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Lemon Area Chart of Long An Province from 2014 ÷ 2020. (Source: Compiled from Sources of Long An Provincial People's Committee, 2014, 2016, 2020)

CHAPTER 1: OVERVIEW OF DOCUMENTS

1.1. Overview of lemon trees


1.1.1. Origin


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Lemon ( Citrus aurantifolia Swingle), lemon originated in Indonesia and Malaysia. The Arabs brought lemon trees to North Africa and Southwest Asia and then to the Mediterranean. By the mid-13th century, lemon trees were grown in Italy and France.

Seedless lemon ( Citrus latifolia Tanaka) is a hybrid of paper lemon ( C. aurantifolia Swingle) and lemon ( C. medica ). According to some reports, seedless lemon has a triploid genome despite having only 18 sets of chromosomes. Seedless lemon was grown in the Mediterranean region under the name "Sakhesli". It was then brought to Brazil by Portuguese traders and from there to Australia in 1824. Seedless lemon trees came to California from Tahiti between 1850 ÷ 1880 and were brought to Florida in 1883 (Morton, 1987). In southern Vietnam, seedless lemons are widely grown and popular in the Mekong Delta provinces, bringing high economic efficiency, not only for domestic consumption but also for export (Ho Cao Viet, 2016).

1.1.2. Classification position


Lemon trees belong to the Rutaceae family, Sapindales order, Citrus genus. There are two types of lemon trees grown today: paper lemon (Vietnamese lemon) and seedless lemon (hybrid lemon). The scientific name of paper lemon is C. aurantifolia Swingle , and the English name is Lime. The scientific name of seedless lemon is C. latifolia Tanaka. The classification of paper lemon is as follows:

Kingdom: Plantae Order: Sapindales

Family: Rutaceae Genus: Citrus

Species: Citrus aurantifolia.

The classification position of seedless lemons is as follows: Kingdom: Plantae

Order: Sapindales Family: Rutaceae

Genus: Citrus


Species: Citrus latifolia


1.1.3. Plant characteristics


Paper lime has been grown in our country for a long time, it is a shrub, about 5 m high. The tree trunk rarely grows straight but spreads many branches from near the base, the trunk has many sharp thorns. The leaves are oval, about 2.5 ÷ 9.0 cm long, looking like orange leaves. The lime flowers are about 2.5 cm in diameter, white to yellow, with light purple veins. The tree flowers and bears fruit all year round but bears the most fruit from May to September. The fruit ripens 5 to 6 months after the flowers bloom. The fruit has a thin skin, is juicy, aromatic, sour and has many seeds (Figure 1.1 A).


A

B


Figure 1.1. Paper lime ( C. aurantifolia Swingle) and seedless lime ( C.

latifolia Tanaka)


(A: paper lemon tree; B: seedless lemon tree)


(Source: Center for Research and Development of Seedlings, Agricultural Academy

Vietnam)

Seedless lemon is a hybrid between paper lemon and lemon, widely grown in the world. In Vietnam, seedless lemon is imported from California (USA). The tree grows up to 6 m high, the trunk has no thorns, the canopy is round, the fruit is in clusters, the fruit is large, 6 ÷ 7 fruits/kg, seedless, thin skin, bright green, juicy, less sour, aromatic and not bitter like paper lemon, so it is favored by many people and has high export value. The tree bears fruit all year round, can yield 150 ÷ ​​200 kg/year/tree (Figure 1.1 B).

1.1.4. Lemon production situation in Long An


In the South, lemon trees are mainly grown in the Mekong Delta (accounting for nearly 60% of the total lemon area in the country). There are 2 lemon crops per year, in the off-season (dry season) the price of lemons increases from 35 ÷ 45% compared to the main crop (rainy season). After 18 ÷ 20 months of planting, lemons begin to be harvested, with the first crop yield reaching about 15 ÷ 20 tons/ha. In the second and third years, the yield increases to 25 ÷ 35 tons/ha (some gardens reach 40 tons/ha), with an average profit of 150 ÷ ​​300 million VND/ha. On average, each lemon tree bears 1,000 fruits/year, about 70 ÷ 100 kg/tree/year. Seedless lemons have large fruits, 6 ÷ 7 fruits/kg. The growth cycle of seedless lemons can be over 10 years.

In recent years, lemon trees have been one of the key crops in the crop restructuring program of Long An province. The province has the largest lemon acreage in the Mekong Delta region (30% of the total lemon acreage in the region). The province's lemon acreage has continuously increased in recent years (Figure 1.2). In 2018, the total lemon acreage in the province was 9,438 hectares, reaching 94.4% of the plan (10,000 hectares), equal to 105.8% over the same period in 2017. Of which, 7,374 hectares of lemons bore fruit, concentrated in the districts of Ben Luc, Thu Thua, Thanh Hoa, Duc Hue and Duc Hoa.

According to the Department of Agriculture and Rural Development of Long An, the province has set a target of increasing the area of ​​lemon trees to 11,000 hectares by 2020. Developing specialized agricultural production areas in the direction of collective economic development, production according to VietGAP and GlobalGAP to help farmers have stable output, bringing lemon products to domestic and foreign markets. In particular, Ben Luc is one of the five largest lemon growing districts in the province, mainly seedless lemons, currently with 5,470 hectares of lemon growing land, an increase of 752 hectares compared to the end of 2017, with a total output of 77,344 tons. Since the end of 2011, Ben Luc district has built an exclusive brand of Ben Luc lemons.


Figure 1.2. Lemon area chart of Long An province from 2014 ÷ 2020. (Source: Compiled from sources of Long An Provincial People's Committee, 2014, 2016, 2020)

To achieve the set plan, the agriculture and rural development sector, Long An has strengthened trade promotion activities, built brands, managed product quality and improved collective economic efficiency; formed good agricultural production processes, value chain models, supplied safe lemon products, contributing to building a comprehensive and sustainable agricultural development. Vietnamese fresh lemon products have been present in the markets of many countries in Asia, the Middle East and especially the European Community (EU), raising the country's lemon export turnover to over 3 billion USD. Of which, Long An contributed about 0.5 million USD (Ho Cao Viet, 2016).

However, in the face of great opportunities, the lemon industry in Long An province as well as the whole country is and will face many challenges in traceability, product quality management according to import market standards, quarantine barriers for some types of diseases on lemons, etc. Therefore, ensuring food safety and hygiene, controlling diseases well and meeting export principles are conditions for Vietnamese agricultural products to reach out to the world.

1.2. Overview of bacterial canker on lemon trees


1.2.1. History and distribution


According to Fawcett and Jenkins (1933), the oldest lesions found on citrus trees ( C. medica collected from India in 1827 ÷ 1831 and C. aurantifolia from Indonesia in 1842 ÷ 1844) were kept in the Herbaria of the Royal Botanic Gardens at Kew, England. Therefore, canker may have first appeared and originated in tropical areas of Asia, such as South China, Indonesia and India, the origin of citrus.

In 1912, in Florida, USA, canker was first noticed, causing serious damage and spreading throughout Florida on seedlings imported from Japan. In 1933, the disease was eliminated through strict inspection and thorough removal of diseased plants. In 1986, the disease reappeared and broke out into an epidemic in Manatee, Florida, south of the Gulf (Stall and Civerolo, 1991). The pathogen then spread widely throughout Asia and then to South Africa (Doidge, 1916), Oceania, South America (Rossetti, 1977) and Australia, (Garnsey et al., 1979). In Iran, bacterial canker was first reported on Mexican lemons, Kahnouj area, Kerman province ( Alizadeh and Rahimian, 1990 ). In Kouli-koro, Mali (West Africa), the incidence of canker on lemons was 50% (Traore et al., 2008). Bacterial canker has been reported in most citrus growing countries in the world (Sharma and Sharma, 2009) (Figure 1.3).



Figure 1.3. Distribution map of bacterial canker disease on citrus trees


X. axonopodis pv. citri


(Source: Cabi.org)

In Vietnam, canker disease on citrus trees was first reported by Whittle (1992). The causative bacteria is X. campestris pv. citri. According to the report of Bui et al. (2009), the bacteria causing canker disease on Mexican lemon trees collected from 14 provinces in both the North and the South is X. citri pv. citri canker type A. Currently, canker disease is one of the common and serious diseases in most citrus growing areas in the country, greatly affecting export products. The disease arises and develops strongly in conditions of high humidity and temperature of 26 ÷ 35°C, spreading very quickly and causing damage to all lemon varieties (Nguyen Van Nga and Cao Van Chi, 2013).

1.2.2. Disease symptoms and economic damage


According to Sharma and Sharma (2009), citrus canker is caused by many different strains of Xanthomonas with different biological characteristics and host plants, but the symptoms and signs of the disease are the same. On young leaves, the lesions are initially round spots with a diameter of about 1mm, clear yellow, wet, often seen on the underside of the leaves, then the lesions expand and break the epidermis of the underside of the leaves, pale white or light brown. Around the lesions is a round halo in the shape of a drop of yellow or dark green oil. When the lesions are old, they become rough, ulcerated, rough, the underside is rough, the upper surface is cracked and ash gray, protruding from the leaf surface (Figure 1.4 A). Lesions can be seen about 7 to 10 days after infection on the underside of the leaves and then clearly visible on the upper surface. Severely infected leaves fall off early, leading to bare branches. The lesions on the fruit are similar to those on the leaves: the lesions are rougher and browner, with raised edges on the outer edges, and dead tissue cracking on the fruit skin in the middle. Severe disease can cause the fruit to deform, become water-poor, dry out early, and fall off easily. Ulcers affect the aesthetic value of the fruit, and do not meet quality standards for export (Figure 1.4 C). Lesions on branches and trunks of young plants are similar to those on leaves, but are relatively clearly raised. In some cases, ulcers on the trunk are up to 15 cm long and on branches up to 5 ÷ 7 cm (Figure 1.4B).


Figure 1.4. Symptoms of canker disease caused by X. axonopodis pv . citri bacteria on lemon trees

(A: leaf ulcer; B: branch ulcer; C: fruit ulcer) (Source: Research student took photo at lemon garden in Ben Luc district, Long An)

Canker is one of the serious diseases on citrus trees caused by bacteria.

X. axonopodis pv. citri is a devastating disease that can destroy entire orchard crops. Millions of dollars are spent each year worldwide on prevention, control and eradication programs. The disease causes heavy losses when infection occurs in the early stages of plant growth. The disease is common during bud formation and early fruit development and is more severe during heavy rainfall and warm temperatures. The proportion of diseased leaves is usually equivalent to the proportion of diseased fruits (Grupta and Sharma, 2008). According to Stall and Seymour (1983), in Argentina during the period 1979 ÷ 1980, 83 ÷ 97% of grapefruit fruits were diseased and the proportion of infected leaves was 88%. Canker seriously limits the production of lemons in particular and citrus in general in Asia and South America. The most serious consequence of canker is the impact on trade due to restrictions on international fruit transport and export, and increased quarantine controls on products originating from infected areas (Rybak et al., 2009).

In Vietnam, in recent times, with the unusual changes in weather and climate, along with the rapid increase in lemon growing area, high level of intensive farming, the rate of gardens using sprinklers is increasing, bacterial canker disease is becoming more common and causing serious damage. According to the annual pest report, canker disease appears in most

Lemon growing provinces across the country with different levels depending on the season. Every year with heavy rain and hot and humid weather, the disease develops strongly into an epidemic. The disease often appears on branches, leaves, and young fruits. Severely infected branches will dry and break, infected young leaves will be underdeveloped and fall off early. After 35 days of flower fall, young fruits about 9 mm in size are susceptible to the disease; fruit diameter from 26 ÷ 32 mm (after flower fall 60 ÷ 80 days) has the highest rate of disease, affecting fruit yield and not meeting export requirements. Currently, some countries that have not reported bacterial canker disease are trying to implement many disease prevention measures. In particular, strict quarantine measures such as restricting or banning the import of seedlings and fruits from epidemic areas are given top priority (Sharma and Sharma, 2009). This has significantly affected the lemon export market of countries that have reported canker disease, including Vietnam.

1.2.3. Source of disease and conditions of disease occurrence


Lemon is a perennial plant with a life cycle of up to 10 years, so the tree can easily overcome the effects of pathogenic bacteria on the stem, leaves and fruit. The bacteria causing canker can survive for up to 6 months in infected leaves (Rao and Higorani, 1963). The disease is transmitted from season to season mainly from infected tree crowns and branches. In lesions on the stem and branches, the bacteria can survive for several years (Chakravarti et al., 1966). In infected plant tissues kept dry and free of soil, the bacteria can survive for many years, with high tolerance to drought and cold (Sharma and Sharma, 2009).

According to Pria et al. (2006), the disease develops and develops strongly in conditions of high humidity and temperature of 25 ÷ 35 °C. Most of the disease occurs on leaves and young branches in the first six weeks after the plant begins to grow. The most important period for the disease on the fruit skin is in the first 90 days after petal shedding. Bacteria causing canker mainly through stomata, wounds created by wind, rain or insects. The incubation period depends on the type of plant but is on average from 6 ÷ 14 days (Schaad et al., 2005). Xanthomonas bacteria have a mucous layer so they easily form a suspension in water and disperse into small droplets. The spread of canker is mainly by wind and rain at close range so the disease becomes more serious in plants exposed to rain, in the same direction of the wind. Rainwater from damaged foliage contains bacteria from 10 5 ÷ 10 8

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