What to Do When Citrus Growth Is Affected by Prolonged Autumn Rain? Detailed Countermeasures

Effects of Prolonged Autumn Rainy Weather on Citrus Growth and Countermeasures

Dangyang City, Hubei Province is a premium growing area for mandarin citrus in China, with an existing citrus planting area of 21,000 ha. The main citrus types are Satsuma mandarin and ponkan, of which ponkan (Egan No. 1, Qianyang seedless) covers an area of 14,000 ha. In the autumn of 2017, a series of abnormal weather conditions occurred within the Yichang area. This article investigates and analyzes the effects of the abnormal autumn 2017 climate on citrus growth and fruit set in Dangyang, and proposes some countermeasures and methods.

1 Actual Conditions of the Abnormal Autumn 2017 Weather

From September 18 to October 19, 2017, Dangyang experienced a pronounced prolonged rainy weather process lasting 30 d, with a daily average temperature of 18.6°C, 1.4°C lower than the same period in previous years; the number of consecutive precipitation days was 28 d, 10 d more than the longest consecutive precipitation days, and the precipitation reached 238.5 mm, ... 3.1 times that of the same period in previous years (77.1 mm); the average relative humidity was 92%, notably high, and the sunshine duration was 7.4 h, only 5% of the same period (148.1 h). The occurrence of abnormal weather greatly affected citrus fruit quality and the normal growth and development of the plants before winter. For this reason, correctly analyzing the many effects of abnormal weather on citrus production and promptly implementing corresponding measures and methods is particularly important and urgent.

2 Observation and Trial Examples

In October 2017, comparative observations were carried out in Wang Yisheng's orchard in Group 2, Yuzhou Village, Banyue Town, the main citrus production area of Dangyang City, with an existing citrus orchard of 4 ha, mainly mandarin citrus. The 4 ha citrus orchard was treated as required with ditching, orchard cleaning, pesticide spraying, fertilization, shoot removal, whitewashing, and other treatments. 667 sq m ... received no treatment at all. Comparative trial period: September 20, 2017 to ... February 2018.

3 Observation and Trial Results

3.1 Trial Results

The observation results for fruit trees with protective measures and those without protective measures during the trial period are shown in the attached table.

Attached Table Observation Results

3.2 Main Harm of Prolonged Autumn Rainy Weather to Citrus Growth

(1) When long-term rainy weather occurs, poor drainage in the orchard leads to insufficient oxygen supply to root growth points, weakened respiration, and difficulty in normally transporting nutrients, with severe cases developing necrosis of root growth points, weakening tree vigor. At the same time, prolonged rain causes massive loss of trace elements and obstacles to their absorption, especially waterlogging of the root system, which reduces the absorption of copper and zinc elements, aggravating nutrient deficiencies, and in severe cases leading to fruit drop and fruit cracking.

(2) The tree body has high moisture, shoots grow vigorously, accumulated temperature decreases, and the fruit ripening period is delayed. The concentration of nutrients accumulated in leaves and buds decreases accordingly, affecting the quantity and quality of flower bud differentiation and causing a reduction in yield the following year.

(3) Continuous rainy weather greatly reduces citrus photosynthesis, lowers carbohydrate synthesis in the fruit, increases moisture in the flesh, affects sweetness and flavor, and makes the peel more fragile and tender, making transport and storage difficult and making the fruit prone to rotting.

(4) Prolonged rainy weather leads to high humidity in the orchard, poor soil aeration, reduced nutrient accumulation in the tree body, and decreased ability to resist pests and diseases, making mature citrus fruit prone to widespread black spot disease, brown rot, anthracnose, blue and green mold, rust mites, and other problems.

(5) It reduces the cold resistance of citrus.

4 Coping Technical Measures

4.1 Promptly Implement the Technical Measures of "One Prevention, Two Cleanings, Three Increases." One prevention means seizing the time after the rain when the sky clears to carry out one thorough disease prevention and control operation throughout the orchard. In view of the current trend of disease occurrence and development in Dangyang orchards, mainly use biological and physical control measures such as solar insect-killing lamps, sticky insect color boards, predatory mites, trap jars, and trunk adhesive tape, combined with necessary emergency control using low-toxicity and high-efficiency pesticides, such as spraying fungicides like prochloraz + fosetyl-aluminum or thiophanate-methyl + fosetyl-aluminum once a week for 2 consecutive times, which can achieve effective prevention and control.

Two cleanings means promptly doing a good job after the rain when the sky clears of clearing ditches and draining waterlogging to reduce orchard humidity, and at the same time removing fallen fruit, diseased and insect-damaged fruit, cracked fruit, diseased and insect-damaged branches and leaves, weeds, and so on in the orchard, creating a clean orchard environment, keeping the soil moderately dry and the air circulating, and ensuring normal growth of citrus roots and absorption of soil minerals.

Three increases means promptly increasing the application of trace elements after the rain when the sky clears to improve the tree body's stress resistance. Research has confirmed that prolonged rain causes massive loss of trace elements and obstacles to their absorption, especially waterlogging of the root system, which reduces the absorption of copper and zinc elements, aggravating nutrient deficiencies and leading to severe fruit drop, fruit cracking, and worsening damage from anthracnose and brown spot disease. Therefore, as an emergency control measure, large-scale orchard operations to supplement zinc and potassium should be carried out, spraying 0.2% zinc sulfate 1 to 2 times; for orchards with severe fruit cracking, dead branches, and gumming, 0.08% copper sulfate should additionally be sprayed for control.

4.2 Promptly Implement the Technical Measures of "One Increase, Two Thinnings"

One increase means applying 1 additional overwintering fertilizer to orchards of late-maturing varieties, with the fertilizer mainly based on organic fertilizer, combined with phosphate and potassium fertilizers. Organic fertilizer mainly comes from farmyard manure, biogas slurry, residue and grass, and forest surface humus, supplemented by high-quality commercial fertilizer. Generally, for adult trees bearing fruit normally, apply 1.5 to 2.0 kg of commercial organic fertilizer and 0.5 kg of high-quality low-nitrogen compound fertilizer per tree. The depth of the fertilization trenches and holes is 25 to 30 cm, and they are applied evenly in layers in the trenches and holes, with the trenches and holes shaped in an arc. Two thinnings means promptly thinning out diseased and insect-damaged fruit, deformed fruit, and especially small fruit on the tree to ensure a reasonable fruit load on the tree body; thinning out surplus and overly dense shoots, especially late-autumn shoots, to create the best possible conditions for ventilation and light penetration in the orchard.

4.3 Promptly Implement Shallow Tillage and Fertilizer Incorporation to Reduce Humidity and Increase Oxygen

Method: Combined with turning in green manure and weeds, carry out shallow tillage of 15 to 20 cm, which helps increase soil organic fertility, accelerate the evaporation of excess soil moisture, and protect the tree body and root system as they smoothly enter the physiological preparation period before overwintering.

4.4 Effectively Carry Out Winter Orchard Cleaning

First, spray pesticides throughout the orchard, with the agents mainly mineral-based pesticides such as lime sulfur mixture, pine-tar alkali mixture, and petroleum oil emulsion, to minimize the pest and disease base; second, deeply plow the whole orchard to improve the orchard soil environment; third, whitewash the whole orchard; fourth, prune the whole orchard at the appropriate time and amount, mainly cutting out diseased and insect-damaged branches, overly dense branches, senescent branches, drooping branches, and so on, to improve orchard ventilation and light penetration conditions to the greatest extent; fifth, according to weather conditions, if strong cold air invades, physical measures such as covering the tree canopy with film, covering the ground with film, and timely irrigation can be adopted to ensure the safe overwintering of the tree body and fruit.