or grass clippings, may help prevent weeds germinating, conserve soil moisture and discourage disease development. Companion plants may also be planted around tomatoes and these are discussed below.
As the plants develop, tie them to the stake with wool or old stockings. Tomatoes grown in the glasshouse may be twisted around a string from the roof and pegged to the ground. Tomato plants usually have a main stem but soon begin to develop side-shoots, known as laterals. These must be removed to prevent the tomato developing too many stems instead of setting fruit. It is usual to keep to one main stem but two or three stems can be allowed to develop as long as they have support. Check tomatoes regularly and remove laterals as soon as they appear.
Supplying the correct level of soil moisture is essential for good tomato development. Both over- and under-watering increase the susceptibility of tomatoes to disease and will cause physiological problems in the tomato fruit. For example, tomatoes stressed by lack of water can develop a condition known as blossom end rot where the fruit has brown dry patches at the base of the fruit and the inside may also be brown and shrivelled. However, over-watering may cause the skin to split. The best time for watering is in the morning, and plants should be watered around the base – not on the leaves.
During the growing season, tomatoes can be given a fortnightly liquid foliar fertiliser (sprayed onto the leaves), such as fish, seaweed or plant extracts. Specialist solid tomato fertilisers that are sprinkled around the base of plants are available from garden centres and the directions on the packet should be followed. Avoid fertilisers with a high nitrogen content as these may favour growth of leaves at the expense of flowers. Tomatoes given adequate fertiliser will not only set lots of delicious fruit, but the plants will also be less susceptible to pests and diseases.
Tomatoes are best left on the plant to ripen but don’t leave them too long or the birds will harvest them first! Once they are picked, tomatoes should be stored at room temperature; tomatoes kept in the refrigerator can lose their flavour. From April it is too cold in most parts of New Zealand for tomatoes to ripen outside. At this time the whole plant should be cut off at ground level, the leaves removed and the vine hung in a warm place to allow the remaining fruit to ripen. Tomatoes can be eaten in so many different ways. Many people think that “fresh tastes best”, but tomatoes can also be dried, made into sauce, chutney, relish, salsa or soup, as well as being preserved in brine.
What defines the flavour of a tomato?
Defining flavour is extremely complex, not only in a physiological sense but also psychologically, as it depends on a person’s perception. For example, people associate tastes and aromas with good or bad situations they have encountered in the past. Taste comes from the sweet, sour, salt and bitter receptors in the tongue, while aromatic or volatile compounds are detected by hundreds of nerve endings in the nose. The brain interprets the input from taste and aroma to give a perception of flavour. To add to the complexity, aromas and tastes can interact, by processes such as masking, where one aroma or taste can override another, and blanking, where receptors become overloaded by an aroma or taste. Small volatile molecules, sometimes called top notes, are perceived as aroma before the large molecules and generally have a greater impact on flavour.
The flavour of tomatoes, like many fruits and vegetables, is primarily determined by sweetness, sourness and aroma. These components can be measured objectively, for example, the simple sugars fructose and glucose (present in roughly equal proportions) are the main determinants of sweetness, while organic acids like citrate (predominant) and malate impart the sour taste. Over 400 volatile compounds have been detected in tomatoes by chemical measurement methods such as gas chromatography-mass spectrometry. It is thought that about 16 of these are present at high enough levels to have a significant impact on flavour. One volatile compound , 2-isobutylthiazole, is unique to tomato. However, it is common to measure flavour subjectively using taste panels. This can be done using a large number of panellists (e.g. 50–100) who rank a small number of different tomatoes on a scale, such as 1-3 or 1-9. Alternatively a smaller number of trained panellists can score tomatoes for the presence and strength of particular flavour attributes.
In the past, selection for new cultivars of tomatoes has focused on characteristics like yield, seasonal growth, colour, size, shape and disease resistance. Some work has been done on texture, for example varieties bred for canning have less water and will hold their shape well, but there has been little work on flavour. This is because it is so difficult to assess but also because there are many genes that influence flavour. In addition, flavour can be affected by non-genetic factors, such as growing, harvesting and storage conditions as well as stage of maturity of the fruit. For example, one study has shown that heavy rain prior to harvest can dilute flavour compounds in tomatoes, while in another study tomatoes treated with higher levels of nitrogen and potassium fertiliser scored lower in sensory analyses than tomatoes with less fertiliser. Other research has demonstrated that tomatoes harvested when immature had fewer volatile compounds than those harvested at a more mature stage.
Storage conditions play a significant role in the flavour of tomatoes. Controlled atmosphere storage, where the concentration of particular gases (e.g. ethylene) in the air is artificially altered to prolong storage or synchronise ripening, has been shown to change the aroma components of tomatoes. The role of temperature is particularly important. One study using trained flavour panellists, showed that tomatoes kept at 2, 5, 10 and 13°C had lower levels of some important volatile compounds, less ripe aroma, and more off-flavour than tomatoes stored at 20°C.
The message from this is … don’t keep your tomatoes in the fridge!
There is a bewildering assortment of tomato varieties available. Rather than try to present a list of them, this section discusses different types of tomatoes and gives some examples. The best idea is to select several different varieties to grow in the garden that will suit your climate and requirements for tomatoes. Packets of seed or young tomato plants can be purchased from garden centres and mail order catalogues. Alternatively, since tomato plants are naturally self-pollinating and the seeds of a tomato will usually produce plants that resemble the parent, you can save the seeds from good performing tomatoes and grow them the following year.
Classifying tomato cultivars
Another way of classifying tomatoes is the time they take to mature. Early or fast-ripening tomatoes will produce fruit within 4 months of sowing the seed (e.g. Early Girl and Moneymaker), main crops take 4–5 months (e.g. Potentate), while late-maturing tomatoes (e.g. Russian Red) will take more than 5 months to produce fruit.
Heirlooms and hybrids
Tomatoes are considered to be one of the most important vegetables in the western diet, based on the amount consumed and the vitamins and minerals supplied. Particularly important are vitamins A and C. At around 22 mg vitamin C/100 g, tomatoes are considered to be a good source of this important nutrient. Tomatoes are also high in carotenoids, such as beta-carotene, which is one of the precursor molecules to vitamin A.
Another important carotenoid in tomato is lycopene, which is responsible for the bright red colour. Lycopene is a potent antioxidant and a number of recent studies have linked high intakes of lycopene with a reduced risk of cancer, particularly prostate cancer. Lycopene is also thought to have a role in preventing degeneration of the eyes and may help in protecting the skin from ultra-violet light damage, while eating tomato-based foods has been associated with a reduced risk of heart disease.
Tomatoes are also an important source of another essential antioxidant, lipoic acid, and contain moderate amounts of flavonoids, a large group of compounds with antioxidant and other health-promoting properties. Tomatoes also contain high levels of the mineral potassium and are a good source of dietary fibre as long as they are eaten with the skin and seeds. Tomatoes contain virtually no fat, are low in sodium and have few calories.
As mentioned above, tomatoes are used in many different ways. Eating a ripe tomato immediately after picking it will ensure that you get the maximum level of vitamin C, as this nutrient tends to decline during storage and can be leached out during cooking. However, some research indicates that cooking tomatoes can increase the absorption of lycopene into the body and that the uptake of lycopene may be assisted by the presence of oils in the diet, such as olive oil. Like most things in life it is probably a question of balance, so aim to eat both fresh tomatoes and cooked tomato products regularly.
Tomatoes can be seriously affected by pests and diseases, particularly in warm, wet climates. In contrast to many other vegetables, e.g. carrots and peas, where lots of plants are grown, a typical home garden may include only 5-10 tomato plants. There is no opportunity for replanting if a plant is lost in January and this will lead to the loss of a significant proportion of the crops. Therefore it is very important to monitor established tomatoes plants regularly and act quickly if there are symptoms of pests or diseases. Some of the more important pests and diseases are described below but first we will look at how the principles of Integrated Garden Management (IGM) can be used to grow healthy tomatoes.
IGM techniques initially rely on preventing pests and diseases attacking the tomato plants. This is not always possible so the tomatoes should be watched closely during growth for the first signs of pests or disease. Once the pest or disease has been identified, an appropriate control technique should be selected. This may involve more than one method and will not necessarily involve the application of chemicals. Chemical control can be very effective for large areas, e.g. a processing tomato crop, or when a pest or disease attack is particularly severe. However, in home gardens, the application of chemicals, especially insecticides, can upset the ecological balance within the garden and lead to further outbreaks of other pests and diseases. Some IGM techniques useful for growing tomatoes are described below. These include crop rotation, resistant cultivars, seed treatment, weed control, fertiliser, irrigation, biological control and chemicals.
One situation where it is difficult to practise crop rotation is in the glasshouse. Particularly in southern New Zealand, growing tomatoes in the glasshouse can improve the chance of getting good tomato yields. It is possible to remove most of the soil from a glasshouse every 1–2 years and replace it with fresh soil that hasn’t had tomatoes growing in it. However, another option is to grow the tomatoes in bags in the glasshouse. Then the bags containing the soil can be removed at the end of the season. If tomatoes are grown in bags, pay particular attention to ensuring the plants have adequate water and nutrients.
Seed treatment and soil conditions
The soil into which tomato seedlings are transplanted should be warm, well aerated and not too wet. Tomatoes are prone to root diseases, such as Phytophthora root rot, in wet, compacted soil.
Irrigation, fertiliser, air circulation and hygiene
The main pests and diseases affecting tomatoes in New Zealand and some suggestions for their control are outlined below.
Seed and seedling diseases (damping off). Some soil fungi, such as Phytophthora, Pythium and Rhizoctoniai, can cause seedlings to die soon after germination. This is particularly a problem in wet soils and cool, damp weather. To prevent or minimise the effects of damping off, high quality seed should be sown into warm, sterile seed-raising mix. Fungicide treatment of the seeds can help prevent these diseases.
Botrytis (Botrytis cinerea). This grey mouldy growth occurs on both the leaves and fruit. Cool, humid conditions are required for the development of the disease. The best way to prevent Botrytis is to allow good air circulation around the tomato plants to reduce humidity and avoid irrigating in the afternoon and evening.
Fusarium wilt (Fusarium oxysporum). The leaves on plants infected with this fungus begin to turn yellow, then wilt and die. The centre of infected stems turns a reddish brown colour. Fusarium can survive in the soil for a long time so crop rotations of 3 years are recommended. The fungus can also be carried on seed. Development of the disease occurs at high soil temperatures. Resistant tomato varieties are available.
Late blight (Phytophthora infestans). Leaves infected with late blight develop water-soaked patches that turn brown. Large, irregular brown-green lesions can be found on the fruit. This disease, which is devastating in potatoes, can also cause severe losses in tomatoes when conditions are favourable (cool, wet weather). The fungus is found on all members of the Solanaceae family and the only means of control is spraying with a fungicide.
Early blight or target spot (Alternaria solani). The fungus causing this disease lives on decayed plant material and on members of the Solanaceae family. Black or brown lesions appear on leaves, which may turn yellow and die. Sunken lesions may develop on fruit. It disperses in the air and can be a problem in moist conditions. Crop rotations, general hygiene and resistant tomato varieties can minimise the risk of this disease. Fungicide sprays are also effective.
Black rot mould (Alternaria alternata). This fungus can cause large, black, sunken lesions in the tomato fruit. Like early blight, black rot mould lives on decaying plant material and requires wet conditions for infection of the fruit. The disease can be minimised by removing plant debris and ensuring the fruit doesn’t get wet. Fungicides can control the disease.
Other root and stem diseases. Other fungi found in the soil that may infect tomato plants are:
The diseases caused by these fungi generally develop when the soil is wet. Crop rotations and good hygiene help to reduce the sources of infection, while resistant tomato varieties are available for some of the diseases.
Bacterial canker (Corynebacterium michiganense). This bacterium can survive on seed and infect the plant during germination. In addition, it can survive on plant debris in the soil. Symptoms are quite variable but include wilting and browning of leaves, a dry, brown, hollow stem and circular, raised, white spots on the fruit that turn into brown, scabby lesions. To minimise the risk of the disease, use good quality seed, practise crop rotation and remove dead plants.
Bacterial speck (Pseudomonas syringae pv. tomato). Wet leaves are required for this bacterium to infect tomato plants. Small dark brown/black spots are found on leaves and superficial, raised, black spots are seen on the fruit. Since the disease may survive on weeds and plant debris, it is important to practice good hygiene and crop rotation.
Bacterial spot (Xanthomonas campestris pv. vesicatoria). This disease can develop in warm, moist conditions. Symptoms and control measures are similar to those for bacterial speck.
Some of the viruses that affect tomatoes include alfalfa mosaic virus (AMV), cucumber mosaic virus (CMV) and tomato spotted wilt virus (TSWV). Once a virus has infected a tomato plant there is nothing that can be done to remove the virus from the plant. Thus, prevention is the main way of ensuring the crop does not become infected. CMV and AMV are transmitted from an infected plant to an uninfected tomato plant by aphids. TSWV is transmitted by thrips. Many common weeds and most other members of the Solanaceae family are hosts to these viruses (see Weed Control in the Integrated Garden Management section ). Prevention of these viral diseases relies on keeping tomato plants away from other host plants and minimising infestation by aphids or thrips, particularly by encouraging biological control of these pests. Symptoms of viral diseases can be quite variable but may include stunting and deformation of leaves, stems and fruit, as well as unusual coloured mottling (yellow, red, brown, black or purple) on leaves, stems and fruit.
A range of insects live on and around tomato plants so it is quite important to identify those that are pests (cause damage to the tomatoes) and those that may be beneficial (biological control agents). If possible, avoid using broad-spectrum insecticides as these will kill most insects – the good and the bad!
Aphids. Aphids cause more damage to plants by transmitting viruses than by direct feeding on plant tissue. Two species of aphid are important on tomatoes. The potato aphid (Macrosiphum euphorbiae) is about 3 mm long and ranges from pale green to green in colour. The green peach aphid (Myzus persicae) is about 2 mm long and ranges from green to pale yellow to pink in colour. As mentioned inBiological Control, there are a number of natural enemies of aphid and these should be encouraged in the home garden. Reflective mulches, such as aluminium foil or white plastic) can deter aphids from landing on a nearby crop, while plants like basil may produce chemicals that repel aphids and can be planted close to tomatoes.
Thrips. Like aphids, thrips are mainly a pest of tomatoes because they can transmit TSWV. Feeding damage by thrips is minor. Thrips are very small (about 1 mm long) and range from dark brown to pale yellow. Adults have two pairs of narrow wings. Onion thrips (Thrips tabaci) is the main vector of TSWV, but New Zealand flower thrips (Thrips obscuratus) is also commonly found on tomatoes. Like aphids, biological control is the best option for minimising thrips infestations. In commercial glasshouses thrips may be controlled by a predatory mite, Neoseiulus cucumeris, which can be bought in large numbers from a company that raises biological control agents. Reflective mulches and companion plants may also deter thrips from landing on the tomatoes.
Tomato fruitworm (Helicoverpa armigera). This caterpillar can cause major damage to tomatoes, especially in the North Island and northern half of the South Island of New Zealand. It ranges in colour from green to yellow to pinkish red to chocolate brown as it progresses through its life-stages. Tomato fruitworm can be found on many vegetable, tree and ornamental plants, and differs from the green looper caterpillar in that it has four pairs of prolegs on its abdomen, whereas the looper only has two pairs. A number of different species of parasitic wasps attack the tomato fruitworm, parasitising either the eggs or the caterpillar. There are also many common predators, such as shield or soldier bugs, the Asian paper wasp, spiders, ants, ladybirds, lacewings, damsel bugs and birds. The tomato fruitworm overwinters as a pupa in the soil so cultivation may kill pupae or leave them exposed to predators. The use of insecticides is not recommended for tomato fruitworm control in home gardens, but you can pick the caterpillars off the tomato plants and squash them!
Green looper (Chrysodeixis eriosoma). The green looper can be found in the North Island and northern half of the South Island of New Zealand. It does not cause major damage of tomatoes since it only feeds on foliage and is largely controlled by parasites and predators.
Green vegetable bug (stink bug) (Nezara viridula). This insect is green and shield-shaped, and emits a pungent smell when disturbed. It is common in warm areas of New Zealand and in hot seasons. The bug feeds on both tomato fruit and foliage but generally only causes minor damage.
Greenhouse whitefly (Trialeurodes vaporariorum). Whitefly adults are white and about 1 mm long, while the immature stages found on the underside of leaves are oval, flat, yellow to white scale-like insects. All life-stages feed on sap from leaves and secrete honeydew on which the black sooty mould fungus can grow. Whitefly is common on all tomato plants but is generally only a problem in tomatoes grown in the glasshouse. There is a wide range of predators that will eat whitefly, including small birds, spiders, lacewings, hoverflies, lady birds and damsel bugs, while the parasitic wasp Encarcia formosa attacks white fly, as described in Biological Control. Whiteflies are strongly attracted to the colour yellow, so yellow sticky traps can be hung inside glasshouses. Chemical insecticides are not very useful in the home garden, as some populations of whitefly are resistant to some insecticides. Organic sprays are available but their use must be balanced against any detrimental effects on beneficial insects. Once the tomato crop has finished then plants should be removed from the garden to eliminate over-wintering sites for whitefly.
Don’t be put off by this large list of pests and diseases. In the home garden, especially where IGM principles are applied, these organisms will rarely cause problems. Your tomato-growing efforts will be rewarded in the summer with bowls of fresh juicy salad tomatoes, tasty tomato sandwiches and delicious sweet treats of cherry tomatoes. In the winter, reach for the jars of dried tomatoes in olive oil, tomato chutney, tomato sauce, tomato relish and tomato soup, and plan what cultivars you will plant next year.