What Black Tomatoes Are and Their Cultivation Techniques
Black tomatoes originated in South America and are a rare treasure of the tomato family, named for the fact that both the fruit color and the fruit itself are red-black. The fruit is both medicinal and edible, with a rich fruity aroma, a sweet-and-sour balanced taste, and high nutritional value. It is especially suitable for fresh eating; it also contains a large amount of lycopene and rich vitamin C and antioxidants, and has the effect for women of nourishing yin and protecting the kidneys, whitening the skin, and reducing weight and slimming the body.
Black tomatoes are both medicinal and edible and are a rare treasure of the tomato family. They have a rich fruity aroma, high nutritional value, and a very sweet taste, making them especially suitable for fresh eating. They contain 10 times more lycopene and vitamin C and antioxidants than red tomatoes, and are easily absorbed by the human body. Black tomatoes especially contain a large amount of anthocyanins, which can produce strong biological effects in the human body and are especially anti-cancer. Eaten in moderation, they can revive declining sexual function in both men and women. Black tomatoes have the effect for men of nourishing yin and strengthening yang, reducing a man's chances of developing prostate cancer and heart disease.
(1) Raising seedlings
Once the planting period has been determined, sow and raise seedlings 60-70 days in advance. Before sowing, the seeds must be disinfected. The seed coat of black tomatoes is relatively thin, and heat disinfection easily scalds the seeds, so chemical disinfection should be the main method. Sowing should not be too dense; 25-35 g of seed per sq m is appropriate, and 300-450 g of seed is needed per hectare.
Black tomatoes tolerate transplanting, but because flower bud differentiation is continuous, it is not advisable to divide the seedlings multiple times during the seedling-raising process. It is recommended to divide the seedlings once and early, completing the division at the latest before flower bud differentiation begins at 2-3 true leaves. Dividing the seedlings into plastic nutrient pots, paper tubes, or similar containers is preferable. After dividing the seedlings, appropriately raise the air temperature and soil temperature by 2-3°C to promote seedling recovery. After recovery, the seedlings gradually grow faster. To prevent excessive vegetative growth, the seedbed temperature should be lowered by 2-3°C compared with the recovery period, but the nighttime air temperature must not be lower than 10°C. Moisture should be kept such that the seedbed soil surface is dry and the lower layer is moist; a light green color of the seedling's new leaves indicates normal moisture.
Before planting, the seedlings should be hardened, gradually lowering the seedbed temperature, especially the nighttime temperature. During the hardening process, do not control moisture excessively; keeping the seedlings from visibly wilting at noon on sunny days is appropriate, otherwise the seedlings easily age.
(2) Land preparation and basal fertilizer application
Choose deep, loose, fertile soil with good drainage for cultivating black tomatoes. To promote the black tomato root system to develop in depth, deep plowing is necessary. For plots left fallow in winter, deep plow 25-30 cm before the soil freezes, and do not harrow after turning, to facilitate soil weathering. Deep plowing should be combined with increased basal fertilizer application; generally apply 5000-7500 kg of organic fertilizer per 667 sq m. While applying basal fertilizer, it is best to mix superphosphate with organic fertilizer; apply 30-40 kg of superphosphate per 667 sq m. Black tomatoes can be cultivated in flat beds or raised beds. In the relatively arid northern regions or in high-density early-maturing cultivation, flat beds are appropriate, generally 1.2-1.5 m wide and 8-10 m long. In the south and in irrigated land with sufficient water or low-lying, easily waterlogged land, raised-bed cultivation should be used.
(3) Planting and density
The open-field planting time for spring black tomatoes should be determined according to local climatic conditions. Generally, planting should be done after the last frost, when the average daily temperature reaches above 15°C and the soil temperature stabilizes above 10°C.
The planting density of spring black tomatoes depends on many factors such as variety, pruning method, and length of the growing period. The suitable density for high-density cultivation of early-maturing varieties is 2800-3000 plants per 667 sq m, and the suitable density for cultivating mid- and late-maturing varieties is 2000-2500 plants per 667 sq m.
(4) Cultivation, topdressing, and irrigation
After planting, black tomatoes should be cultivated in a timely manner. Early and deep cultivation helps raise soil temperature and promotes rapid rooting and seedling recovery growth. Cultivation should be carried out 3-4 times in succession, with the cultivation depth becoming shallower each time, and appropriate hilling can be done to promote adventitious roots at the stem base and expand the root mass.
Black tomatoes have a relatively large fertilizer requirement. In addition to heavy basal fertilizer application, quick-acting fertilizers should be applied as needed. When the fruit of the first truss begins to swell, apply a stage of vine- and fruit-promoting fertilizer together with watering. Per 667 sq m, apply 15-20 kg of urea, 20-25 kg of superphosphate, or 20-30 kg of diammonium phosphate; when potassium is deficient, apply 10 kg of potassium sulfate, or use 1000 kg of decomposed human manure and urine and 100 kg of plant ash instead of chemical fertilizers. Apply topdressing once each when the second and third trusses begin to swell rapidly. In addition to soil topdressing, foliar topdressing can be used during the peak fruiting period, spraying the leaves with 0.2%-0.5% monopotassium phosphate, or 0.2%-0.3% urea, or a 2% superphosphate aqueous solution, or spraying a multi-element compound fertilizer. Trace elements such as 50-100 μl/L boric acid or zinc sulfate can also be sprayed on the foliage.
Black tomatoes have a certain drought tolerance, but to obtain high yields, attention must be paid to the supply and regulation of water. 5-7 days after planting, water once to help the seedlings recover, then cultivate to preserve moisture, control watering, and appropriately restrain growth. Mid- and late-maturing varieties flower and fruit later and have stronger vegetative growth, so water should be controlled before fruiting. The water uptake of black tomatoes peaks during the peak fruiting period; during this period, irrigate once every 4-6 days to keep the soil relatively uniformly moist throughout the fruiting period, preventing alternating dryness and wetness and reducing fruit cracking and blossom-end rot. Black tomatoes have relatively strict requirements for soil aeration; after rain, drain promptly to prevent root rot.
(5) Plant adjustment
Black tomatoes have the characteristics of lush stems and leaves, strong branching ability, rapid growth and development, and easy flower and fruit drop. To regulate balanced growth among the various organs and improve light and nutritional conditions, a series of plant adjustment measures should be taken during cultivation, such as trellising, tying vines, pruning, removing side shoots, pinching, preserving flowers and fruits, and thinning flowers and fruits. Except for a few upright varieties, black tomatoes all require trellis cultivation. Generally, conical trellises (three-cornered or four-cornered conical trellises) are mostly used, as they are relatively simple and sturdy.
Black tomatoes have many pruning methods, each with its own characteristics. The common pruning methods in open-field cultivation include single-stem pruning, improved single-stem pruning, and double-stem pruning. In single-stem pruning, only the main stem is retained and all side branches are removed. This method is suitable for dense planting and can achieve higher yield per unit area under conditions of a shorter growing period. The disadvantages are that more seedlings are used per unit area, root development is somewhat restricted, and the plants tend to age prematurely. To overcome these disadvantages, the improved single-stem pruning method can be used, that is, on the basis of single-stem pruning, retain one side branch below the first truss and pinch it after one truss of fruit. There are many variations of this improved single-stem pruning method, which can be applied flexibly according to cultivation needs. In double-stem pruning, in addition to retaining the main stem, retain one side branch below the first truss and remove all other side branches. With this pruning method, early yield is not as good as with single-stem pruning, and fruiting is later, so it is suitable for mid- and late-maturing varieties with a longer growing period and vigorous growth. During pruning, excess side branches are removed, that is, shoot removal. Removing shoots too late consumes too many nutrients; in the early stage of plant growth, removing shoots too early affects root development, especially for early-maturing varieties with weaker growth, so side branches should be removed in stages and batches when they grow to 3-6 cm long.
For indeterminate varieties, when a certain number of trusses has grown, the top of the plant must be removed, called pinching or topping, to ensure that the fruits set within the limited growing period can fully enlarge and mature. Pinching should be determined according to the cultivation method. When cultivating in the south, retain 3-4 layers of trusses; when cultivating in the north, retain 4-5 layers of trusses before pinching. In principle, black tomato cultivation does not involve leaf removal, so as to maintain a relatively large assimilation area of the plant. Only after the peak fruiting period can diseased and yellow leaves at the base be gradually removed to improve ventilation and reduce respiration consumption.
(6) Preserving flowers and fruits
Flower drop is relatively common in black tomatoes. The causes of flower drop are mainly the following two aspects: ① Nutritional flower drop. Flower and fruit drop are caused by insufficient soil nutrients and water, excessive plant damage, poor root development, untimely pruning and shoot removal, excessive nutrient consumption under high nighttime temperatures, excessive vegetative growth of the plant, and imbalanced nutrient supply. ② Reproductive development disorder flower drop. Temperatures that are too low or too high, and rainy or overly dry flowering periods, all affect pollen tube elongation and pollen germination, producing deformed flowers (such as long-styled or short-styled flowers) and causing flower drop. The main cause of early flower drop in open-field spring black tomatoes is low temperature or planting injury; the main cause of summer flower drop in black tomatoes is high temperature and high humidity.
To prevent flower drop, using plant growth regulators can effectively prevent flower drop and can also stimulate fruit development, forming seedless fruits the same size as, or even larger than, pollinated fruits. Commonly used growth regulators include: 2,4-D (2,4-dichlorophenoxyacetic acid), used at a concentration of 10-20 μl/L, for dipping or painting flowers; PCPA (p-chlorophenoxyacetic acid, also called fruit-setting hormone, black tomato spirit), used at a concentration of 25-30 μl/L, which can be used for flower spraying.
(7) Physiological diseases and their control
Physiological diseases in the fruit development of black tomatoes are one of the main problems in cultivation. Common physiological diseases include deformed fruit, puffy fruit, blossom-end rot, fruit cracking, rib rot, and sunscald, which greatly affect product quality.
① Deformed fruit: Deformed fruit mainly occurs during flower bud differentiation and development, that is, under low temperature, excessive fertilizer (especially excessive nitrogen), and sufficient water and light, the growing point accumulates too many nutrients, the dividing cells of the developing flower bud divide excessively, and the number of carpels is too many. After flowering, uneven development of the carpels forms multi-locular deformed fruit. Among deformed fruits, top-cracked or cross-cracked fruits are mainly caused by unfavorable conditions during flower bud development inhibiting the transport of calcium to the floral organs. In addition, because fruit growth is mainly longitudinal at first and then gradually expands laterally, fruits developed under conditions of nutritional deficiency are often pointed-top deformed fruits. To prevent the occurrence of deformed fruit, the temperature during seedling raising should not be controlled too low, and water and nutrients must be regulated appropriately.
② Puffy fruit: That is, the fruit flesh is not plump, the placental tissue grows insufficiently, and the seed cavity becomes hollow, seriously affecting fruit weight and quality. Poor fertilization and excessively high concentrations of growth regulators can easily produce puffy fruit. In addition, during fruit growth, excessively high temperatures, insufficient sunlight, or excessive application of nitrogen fertilizer, excessive vegetative growth, and low carbohydrate accumulation in the fruit can also form puffy fruit. During cultivation, management should be strengthened, environmental control techniques improved, and suitable conditions created for fruit growth to avoid the appearance of puffy fruit.
③ Blossom-end rot: Also called navel-end rot or bottom rot, dark brown lesions occur at the top of the fruit, and rot occurs in rainy weather or when air humidity is high. It is a physiological disease caused by calcium deficiency in the fruit. The causes of calcium deficiency in the fruit are: first, calcium deficiency in the soil; second, dry soil and excessively high soil solution concentration, especially excessive potassium, magnesium, and ammonium nitrogen, affecting the plant's absorption of calcium; third, under high temperature and dry conditions, calcium moves slowly within the plant. To prevent blossom-end rot, apply more organic fertilizer, apply lime to adjust acidic soil, maintain an appropriate soil solution concentration, and appropriately control the amount of ammoniacal nitrogen. Try to avoid excessively high soil temperature and drastic changes in soil temperature. Water supply should be uniform, preventing alternating dryness and wetness. During the fruiting period, spray 0.5% calcium chloride on new leaves and newly grown trusses to supplement calcium content.
④ Fruit cracking: Fruit cracking is likely to occur in the later stage of fruit development. Fruit cracking appears as ring-shaped cracking and radial cracking. The main causes of fruit cracking are: drought in the soil in the early stage of fruit growth, slow fruit growth, and then rainfall or heavy irrigation, causing the flesh tissue to expand and grow rapidly while the skin cannot grow correspondingly, causing cracking. To prevent fruit cracking, in addition to paying attention to selecting varieties that do not easily crack, cultivation management should increase organic fertilizer application, water reasonably, supply water evenly, and avoid direct exposure of the fruit to strong light.
⑤ Rib rot: It is a physiological disease during the fruit swelling period. According to the symptoms, it can be divided into two types, the browning type and the whitening type. In the former, the vascular bundles and surrounding tissues inside the fruit turn brown; in the latter, the fruit skin or fruit wall hardens and turns white. The onset conditions of the two types are similar, and it is a disease induced by a variety of unfavorable conditions. To prevent the occurrence of rib rot, special attention should be paid to fertilizer use, appropriately increasing potassium fertilizer and mainly using nitrate nitrogen for nitrogen fertilizer, etc.
⑥ Sunscald: In the high-temperature summer season, due to direct strong light, the temperature of the shoulder of the fruit rises, part of the tissue is scalded and dies, producing sunscald. The harm of sunscald varies greatly among varieties. Varieties with smaller leaf area, exposed fruit, or thin skin are prone to the disease. To prevent sunscald, avoid sunburn of the fruit during the fruiting period, and it is best to use conical trellises or A-frame trellises, placing the trusses in the leaf shade within the trellis when tying the vines. Appropriately increasing potassium fertilizer can enhance its resistance.