Detailed Explanation of Key Techniques for Summer Management of Citrus Orchards
From the perspective of the phenological stages of citrus growth and development, summer occurs during the young fruit stage (second physiological fruit drop period) through the summer shoot emergence period. Citrus fruits enter the expansion stage, requiring larger amounts of fertilizer and water. This coincides with the hot and humid season, which is also the peak period for citrus pests and diseases. Therefore, strengthening summer management of citrus orchards is very important. Whether management measures are adequately implemented has a great impact on the yield and fruit quality of the year. Specific management should focus on the following aspects:
1. Apply sufficient fertilizer to promote tree growth and protect fruit. During the citrus fruit expansion stage, the root system has strong fertilizer absorption capacity. Applying sufficient fertilizer in time allows young fruits to receive adequate nutrition, which can reduce fruit drop, increase fruit set rate, and ensure a bumper harvest. According to fruit growers' practice, deep application of slow-release fertilizer: each bearing tree should be applied according to tree age and fruit load, with livestock and poultry manure 20-25 kg, cake fertilizer 1-15 kg, superphosphate or bone meal 0.5-1 kg. For bearing trees, 1-2 foliar feedings should be carried out, spraying 0.2% urea and 0.2% monopotassium phosphate on the leaf surface. If deficiency symptoms appear in new leaves, spray accordingly - supplement whatever is deficient. During high temperature and drought, spraying hormones can also promote root cell division, improve absorption capacity, and has a certain effect on preventing fruit drop. Note: Fertilization should be based on tree condition. For varieties prone to June shoot emergence, this top-dressing should be avoided to prevent fruit drop; for first-year bearing trees or trees with few flowers and fruits and vigorous trees, fertilizer can be omitted or reduced to avoid triggering summer shoot emergence, which would cause new shoots to compete for nutrients and lead to fruit drop. In mid-July (about 15 days before releasing autumn shoots), apply sufficient fruit-strengthening and shoot-promoting fertilizer. The application rate should account for 40-50% of the annual fertilizer amount, with organic fertilizer as the main type [organic fertilizer accounting for 80%]. Apply 25-50 kg of decomposed manure water per tree, plus 0.5-1 kg of composted cake fertilizer, 150-250 g of urea, 100-150 g of potassium fertilizer, or 250-350 g of compound fertilizer.
2. Strengthen water management, drought resistance and cooling. Water plays a decisive role in citrus fruit size and yield. Citrus fruit expansion has three peaks from June to October: the first in June; the second from early July to early August; the third from late September to early October. Overall, although the first peak has the largest fruit growth, it coincides with the rainy season, making water deficit unlikely. The second peak's fruit expansion rate is second only to the first, but it coincides with the summer-autumn drought season, making it most vulnerable to water stress, resulting in large numbers of irreversible small fruits. Therefore, this stage is the critical period determining yield and large-fruit rate. Every effort must be made to satisfy the fruit's water requirements. If irrigation is delayed, small fruits will be difficult to expand further. Therefore, in water management from May to July, the early period requires attention to drainage to prevent waterlogging in the orchard causing "root suffocation"; the mid-to-late period requires attention to irrigation to prevent small fruits, which affects quality and yield. Soil moisture content should be maintained at around 20%.
Mountain orchards should repair and reinforce drainage and irrigation systems to prevent soil erosion caused by heavy rain. At the same time, drought resistance and cooling should be done well to prevent high-temperature fruit drop. When citrus growth temperatures exceed 35°C, photosynthesis weakens and physiological functions are impaired. Therefore, when high temperatures arrive and citrus leaves begin to curl, water should be promptly introduced into the orchard to regulate temperature and increase humidity through water. Where conditions permit, sprinkler irrigation can be used to increase air humidity and soil moisture. Cultivate and loosen the soil to cut off soil capillaries, improve water and fertilizer retention capacity, thereby improving the orchard microclimate, reducing fruit drop, and promoting fruit enlargement. During the hot and dry summer heat, grass should be cut and used to cover the orchard floor to reduce direct sunlight and water evaporation, lower surface soil temperature, protect root groups, enhance root vitality, and reduce fruit drop rate.
3. Remove summer shoots to promote autumn shoots. For bearing trees, summer shoots should be removed, continuing until just before autumn shoots are released, to reduce consumption of large amounts of nutrients and ensure robust autumn shoots. For old trees and trees with heavy fruit load, autumn shoots should be released earlier; for young trees, low-yield trees, and non-bearing trees, slightly later. Generally, shoots should be released from late July to mid-August. For sweet oranges, releasing shoots in August is appropriate.
4. Do a good job in pest and disease control to promote high yield and quality. The main diseases likely to occur at this stage include scab, canker, and anthracnose. The main pests include scale insects, longhorn beetles, red spider mites, rust mites, and leaf miners. Locally, aphids, psyllids, and whiteflies may cause damage. During July and August, citrus fruits expand and are vulnerable to fruit-piercing moths, often causing massive fruit drop. Severe rust mite infestation can cause defoliation and fruits turning purplish-brown. Red spider mites can cause citrus tree growth to shrink, defoliation and dead branches, turning leaves white-gray. Leaf miners damage new shoots; larvae burrow under the epidermis of tender leaves to feed, causing severe leaf curling and affecting growth.
Canker disease: Combine shoot control with leaf miner prevention, achieving "remove scattered shoots and keep uniform ones, remove early shoots and keep synchronous ones, suppress strong and support weak, and concentrate spraying during shoot release period to prevent leaf miners." For leaf miners, use 5% abamectin EC at 10000 times dilution, or pyrethroid pesticides at 3000-4000 times plus 312, and spray again after 5-7 days. When shoots grow to about 12 leaves, pinch the tips and spray 3% Kechunkang wettable powder at 1000 times dilution or 600-1000 units/ml agricultural streptomycin plus 1% alcohol as an adjuvant. If mixed with 80% Dacomit M-45 wettable powder at 600 times dilution, it can both improve efficacy and simultaneously control fungal diseases. For young trees, 77% Duoning at 400-600 times dilution can also be used to prevent canker disease.
Longhorn beetles: Based on the habits of adults mostly resting on branch tips at noon on sunny days and laying eggs near the trunk base around dusk, manually capture and kill adult longhorn beetles. For chemical control, in mid-May, use 5% Regent suspension concentrate at 1000-1500 times dilution to spray the trunk, with a 15-day interval. Additionally, frequently patrol the citrus orchard, promptly scrape off egg masses and larvae, hook-kill larvae, or plug insect holes with medication.
Red spider mites: Regularly check pest conditions. When more than 20% of leaves are infested, or when there are more than 2 red spider mites per leaf on average, spray promptly. Pesticides can include 50% Torque wettable powder (fenbutatin oxide) at 2000-2500 times dilution, 5% Nissorun EC at 3000 times dilution plus sulfur colloidal suspension at 400 times dilution, or 20% fenpyroximate wettable powder at 4000 times dilution, or 15% fenpyroximate EC at 3000-4000 times dilution, or 25% azocyclotin wettable powder at 1500 times dilution.
Rust mites: Pay attention to checking pest conditions. When 20% of examined leaves and fruits are found to have rust mites, spray immediately for control. Pesticides can include 1.8% abamectin EC at 2000-3000 times dilution, or 80% Dacomit M-45 wettable powder at 600 times dilution, or 70% Antracol wettable powder at 600 times dilution, or 65% zineb wettable powder at 1000 times dilution, or 2.5% Decis EC at 3000-5000 times dilution, or 73% propargite EC at 2000 times dilution, or lime sulfur mixture at 0.3 degrees Baumé. Rotate pesticides to avoid developing resistance. Use Maidaizhi at 1500 times dilution or Dacomit M-45.
Leaf miners: Besides removing shoots to control new growth and cutting off pest sources, when shoots are 2-3 cm long during shoot release, spraying should begin, then continue spraying 2-3 times at 5-7 day intervals. Pesticides can include pyrethroid insecticides [such as fenvalerate, fenpropathrin, Baichongning, Dipai] plus "312" or abamectin-type [such as Kunde, Keweding, abamectin] pesticides for control.
5. Interplant green manure crops to use and nourish the land. From the fertilization practices in our city's citrus production areas, there is a widespread situation of severe organic fertilizer shortage, excessive nitrogen and phosphorus, and potassium deficiency. Excessive nitrogen not only affects quality improvement but also reduces the availability of copper, zinc, manganese, and molybdenum, and reduces the occurrence of absorbing roots. Planting green manure crops on a large scale should be promoted to use and nourish the land and build soil fertility. Interplanting one mu of green manure in a citrus orchard, if it can produce 4000 jin of fresh grass, can increase pure nitrogen by 15-25 jin per mu, equivalent to 32-54 jin of urea or 88-147 jin of ammonium bicarbonate, and can also reduce soil erosion, stabilize orchard soil temperature and humidity, and improve the orchard ecological environment. Therefore, interplanting leguminous green manure crops in orchards should be promoted, such as pinto peanut, round-leaf cassia, sicklepod cassia, ageratum, black mung bean, Indian cowpea, and other green manures, and timely cutting green material for mulching or incorporation to improve the soil.
6. Do a good job in summer pruning and fruit thinning to improve fruit quality. Summer pruning focuses on improving canopy distribution, increasing light penetration and air circulation, and regulating the contradiction between fruiting and shoot growth. Adopt pruning measures combining bud removal and shoot control, pulling, propping, hanging, and thinning. Fruit thinning can regulate tree load, effectively overcome alternate bearing, and increase individual fruit weight. Fruit thinning should be conducted when the second physiological fruit drop ends, with a supplementary thinning in early August. Method: first thin diseased and insect-damaged fruits, deformed fruits, and fruits on water sprouts and weak slender branches. Standard: Through fruit thinning, depending on variety, the corresponding leaf-to-fruit ratio should be achieved, such as 25:1 for common satsuma, 45:1 for early satsuma and sweet orange, and 60-70:1 for Ponkan. Too high a leaf-to-fruit ratio can also cause thick-skinned large fruits and reduce quality.