Key Points of Breeding and Cultivation Techniques of Wushan Crisp Plum, a New Cultivar of Green Crisp Plum

Wei XIONG, Ming HUANG, Ming ZENG, Linling KOU, Wenbin KONG, Jun TANG, Fang XIANG, Tao HE

1. Chongqing Agricultural Technology Extension Station, Chongqing 401121, China; 2. Fruit Industry Development Center of Wushan County, Chongqing 404700, China; 3. College of Horticulture and Landscape Architecture, Southwest University, Chongqing 400715, China; 4. Economic Crop Technology Extension Center of Wuxi County, Chongqing 405899, China; 5. Economic Crop Technology Extension Center of Yubei District, Chongqing 401120, China

Abstract Wushan crisp plum is a new cultivar of green crisp plum with dignified fruit shape, crisp and tender flesh, pit separability and sweet taste. It is bred from natural bud mutant of Jiang’an Dabaili, a local green crisp plum cultivar by Wushan County Fruit Industry Development Center, Southwest University and Chongqing Agricultural Technology Extension Station. Wushan crisp plum has won the gold award in the National High-quality Plum and Apricot Appraise and Elect and the tile of "the king of fruit" in the "Three-Gorge Cup" High-quality Crisp Plum Appraise and Elect. In the southwestern region of China, the planting area of Wushan crisp plum has been extended to 33 000 ha. The cultivar has strong robust and fast growth. In the area along the Yangtze River in the Three Gorges Reservoir area of Chongqing, it ripens from early to middle July. The fruit is ovoid and of medium size, with average longitudinal diameter of 3.3 cm, average horizontal diameter of 4.0 cm, average single fruit mass of 37.2 g, carpopodium length of 1.5 cm, and carpopodium diameter of 0.1 cm. The top of the fruit is slightly concave. The peel is green to green-yellow in background color and medium in thickness, with obvious fruit dots and thick white fruit wax. The flesh is light yellow, compact, short in fiber length, juicy, crisp, tender, pure and sweet, with soluble solids content of 12%-15%, titratable acid content of 0.43%-0.72% and vitamin C content of 6.12-8.99 mg/100 g. The pit is small, separable, and oblate. The percentage of edible flesh is up to 94.80%-96.88%. There is no obvious pre-harvest cracking and intra-fruit pectin. The cultivar is mid-ripening and high in yield. In the firm ripe stage, the flesh is crunchy; and after entering the soft ripe stage, the flesh becomes powdery, and the hardness decreases. At room temperature, the fruit is not resistant to storage and transportation. Wushan crisp plum is tolerant to the climate with high temperature, high humidity and less sunshine, with good adaptability. It is suitable for promoting in the upper reaches of Yangtze River within 180-1 000 m elevation.

Key words Plum, New cultivar, Wushan crisp plum, Variety breeding

1 Introduction

Green crisp plum is the main plum variety in the upper reaches of Yangtze River. It has a long cultivation history and a wide planting range. Jiang’an Dabaili is the dominant cultivation variety. The maturity period is mainly concentrated from late June to early July, which overlaps with the annual "plum rain season". Due to pre-harvest fruit drop, fruit cracking, light fruit taste, poor quality, poor storability[1], the development of the industry has been lingering for a long time, and is basically in a state of sporadic planting. The planting scale of plum in Chongqing has been maintained at about 13 000 ha for a long time. In recent years, targeting the problems existing in the cultivar of Jiang’an Dabaili, Chongqing Municipality has carried out the breeding of new middle to late-ripening crisp plum cultivars to dig up the local excellent plum germplasm resources and breed new green crisp plum varieties characterized with large fruits, high sugar content, crisp taste and late ripening (delayed to July and later) using molecular assisted breeding[2], so as to effectively avoid the "plum rain season" from June to early July and promote the development of plum industry in Chongqing.

2 Breeding process

In the 1970s, in a Jiang’an Dabaili (also known as Jiang’an plum) plant in Ganyuan Village, Quchi Township, Wushan County, it was found that the fruit on a large branch was significantly different from other Jiang’an plums, mainly manifested in the following aspects: high yield, more fruiting branches, large fruit, dignified fruit shape, obvious fruit dots and fruit wax and late ripening (early to middle July). The local villagers spontaneously propagated and self-produced by using methods such as supporting seedling and high grafting, and a larger scale has been gradually formed. In 2007, Wushan County Fruit Tree Station (now Wushan County Fruit Industry Development Center) collected the mother tree as scion and carried out research on the genetic stability and high productivity of the progeny of the variety by grafting the progeny seedlings and high grafting in Ganyuan Village, Quchi Township. In July 4, 2012, the new cultivar passed the field appraisal organized by the Chongqing Crop Variety Appraisal Committee. In the regional trials in townships and towns such as Daxi and Wuxia of Wushan County, it all appeared to be large in fruit size, early in fruiting, high in yield and strong in resistance. Using the molecular identification method of DNA barcoding technology, Chongqing Agricultural Technology Extension Station and the College of Horticulture and Landscape Architecture of Southwest University carried out the identification of the relationship between Jiang’an Dabaili and Wushan crisp plum to confirm that there are differences between the varieties[3-4]. On July 9, 2016, the new cultivar was named as Wushan crisp plum by the Chongqing Crop Variety Appraisal Committee (Yu Pin Shen Jian 2016015)[5]. On April 23, 2018, Wushan crisp plum was awarded theNewPlantVarietyRightsCertificateby the Ministry of Agriculture and Rural Affairs (CNA20161163.9)[6].

3 Source of female parent and analysis of genetic relationship

3.1 Source of female parentThe female parent is Jiang’an Dabaili, which is an excellent green crisp plum variety bred in Jiang’an County, Sichuan Province more than 300 years ago. It grows vigorously in the sapling stage, and the posture becomes more expansive after entering adulthood, with naturally semicircular crown. The blade is inverted lanceolate or narrowly elliptical, and the base is wedge-shaped, long in tip and gradually pointed in bottom. Generally, the leaves are 9.45 cm long and 3.16 cm wide. The flowers are white in color, and the five petals are rectangular oval. The cultivar has strong adaptability, strong barren tolerance, and good productivity. The average plant yield is about 50 kg. The full blooming period is about mid-March. The fruit ripens from late June to early July.

According to information, the fruit of Jiang’an Dabaili is medium in fruit size, nearly spherical, delicate in flesh, crisp, tender, concave in the bottom and separable between flesh and pit. The average single fruit weight is 25-35 g. The top of the fruit is flat or slightly convex to one side. The suture is shallow. The carpopodium concave is deep and wide. The fruit wax is thick and white. After peeled off, it could be seen that there is short needle-shaped pulp attached to the pit. Some pits also have yellow-white gel-like exudates. In the firm ripe stage, the flesh is thick and yellowish green or light yellow and the pit is small. In the soft ripe stage, the flesh can be separated from the pit, and the pit is short and oval. While shaking lightly by hand, the impact of the pit and the flesh can be heard. For fine and crisp flesh, abundant juice, moderate sweetness and sourness, slight aroma and high quality, Wushan crisp plum has become a traditional specialty product of Jiang’an and has been popular among consumers[7].

3.2 Analysis of genetic relationshipIn April 2014, 5 fresh leaf samples of Wushan crisp plum were collected in Wushan and 5 fresh leaf samples of Jiang’an Dabaili were collected in Jiang’an County, Sichuan. All the samples collected were sent to the College of Horticulture and Landscape Architecture of Southwest University. On the basis of observing and recording the botanical characters of Wushan crisp plum, the characteristic sequence of the DNA of the descendants of Wushan crisp plum and Jiang’an Dabaili was identified using DNA barcoding technology[8-9]. The total DNA of the leaves was extracted, respectively. After amplification, the characteristic sequences of matK, rbcl and ITS were obtained[10-12]. The variation degree of bases in the sequences of the two varieties was analyzed. The genetic distance between the varieties was calculated. A phylogenic tree was built to compare the genetic differences between Wushan crisp plum and Jiang’an Dabaili from the molecular level and lay a foundation of molecular biology for the identification of Wushan crisp plum[3]. The comparison analysis of matK sequence shows that Wushan crisp plum has a significant difference in the matK sequence from Jiang’an Dabaili, which provides a basis for the identification of Wushan crisp plum in Chongqing[4].

4 Biological characteristics

4.1 Botanical characteristicsThe young tree of Wushan crisp plum has strong growth, with strong branching capability, conducive to early shaping and early production. After putting into production, the growth becomes moderate. The adult tree has natural heart shape, relatively large branching angle, strong branching capability and compact crown. The one-year-old branches are pale brown, and the three-year-old branches are brown. New shoots and lateral branches have strong branching capability. Combined with trimming in summer, high-yielding tree structure can be formed that year. Leaf blades are inverted lanceolate or narrowly elliptic, 10 cm long and 3 cm wide. Leaves are gradually pointed in tip, narrow in base, 1 cm long in petiole, green in color and obvious and dense in veins. The main veins are yellowish green and are reticulate. The edge of leaves is jagged.

4.2 Growth and fruiting habitThe flowering rate of the branches is high. In early years, fruiting branches are dominated by medium and long fruiting branches. In the full bearing period and later, fruiting branches are dominated by short fruiting branches and bouquet-shaped fruiting branches. The self-flowering fruit setting rate is also high. The axillary buds have a high flowering percentage. The percentage of flowers which set single fruit was 68.9%, and the percentage of flowers which set double fruit was 31.1%. High percentage of single bud and double fruit is the most distinctive fruiting characteristic of this variety. The flowers are monopetalous, with five white petals. The flowers are small, lush, complete and pollen-rich. The fruit is in medium size, nearly spherical or ovoid, 3.3 cm in longitudinal diameter and 4.0 cm in horizontal diameter, with average single fruit mass of 37.2 g (Table 1). The carpopodium has a length of 1.5 cm and a diameter of 0.1 cm. The top of the fruit is slightly concave. The peel is green to greenish yellow in background color and medium in thickness, with obvious fruit dots and thick white fruit wax. The pit is separable, small, and oblate. After years of fruit quality testing by the Quality Supervision, Inspection and Testing Center of Citrus and Seedlings of the Ministry of Agriculture and Rural Affairs, Wushan crisp plum has light yellow, compact, crisp and tender flesh, which has less crude fiber, more juice and delicious taste. The average percentage of edible portion, soluble solids content, titratable acid content and vitamin C content are 96.88%, 12.4%, 0.43%-0.95% and 6.12-8.99 mg/100 g, respectively, with good quality. The variety has wide adaptability, strong resistance, early fruiting, high yield, and resistance to barren and drought. The trees can bloom and bear fruit in 2 years and enter the full bearing period in 4-5 years. In the areas along the Yangtze River in the Three Gorges Reservoir area, Wushan crisp plum ripens in early and middle July. Basically, there is no pre-harvest fruit cracking and intra-fruit pectin. The color, aroma, taste, and shape of the fruit are all good. Cold-chain transportation is recommended. At the temperature of 0-2 ℃, the fruit can be stored for more than 20 d, indicating good storability.

Table 1 Comparison of yield traits between Wushan crisp plum and Jiang’an Dabaili

4.3 Phenological periodIn the low mountain valley area along the Yangtze River in Wushan, Chongqing, it sprouts at the end of February, begins to show buds in early March, begins to bloom on March 5, enters the full blooming period on March 5-12 and ends the blooming in middle March. The fruit ripens in early to mid-July and the leaves fall in mid-October.

4.4 Evaluation of stress and disease resistanceIn field trials, it is observed that the variety can be distributed in the area with 180-1 750 m elevation, and its requirements on the soil are not strict. The seedling breeding is mainly based on wild peach stock and rootstock. The variety has strong resistance to drought, cold and high temperature, and it is also resistant to bacterial shot-hole disease and early defoliation. According to the requirements of crop variety identification, from 2012 to 2014, the resistance of Wushan crisp plum to the two main diseases of bacterial shot-hole disease and red spot disease was tested and evaluated.

4.4.1Location and method of evaluation. The experiment was conducted in a Wushan crisp plum orchard in Ganyuan Village, Quchi Township, Wushan County, with 8-year-old wild peach as the stock and Jiang’an plum as the control.

Soil and fertilizer management was carried out according to the traditional habits. There were two treatments for the prevention of diseases and insect pests in the experimental area. Treatment 1: the orchard was cleaned in winter to reduce the source of dissemination; before germination, lime-sulphur mixture of 5 Baume degrees was sprayed; after leaf spreading, lime-sulphur mixture of 0.3-0.5 Baume degree was sprayed; and from June to July, liquid of 65% mancozeb wettable powder (500×) was sprayed every 15 d, for 4 times. Treatment 2: the orchard was cleaned in winter to reduce the source of dissemination; before germination, lime-sulphur mixture of 5 Baume degrees was sprayed; and after leaf spreading, lime-sulphur mixture of 0.3-0.5 Baume degree was sprayed.

Evaluation methods: Incidence rate refers to the percentage of diseased plum leaves in the total plum leaves surveyed, and it is used to indicate the prevalence of the disease. The specific survey method is as follows: Five sampling points were arranged in the eastern, western, southern, northern and central parts of the demonstration orchard, respectively. In each sampling point, 3 plum trees were selected. In each plum tree surveyed, total 50 leaves were collected from each of the eastern, western, southern and northern parts, respectively. Thus, a total of 3 000 leaves were sampled for investigation on the incidence and severity. Severity refers to the percentage of diseased leaf area in the total leaf area. It is expressed by the leaf grading method. The specific grading criteria are as follows: grade 0, no disease; grade 1, area of lesions accounted for 0.1%-5.0% of the entire leaf area; grade 3, area of lesions accounted for 5.1%-15.0% of the entire leaf area; grade 5, area of lesions accounted for 15.1%-33.3% of the entire leaf area; grade 7, area of lesions accounted for 33.4%-50.0% of the entire leaf area; and grade 9, area of lesions accounted for more than 50.1% of the entire leaf area.

Disease index=∑ [Number of diseased leaves at each grade × Relative value of each grade]/(Total number of leaves surveyed×9)×100.

4.4.2Resistance to bacterial shot-hole disease and red spot disease. If prevention and treatment were not performed during the onset period, bacterial shot-hole disease and red spot disease were more common in Wushan crisp plum and the control. In treatment 1 with preventive measures, the occurrence of the diseases was better controlled. In each treatment, Wushan crisp plum had a lower incidence than the control, and the disease index was lower than that of the control (Table 2).

5 Regional comparative tests

In order to verify the adaptability of Wushan crisp plum in different regions, in February 2010, through high grafting and planting one-year-old grafted seedlings, simultaneous multi-point trials were conducted at Longping Village of Gulu Town and Tianshan Village of Bayang Town in Wuxi, and Xuehua Village of Duping Township in Wushan County. Sampling were performed from 2013 to 2015 for analysis.

5.1 Observation of botanical charactersGrowth habit: The trees are naturally heart-shaped, with strong branching capability and a compact crown shape. The one-year-old branches are light brown, and the 3-year-old branches are brown. The leaves are oblong, 10 cm long and 3 cm wide. The tip of the leaves is gradually pointed, and the base is narrow. The petioles are 1 cm long and greenish-blue. The veins are obvious, dense, and reticulate. There are obvious serrations on the margin of the leaves. The flowers are monopetalous, small, lush, complete, and white, with a lot of pollen.

Table 2 Occurrence situation of bacterial shot-hole disease and red spot diseases in Wushan crisp plum

Fruiting habit: The tree posture is open. The sprouting rate and branching capability of young trees are high. The flowering rate of branches is high. In juvenile fruiting trees, the fruiting branches are mainly medium and long fruiting branches which were self-fruitful.

Phenological period: The phenological period varies with altitude. The trees sprout in the end of February, the buds are starting to show up in the beginning of March, and blooming starts in March 5-10, enters the full blooming period in March 10-13, and ends the blooming in mid-March, the fruit ripens at the end of mid-July and the leaves fall in mid-October.

Resistance: Field observation of Wushan crisp plum shows a strong resistance to drought, cold and high temperature, as well as bacterial shot-hole disease and red spot disease.

5.2 Yield and quality analysisThe results of the regional trials show that this variety has the advantages of early fruiting, high yield, middle ripening, easy cultivation and management, strong resistance, and strong adaptability, and is suitable for promotion as an excellent medium-ripening plum. The yield and fruit quality of Wushan crisp plum in different regions are shown in Table 3.

Table 3 Yield and fruit quality of Wushan crisp plum in different areas of Chongqing in 2015

6 Key points of cultivation techniques[13-14]

6.1 Orchard establishment and cultivationIt is better to choose leeward and sunny gentle slope or flat land with loose and fertile soil, deep soil layer, good permeability, good irrigation and drainage, air circulation, soil pH of 6-8 and slope of less than 25° for establishing orchard. Planting is appropriate to be conducted in late autumn and early spring. In general, the plant and row spacing is 3 m × 4 m or 2.5 m × 4 m, with 840-990 plants per hectare. For transplanting, one-year-old disease and pest-free seedlings with a stem diameter of more than 1 cm should be selected. After the seedlings are planted, it is required to make a tree disc with a diameter of 1 m around every seedling and pour enough root water. Pour enough root water. The planting depth should be that the graft union is 5-8 cm above the ground.

6.2 Fertilizer and water managementYoung trees are drenched with rotted manure water and compound fertilizer, and fruiting trees are generally fertilized three times a year, that is, fruit picking fertilizer, pre-flowering fertilizer and fruit robustizing fertilizer. Wushan crisp plum should be supplemented with enough water according to different growing stages. Plum trees are not resistant to waterlogging, so drainage system should be built to avoid flooding.

6.3 Trimming and pruning

6.3.1Trimming. In winter trimming, 3-4 branches with uniform distribution and good growth should be selected as the main branches and cut short, and the remaining branches are all cut off. In winter trimming of the second year, the extended branch of the main branch should be cut short. At the lower part of the extension branch, one branch growing outwards is chosen as the first lateral branch, of which about 1/3 of the original length is cut off. Branches with length less than 5 cm are all reserved to cultivate into bouquet-shaped fruiting branch or short fruiting branch. Medium and long branches can be cut short to promote their branching. Later, the extension branch is cut short moderately, the crown is expanded, and the second and third lateral branches are trimmed. Thus, trees are shaped in 3 years.

6.3.2Pruning. Pruning in the initial fruiting period: The pruning of trees in the initial fruiting period is dominated by pruning in the growing period, supplemented by the pruning in the dormancy period. For trees that just begin to bear fruit, the pruning should try the best to promote blooming and fruiting of the branches. The main methods include hanging, branch pulling, branch removing and branch twisting. Pruning during full fruiting period: The main point of pruning is to maintain the tree shape, coordinate the relationship between vegetative growth and reproductive growth. Sparseness and elimination are the dominant. Shrinking and expanding are combined.

Pruning during aging: The main lateral branch should be retracted and rejuvenated in time. The branches that are too long or too old should also be properly retracted. After retraction, more strong branches will appear generally. Thus, new backbone branches and branch groups will be cultivated to form a new canopy.

6.4 Flower and fruit managementWhen the number of flowers is too large, excessively dense flowers and weak flowers should be removed. At the same time, 0.1% borax solution or 0.1% borax solution + 0.1% Lufenwei No.1 should be sprayed at the flowering stage. After flowers fall off, fruit should be thinned when large and small fruit is distinguishable. According to fruiting amount, thin and weak fruit, deformed fruit, fruit infected with diseases or insect pests and twin fruit are removed. The yield per hectare is controlled at about 22 500 kg.

6.5 Comprehensive prevention and control of major diseases and insect pestsThe main pests and diseases of plums are bacterial shot-hole disease, scab, anthracnose, brown rot, gummosis, red spider, plum sawfly, aphid, peach borer, plum borer, weevil, chafer, fruit sucker and thorn moth. To enhance the resistance of plum trees and control diseases and insect pests comprehensively, one should strengthen soil, fertilizer, and water management, combined with pruning, flower thinning and fruit controlling, improve the ecological environment of plum orchards. When pesticides are used for prevention and control, it is necessary to select mineral-based pesticides, plant-based pesticides, pesticides of biological source and insect growth regulators as much as possible, pay attention to protecting natural enemies of insect pests, and give full play to the role of comprehensive prevention and control. Cleaning the orchards in winter is an important measure for the integrated prevention and control of pests and diseases.

Clearing plum orchards thoroughly. Pruned leaves, dead branches, weeds and diseased or pest-infected branches, spores and still fruit pruned in winter should be clear out of plum orchards and burned or buried deeply. When clearing orchards in winter, lime-sulphur mixture of 3-5 Baume degrees can be sprayed to eliminate the source of overwintering pests. Scraping the old bark of trunk and removing root nodules. Before scraping the bark, a piece of plastic film needs to be laid under the tree trunk to catch the scrapes. Then, the scraped objects are cleared out of the orchard for centralized treatment. At the same time, scars are smeared with liquid of 75% chlorothalonil wettable powder (50×-100×) to eliminate the source of wintering pests. Painting the trunk white. The trunk of normal plum trees is painted white with 10% limewater, while the trunk of plum trees scraped is painted with a special whitening agent. Combined with mulching under trees, overwintering pests such asGrapholithafunebranaTreitscheke and plum sawfly will be prevented.

7 Prospects of comprehensive application

Wushan crisp plum is a high-quality early and middle-ripening green crisp plum cultivar, characterized by large fruit, beautiful appearance, excellent quality, early fruiting, high yield, and disease resistance. It is suitable for promoting in the areas suitable for green crisp plum, especially in the area along the Yangtze River from Wanzhou to Wushan in the Three Gorges Reservoir area in Chongqing[15].