Key Techniques of Stereoscopic and Efficient Cultivation of Strawberry in Southern Xinjiang

Zehui LIU, Yong SHUI, Jianyong XU, Qiang CHEN, Shujuan REN, Ping ZHANG, Xian CAO

1. Alaer National Agricultural Science and Technology Park, Xinjiang Production and Construction Corps, Alaer 843300, China; 2. Institute of Agricultural Sciences of the First Division of Xinjiang Production and Construction Corps, Alaer 843300, China

Abstract Strawberry is rich in nutrition, which is the only fresh fruit in winter in southern Xinjiang, with good prospect of stereoscopic cultivation. In this paper, the key techniques such as stereoscopic cultivation pattern, the combination of different modes, the substrate of strawberry cultivation, the drip irrigation system, the formula of nutrient solution, the selection of varieties, the propagation of seed and seedling, the induction of flower buds, the planting and management of plants, and the green control of diseases and insect pests are introduced, and greenhouse space and solar energy are fully used to maximize the economic and social benefits of strawberry farming combined with tourism. The results will further promote the quality and efficiency of strawberry industry in southern Xinjiang, and provide a reference for strawberry growers.

Key words Strawberry, Stereoscopic cultivation, Pattern combination, Seed and seedlings breeding, Flower bud induction, Plant management, Green prevention and control

1 Introduction

Strawberry, featured by red and tender appearance, juicy flesh and high nutritional value, is favored by consumers. Strawberry stereoscopic cultivation is making full use of space and solar energy to improve land utilization rate through various cultivation pattern of stereoscopic and spatial dislocation collocation, to avoid the harm caused by continuous cropping of strawberries, to significantly reduce the use of pesticides and fertilizers, and to improve the yield and quality of strawberries; The cultivation patterns are novel and beautiful, and the economic and social benefits are significant combined with picking and sightseeing[1-5]. Xinjiang region is chill in winter, with less fresh fruits in the market, and winter strawberry has broad prospects for development. After years of research and promotion, this study sums up a relatively complete set of key technologies of strawberry stereoscopic cultivation in winter warm greenhouse, which would provide a reference for improving the technical level of strawberry stereoscopic cultivation in southern Xinjiang and promoting the quality and efficiency of strawberry industry in Xinjiang.

2 Strawberry stereoscopic cultivation pattern

2.1 Columnar patternThe columnar cultivation mode adopts environment-friendly plastic cultivation bowl or other forms of plant growth carriers and drip boxes, which are connected by columns. The cultivation bowl is hollow, 4- or 6-valved structure, made of PVC material. Cultivation bowls are stacking on the column, making full use of greenhouse space and solar energy. The biggest characteristic of columnar cultivation is that water and fertilizer can be directly applied to the root; the cultivation nutrient solution can penetrate into the next layer one by one from the upper end, and be recycled through the liquid pipe to reduce fertilizer loss and improve fertilizer utilization rate. The cultivation bowls on the column can rotate freely, and the cultivation column is rotated once every 3-4 d, so that strawberry can be lightened uniformly. Because each cultivation column is independent from each other, it also reduces the spread of pests and diseases. Columnar cultivation is suitable for small space in the corner of greenhouse.

2.2 Pipe cultivation patternStrawberry pipe cultivation is a cultivation pattern that uses the pipe as the cultivation carrier and allows the cultivation nutrient liquid to circulate in the pipe. PVC pipe (about 20 cm in diameter) is mostly used. The pipe with holes drilled at a certain distance is placed on a specific vertical rack, and the nutrient liquid is injected from the holes and flows in the pipe. This cultivation pattern is attractive, but not suitable for large-scale cultivation.

2.3 Trough cultivation patternTrough cultivation pattern is composed of cultivation trough, trough support frame, non-woven fabric, plastic film, substrate, and feed and return line. The plastic cultivation trough is 30 cm wide, 15 cm deep in the inner side and 25 cm deep in the outer side. There is a drainage ditch at the bottom of the cultivation trough. The inner layer is non-woven trough, the middle layer is absorbent cloth, and the outer layer is waterproof film. The support frames of the trough can be divided into A type cultivation frame, double layer culture frame, H type scaffolding, hanging trough cultivation (suitable for aerial corridor), wall-hanging trough cultivation (greenhouse back wall),etc.For example, there are 3-4 rows of cultivation troughs installed on both sides of A type cultivation frame, with the layer spacing of 40 cm and the height of 1.55 m; the cultivation frame is about 1 m wide. The vertical frame is placed north to south with a spacing of 70 cm. There are two planting methods: single-trough row and double-trough row in parallel[6].

In a cultivation space, a variety of cultivation model spaces are dislocated and collocated, which not only are beautiful, but also can make full use of greenhouse space and solar energy. The land utilization rate is improved by 3-5 times and the unit area yield is improved by 2-3 times, so the economic and social benefits are significantly improved combined with picking sightseeing.

3 Strawberry stereoscopic culture substrate

The basis of strawberry stereoscopic cultivation is the selection of substrate. A good substrate should have strong water retention, good air permeability, not easy decomposition and decay, cheap price, convenient materials, good safety, little environmental pollution, which is an important guarantee for high yield and good quality[7]. After several years of exploration, southern Xinjiang generally use mushroom waste, peat soil, rice husk and other organic materials as the main materials, and supplemented with perlite, vermiculite and other inorganic materials. The best formula for local area is the mixture of peat soil, mushroom waste, rice husk and perlite at a ratio of 2∶2∶1∶2. Approximately 8-10 kg/m3fermented soybean fertilizer is added to the culture substrate. The substrate must be strictly disinfected before use.

4 Drip irrigation system for strawberry stereoscopic cultivation

The strawberry drip irrigation developed by the project is a semi-automatic mechanical irrigation method, mainly composed of fertilizer pool, water pump, filter, main pipe, hair canal, water dropper, regulator, fertilizer applicator and so on. One row of strawberry is equipped with a hair canal, or two rows a hair canal. The water dropper spacing has a variety of specifications, and 30-40 cm is the commonly used spacing. The spacing can be selected according to the actual spacing of different varieties. Drip irrigation can use pipes to directly transport water to the root of strawberry, and control the drip as required. The drips are infiltrated into the substrate, which not only meets the growth needs of strawberry, but also does not reduce the temperature due to irrigation.

5 Nutrient solutions for strawberry stereoscopic cultivation

The preparation of nutrient solution is a key step in strawberry soilless cultivation technology. Based on previous studies and combined with soil, water quality conditions and cultivation pattern in Xinjiang, the project developed a formula of strawberry nutrient solution, which is mainly consisted of A, B, C and D parts (g/t).

A: Calcium nitrate tetrahydrate 236 g, potassium nitrate 303 g;

B: Ammonium dihydrogen phosphate 57 g, magnesium sulfate heptahydrate 123 g;

C: EDTA-Fe 20-40 g, boric acid 2.86 g, manganese sulfate tetrahydrate 2.13 g, zinc sulfate heptahydrate 0.22 g, copper sulfate 0.08 g, ammonium molybdate 0.02 g;

D: Humic acid 50 g, acid-base balancing agent 5-10 g.

One third of water is usually added to dissolve A completely; then, B is dissolved and diluted to a certain extent and slowly injected into the inlet when continuously stirring; C and D are added in the same way, which are all circulated evenly. The control of the electrical conductivity (EC) and pH of strawberry nutrient solution is also the focus, and it is ideal to maintain the pH between 5.5 and 6.5[8]. Generally, the EC of the nutrient solution before flowering should be controlled at 1-1.7 Ms/cm, which can be properly increased to 2.5-3.5 Ms/cm at flowering stage. After two years of application of strawberry nutrient solution, this formula can improve the growth, yield, sugar content and quality of strawberry.

6 Key technologies of strawberry stereoscopic cultivation and management

6.1 Variety selection and seedling breedingStrawberry stereoscopic cultivation should select the varieties with shallow dormancy, early flower bud differentiation, early maturity, high quality, high yield and disease resistance, fragrant and sweet fruit, and comfortable taste. According to the environment of southern Xinjiang, sales pattern and strawberry variety characteristics, the excellent varieties with self-pollination ability, low temperature resistance, less black flowers, large and neat fruit, less malformed fruits, fruit flavor are selected, such as Hongyan, Zhangji, sweet Charlie.

Stolon propagation is usually adopted for the breeding of strawberry seedlings, which generally goes through three stages: seedling propagation, seedling cultivation and flower bud differentiation of seedlings. Transplanting after flower bud differentiation will lead to neat and more flowering. It is recommended that flower bud differentiation should be conducted 15-20 d prior to transplanting. The conditions of flower bud differentiation of strawberry are low temperature and short sunlight. The temperature is generally 17-24 ℃, and the day length of sunshine is 8-12 h. Low temperature treatment and short sunlight treatment are the major methods to promote flower bud differentiation of strawberry. The low temperature treatment methods mainly include shading and cooling seedling, seedling cultivation at low temperature, seedling cultivation in alpine areas, night-chilling seedling cultivation and temporary planting. The simple methods for short sunlight treatment mainly include shading or seedling cultivation using natural shading in mountains and valleys[9-11].

The standards for strawberry strong seedling are as follows: virus-free seedlings, 40-50 d old, with 4-5 intact leaves without disease and insect pests; seedling stem 0.8-1.5 cm in diameter; root system developed, fibrous roots numerous and thick, yellow and white; central bud full, differentiation of terminal flower bud complete; weight per plant about 25-30 g. The morphological standard of flower bud differentiation is that the petiole is thick and short; the central bud is full, and has "stiff seedling" (the base of petiole is half open); and most of the flower buds in the upright leaves fail to differentiate.

6.2 Strawberry planting, film mulching and heat preservationThe planting time of strawberry in southern Xinjiang is generally from early August to middle September, and 50% of strawberry seedlings with flower bud differentiation is the suitable planting period. Seedlings are planted in single or double rows depending on the size of trough, with the plant spacing of 20 cm. Seedlings are transplanted with soil blocks or substrate. The hollow back of new stem should be placed towards the trough, so that the fruits will hang on the edge of the trough, which is conducive to sunlight exposure and ventilation, reduces the humidity of the fruit surface, and improves the fruit quality. The seedling should be planted at a depth not reaching heart leaf and not exposing roots. Drip irrigation is installed immediately after transplanting, and the transplanting water is thoroughly irrigated.

The rejuvenation of strawberry seedlings generally lasts 7-10 d, and 2 pieces of old leaves should be removed timely after rejuvenation. Afterwards, mulching film is covered immediately, and the field is irrigated through the water pipe under the film once every 4-5 d to ensure that the cultivation substrate is wet.

When the temperature of strawberry is below 5 ℃, strawberry plants will enter dormancy. The proper insulation period of buckling shed is when the outside night temperature drops to 8-10 ℃. The sheds are buckled in early and middle October in southern Xinjiang region, but the temperature will be raised in early November. Spraying gibberellin at the initial stage of heat preservation is an important measure for stereoscopic cultivation, which can induce flower bud development, promote growth and break dormancy Creating.

6.3 Creating a good environment for growthStrawberry is suitable for cool climate conditions. The most suitable temperature for strawberry growth and development is 18-25 ℃, and the lowest temperature is above 12 ℃ at night. The temperature at the germination stage should be 26-28 ℃ during the daytime and above 8 ℃ at night. At the squaring stage, the temperature should be kept at 25-28 ℃ during the daytime and 8-12 ℃ at night (the temperature should be kept above 13 ℃ at night to prevent flower bud degradation). At the flowering stage, the temperature should be 22-25 ℃ during the daytime and 8-12 ℃ at night. During the fruit enlargement period, the temperature is about 20-25 ℃ during the daytime and 8-12 ℃ at night, and the lowest should not be lower than 5 ℃. In case of high temperature above 30 ℃, timely ventilation and cooling should be carried out. It is very important to keep warm in winter in cold area of southern Xinjiang, and cold cover is overlapped to improve windproof and insulation effect.

The relative humidity of strawberry plants should be controlled at 70%, which should be below 60% in flowering period. At the same time, ventilation must be attached importance to effectively control the humidity in facilities and to reduce the occurrence of diseases. The optimum humidity of substrate is 50%, while too large or too small humidity will affect the root activity of strawberry and normal growth and development of fruits. The humidity in the shed can be controlled by using drip irrigation flexibly.

Long sunlight and strong light can promote pollination, fertilization and fruit development. Light conditions can be improved by hanging reflective curtain and cleaning shed membrane.

6.4 Strawberry plant management and flower and fruit thinningRemoving bottom leaves and picking buds should be timely conducted during growth period and fruiting period of strawberry, and yellow leaves, sick leaves and stolons must be removed. Only one terminal bud is retained before extraction of terminal inflorescence of strawberry. After the terminal inflorescence is extracted, two strong tiller buds with good orientation are retained.

Strawberry plants usually have 2-3 inflorescences, and each inflorescence may bear 3-30 flowers. High-node flowers bloom late, and often become sterile and ineffective. Even if some can form fruits, they become ineffective because the fruits are too small and have no harvest value. Therefore, during the flowering and bud separation period, the buds of high nodes should be thinned at an appropriate amount no later than the opening of the first flower. Each inflorescence with 7-8 flowers can concentrate nutrients to promote the extraction of new inflorescence, reduce nutrient consumption, ensure nutrient supply to fruit, and improve fruit quality.

6.5 Fertilizer and water management of strawberry stereoscopic cultivationFrom the 2ndd to the 15thd of field planting, water is dripped three times every 2 d, that is, the field is watered at 10:00 and 15:00 on one day, and watered once at noon on another day. The substrate must never dry out and turn white. When new leaves are expanding, the plants are watered once a day. After transplanting and survival, strawberry cultivation nutrient solution could be applied once every 15 d through drip irrigation system of water and fertilizer integration. In the process of fertilization, the conductivity (EC) and pH of strawberry nutrient solution should be well controlled, and the pH should be kept between 5.5 and 6.5, which is ideal to prevent salt damage that will cause poor growth. Suitable foliar fertilizer (such as potassium dihydrogen phosphate and calcium fertilizer) can be applied once every 2 weeks at fruit-bearing stage. Water should be properly controlled at flowering stage, and more water is needed during fruit expansion stage. In the middle and late stage, spraying organic nutrient solution such as multiple micro-fertilizer and amino acid fertilizer combined with pesticides can promote the development of fruits in the middle and late stage and improve single fruit weight and quality.

To enhance the flavor of strawberries, milk powder is diluted to 1 000 times dilution and then dripped once or twice a month. The purpose of watering diluted milk is to improve the organic composition and to strengthen the beneficial bacteria community of the substrate, thereby providing a good substrate environment for strawberry seedlings.

6.6 Management of bee pollination, flower promotion and fruit protectionIn order to improve the fruit setting rate of strawberry pollination, reduce malformed fruit and increase the yield of commercial fruit, it is necessary to adopt the combination of natural pollination, artificial assisted pollination and bee release. When the greenhouse temperature exceeds 20 ℃ during the day, the window can be properly opened to promote pollination. Artificial assisted pollination is performed after 10:00. When the greenhouse humidity is low and the plant leaf surface is relatively dry, household feather duster is gently brushed the strawberry flowers once a day. Bumblebees or bees are released at flowering stage about 1 week prior to flowering, that is, 60%-70% of strawberries are in bud. About 75 000 bees/ha will receive good pollination effect, which can further improve the quality of strawberry.

Spraying gibberellin is an important measure for stereoscopic cultivation of strawberry, which can induce flower bud development, promote growth and break dormancy. 150 mg/kg Gibberellin is sprayed at the dose of 70 L/ha when inflorescence emerges. After a week, 75 mg/kg gibberellin is sprayed at the dose of 70 L/ha. The liquids are sprayed on heart leaves, and the temperature of the shed is slightly increased after spraying, which can make the top inflorescence blossom in advance. However, too early spraying will turn axillary buds into stolons, while too late spraying can not promote flowering but can only promote petiole growth.

7 Green prevention and control of diseases and insect pests in strawberry stereoscopic cultivation

The common strawberry diseases mainly are powdery mildew, anthrax, root rot, yellows, black spot and viral disease. The main insect pests are aphids, red spider, thrips, whitefly, slugs,Spodopteralitura,etc.Prevention and comprehensive control is the major principle against diseases and insect pests, while physical prevention and control, biological pesticide or plant preparation are major green prevention and control technologies against diseases and insect pests[12].

The agriculture measures include selecting disease-resistant varieties, using virus-free seedlings, selecting higher land plot for planting strawberry, covering plastic film in the whole garden, controlling humidity; reasonable dense planting, controlling the amount of fertilizer; clearing pathogens, timely removing sick, old, residual leaves and sick inflorescence, eliminating and destroying sick fruits. At the end of one season, the garden should be thoroughly cleared to reduce the source of primary infection and insect sources.

The physical control measures include installation of insect proof net and sticky trap or light trap, disinfection with limewater or pasteurized disinfectant, and utilization of sexual attractants or sweet and sour liquids. The 60-mesh insect proof nets are installed at all air vents. Yellow sticky traps are hung at 50 cm above the plant at an interval of 4 m2to kill aphids and whitefly, and blue sticky traps are hung to kill thrips. A total of 1 245 yellow and blue sticky traps, with the specification of 20 cm × 30 cm, are hung alternatively every ha, and 6 light traps can also be hung 1.5-1.8 m above the ground. Disinfection pool is laid at the entrance and exit of the shed, which is added with limewater or pasteurized disinfectant to a depth of about 1 cm. Sex attractants or sweet and sour liquid can be used to reduce the damage of aphids, whitefly and other pests.

When the symptoms of gray mold, powdery mildew and anthracnose occur in the field, 15 kg ofTrichodermasp. is mixed with 75 kg of amino acid fertilizer and artificially sprayed per ha once every 10-15 d, or petroleum ether camphor leaf extract, garlic liquid, washing diluent, pepper liquid,etc.can be sprayed. Besides, predator mites can be released to control red spider, thrips, whitefly,etc.

Safe, low-toxic, efficient pesticides can be selected appropriately and sprayed in time based on the situation, and agents should be used alternatively. All weeds around the shed should be removed, and insecticides or fungicides are sprayed once every week within 5 m of the shed.

8 Fruit harvest

When the surface coloring of fruits reaches more than 80% and fruits change from hard to soft, fruits are harvested in batches. Fruits are picked from 7:00 to 10:00 and from 16:00 to 18:00 on a sunny day. The worker should wear rubber gloves, gently hold the strawberry in hand and fold it in half opposite to the stem to break the stem, or cut it at the joint between the stem and the fruit. Fruits are gently placed in the container according to their size. The harvest strawberries should have complete sepals and clean fruit surface, and should be free from the damage caused by diseases and insect pests. Within 2 h after harvest, the fruits are pre-cooled immediately in a refrigeration house for 1 h, and then graded and packaged.