Latest Technical Methods and Tutorial for Citrus Cultivation

Citrus fruits are rich in nutrients and excellent in color, aroma and taste. They can be eaten fresh or processed into various processed products, mainly juice. Citrus ranks first among all fruits in production, and citrus juice accounts for 3/4 of fruit juice, making it widely favored by consumers. Citrus is long-lived, high-yielding and stable in production, with high economic benefits, and is the most important tree species among fruit trees in southern China, playing a major role in helping fruit farmers escape poverty and become prosperous and in rural economic development.

Seedling Raising Techniques

1. Rootstock seedling cultivation

The seedling nursery should be selected in a location with convenient transportation, flat terrain, water sources, and gentle slopes below 5° or paddy fields with convenient drainage and irrigation. The soil is required to be deep, well-structured, sandy loam or loam, with a pH value in the range of 5.5 to 6.5. Water source and air pollution should be avoided, and it should be far from old citrus orchards. The rootstock should be a variety or type that adapts to local climate and soil conditions, has strong affinity with the main cultivated variety, grows normally, has a well-developed root system, strong stress resistance, and can produce early fruiting, high and stable yields, and high quality after grafting, with no quarantine pests or diseases. In Guangdong, Guangxi and Hainan, for sweet oranges, banana oranges and ponkan, sour mandarin, Zhu mandarin (Jiangxi red mandarin) and red lemon are mostly used as rootstocks; in recent years, ponkan and navel orange using trifoliate orange as rootstock have shown early fruiting and high yield. In Fujian, red mandarin is used as rootstock for loose-skin mandarins, and mandarin is also used as rootstock for ponkan, banana orange and sweet orange. In Taiwan, China, banana orange and ponkan mostly use sour mandarin and red lemon as rootstocks. In Guangdong, Guangxi and Fujian, sour pomelo is mostly used as rootstock for pomelo. Rootstock seeds are collected from mature fruits by using a specially made fruit-splitting board or a small knife to cut vertically into 6-8 segments and squeeze out the seeds. Then put them into a bamboo basket or coarse cloth bag, add a small amount of plant ash and rub gently, then rinse with water to remove fruit residue and pectin on the seed coat. Spread the cleaned seeds in a cool and ventilated place, and sow when the seed coat turns white. Fresh sowing is best; otherwise, dry storage or sand storage methods should be used for preservation. Seed content of rootstock fruits and sowing rate. The sowing land should be finely cultivated, fully plowed and harrowed to break up the soil, and sufficient base fertilizer should be applied evenly. Generally, 3750-4500 kg of pig manure, cow manure or high-quality compost, 225-375 kg of superphosphate, and 375-450 kg of lime are applied per hectare. There are two sowing methods: broadcast sowing and drill sowing. After sowing, the seedbed soil should be kept moist. After the seedlings grow 3-4 true leaves, water and fertilizer should be applied frequently and lightly, and attention should be paid to the prevention and control of anthracnose, damping-off and leaf miner and other pests and diseases. Rootstock seedling transplanting is divided into two types: newly planted seedlings and old planted seedlings. Newly planted seedlings are sown in the first winter and the following spring, transplanted in mid-May to July of the next year at a spacing of 20cm×12cm, grafted in winter and spring, and released from the nursery in autumn, with the whole seedling raising period lasting two years. This method requires strengthened fertilizer and water management so that the rootstock grows rapidly and can be released from the nursery in time. Old planted seedlings are sown in winter and transplanted in the following autumn and winter or the third spring at a spacing of 25cm×12-15cm; those sown in spring are transplanted in autumn and winter or after the spring shoots mature in April-May of the following year. After growing for 9-15 months, winter-spring grafting is carried out, with the whole seedling raising period lasting two years.

2. Cultivation of grafted seedlings

Grafting can be done in all four seasons, but it is best before the Beginning of Spring. If the winter is warm and the branches are full early, grafting can be done earlier; otherwise, it should be delayed. For rapid seedling raising, summer grafting and autumn grafting can also be used, and should be carried out on sunny and warm days. Rainy days, days when the soil is too wet, and days with strong north winds are not suitable for grafting. Summer and autumn grafting should avoid windy and rainy days and times when the midday sun is strong. In Guangdong, Guangxi, Fujian, Hainan and other places, single-bud cut grafting or single-bud side grafting is mostly used, with a survival rate of over 90%. Grafted seedlings should be shaped in the nursery, mainly to determine the trunk height and cultivate a certain number of scaffold branches. And by pruning the top of summer shoots, autumn shoots are promoted. In hot regions, top pruning is generally carried out around the Beginning of Autumn. In places where the Cold Dew wind comes early and drought is likely, or where seedlings are to be released from the nursery in late autumn and early winter, it can be appropriately advanced. The top can be pruned at 25-30cm above the ground at the middle-upper part of the first summer shoot, choosing a position with uniform internode spacing and complete leaves. If the spring shoot exceeds this height, it should be pruned on the spring shoot. After top pruning, remove in time the scattered autumn shoots that sprout early. Only after most seedlings have 3-6 buds sprouting should the shoots be released, so as to maintain balanced branch growth. When the buds grow to about 3cm, select and retain 3-5 robust main branches evenly distributed in all directions, and remove the rest in time. The standards for excellent grafted seedlings are: excellent and pure variety, free of huanglongbing and other quarantine diseases, strong branches and trunk, well-healed graft union, newly planted seedlings over 10cm tall (old planted seedlings over 50cm tall), stem diameter over 0.8cm, and 3-5 evenly distributed main branches, leaves dark green and glossy, main root over 15cm long, with the most fibrous roots of uniform thickness and bright yellow color, and balanced growth of the aboveground and underground parts of the whole plant.

3. Virus-free seedling cultivation. Hot regions have high temperature and high humidity, and citrus diseases, especially dangerous diseases, pose a great threat. Raising virus-free seedlings is an important way to solve this problem. The main methods are:

① Heat treatment. Rootstock seeds are treated with 56℃ hot water for 50min, scions are treated with 50℃ steam for 45min, and seedlings before spring sprouting are treated with 49℃ steam for 50min.

② Antibiotic treatment. Scions are soaked in 700u/ml streptomycin solution for 1h; soaked in 1000u/ml tetracycline solution for 2h; if the 1000u/ml tetracycline solution is heated to 46℃, the soaking time can be shortened to 20min.

③ Shoot tip culture. Using tissue culture technology, after removing the leaf primordia from the shoot tip of the scion variety, take its shoot tip of about 0.1-0.2mm (conical meristem without vascular bundle tissue) and place it in an appropriate medium to cultivate into a seedling.

④ Micrografting. Use heat treatment and test-tube sowing methods to cultivate virus-free rootstock seedlings, then place the potted seedlings of the scion variety in an artificial climate chamber (30-40℃) and maintain for 2-4 weeks to promote new shoot growth. When the new shoots grow to several centimeters, take their <0.2mm growing points and graft them onto test-tube rootstock seedlings to raise virus-free seedlings.