Phenological Study of Chinese Jujube Trees Using Biologische Bundesanstalt, Bundessortenamt and CHemische Industrie (BBCH) Scale

Chinese jujube is a perennial fruit tree, famous for its nutritional and medicinal value. However, there is a lack of information on its growth cycle. The study was carried out in Portales, New Mexico, from 2018-2019, to define the phenophases of Chinese jujube according to BBCH scale. We observed eight major growth stages for Chinese jujube: bud growth, leaf formation, leaf growth, shoot growth, formation of inflorescence, flowering, fruit formation, growth and development, fruit maturation, and winter dormancy. The developed code could help jujube farmers perform orchard management practices such as pruning, water and fertilizer management, pest control, and harvesting.


Introduction
derstood. To date, no particular phenological scale has been established for the jujube plant. Southern regions of New Mexico are potential regions for farming jujube, but the lack of information about crop species have restricted production. A very few studies have been conducted using jujube plants in these regions. Sapkota, et al. [23] reported the best size of jujube suckers (height: 50-100 cm; diameter: 0.36-0.75 cm) in the semi-arid region of New Mexico and suggested the need of more plant-ecological studies. The research about phenological stages is critical, not only for introducing a crop to a new region but also for enhancing crop production. Thus, this study aims to describe phenological growth stages of Chinese jujube (Ziziphus jujuba) using BBCH scale in Portales, New Mexico, USA.

Materials and Methods
The study was conducted in Portales, New Mexico (latitude: 34°11'10.29" N; longitude: 103°20'3.83"W, elevation: 1222 m), which represents the south western USA. The experimental area has hot summer (May to August), less rainfall, and mild winter (October to January). The minimum, maximum, mean annual temperatures, and rainfall of the study area in 2018 were 7 °C, 25 °C, 16 °C, and 367 mm whereas in 2019 were 6 °C, 24 °C, 15 °C, and 440 mm, respectively. Orchard soil was sandy loam (pH: 6.95) with potentially mineralizable carbon and nitrogen values 7.64 and 135.96 mg/kg, respectively. Different growth stages of Chinese jujube trees were closely observed using the BBCH scale during 2018-2019. We recorded data at three-day intervals.

Results
The growth cycle of Chinese jujube consisted different stages between bud growth and winter dormancy (Table 1).  stages. Interestingly, Chinese jujube had forty-six secondary growth phases in our location ( Table 1). The major growth stages of Chinese jujube are as shown in Figure 1. The detail of phenological stages of Chinese jujube is as shown in Figure 2.

Discussion
The BBCH scale precisely describes morphological char-Notably, eight major phenological stages were observed for Chinese jujube; three for vegetative stage, four for the reproductive phase and one for winter dormancy. The major stages were bud growth, leaf formation and growth, shoot growth, formation of inflorescence, flowering, fruit formation, growth and development, fruit maturation, and winter dormancy. Each major stage consisted several secondary  removing weak, old, disease infected, and crowed shoots so that trees can produce satisfactory fruit yield and quality. During the summer season, crowded buds, shoots, and infected/damaged shoots should be removed. The best time for fertilizing jujube is immediately after fruit harvest to provide necessary nutrients. For jujube orchards, natural pollination mainly depends on bumblebees and houseflies [1]. Hand and machine pollination may not be feasible in the farmer's field, but farmers can apply natural pollinators (introduction of bee hives).
Moreover, BBCH scale is useful in dealing with phytopathological problems in jujube orchards. For example, witches broom, caused by phytoplasma, is the severe disease of jujube, where shoot appears as a witches broom, flower undergoes metamorphosis, and the tree dies 4-5 years early. The effective way to control witches broom disease is girdling the tree trunk after the bud break stage. This practice obstructs the pathogen movement from root to upper part of the tree [25]. Furthermore, peach fruit moth is a severe insect of jujube. The insect lays eggs inside immature fruit by making a hole. The eggs hatch into larvae, feed on fruit pulp, and finally emerge as an adult moth. Infected fruits drop and responsible for significant yield reduction. Thus, throughout the fruiting stage, the trees should be carefully inspected to find out the symptoms. The fallen infected fruits should be destroyed to stop further spreading of pathogens. Timely spraying of pesticides could be an effective method to control peach moth [1].

Conclusion
In summary, an understanding of plant phenology helps in effective management of fruit orchard. The proposed guidelines could act as a unique tool for the phenological description of jujube. The findings further help jujube growers to recognize critical growth stages and perform management practices, including irrigation, fertilization, disease pest control, and harvesting to enhance yield and quality. Phenological documentation also aids the jujube researcher to conduct experiments in various climatic conditions. acteristics of the jujube plant at different growth stages. The scale provides ideas to growers about critical growth phases and helps them to enhance fruit yield and quality. The vegetative growth of Chinese jujube consisted of three major stages: Bud, leaf, and shoot development at our location in Portales, New Mexico. The jujube growth consists of the formation of primary, secondary, mother bearing, and bearing shoots. The primary shoots formed the major structure of the plant, whereas secondary and bearing shoots were meant for reproduction. The vegetative growth of Ziziphus jujuba extends about two months, from mid-March to mid-May. During March, new green leaf tips started to emerge from the trees (7-10). The leaf continued to elongate and expand till it attains its typical size and color (bright green). Maximum shoot growth and maturation took place during mid-May (39). Notably, the vegetative growth stage proceeded in parallel with the reproductive stage. The reproductive stage started with the development of bearing shoots (4,5) from the mother bearing shoots. The flowers were observed in a cluster on bearing shoot at the axils of the leaf from late May to July (60-67). The growing pattern of flower bud showed slower growth for the first week and then rapid growth and resulted in fully expanded flower within three to four weeks (63). The flowering cluster consisted of 10 to 12 flowers. Each flower was yellowish-white (60), about 2.5 mm in diameter, and consisted of five petals.
Furthermore, different secondary growth phases, including pollination, end of flowering, and flower fading, were observed. Pollination (64) happened all the time during the day by nectar-feeding housefly and bumblebee. When the first few flowers faded, the fruit began to develop (71). Flowering, fruit setting, and their growth proceeded in a parallel way. Fruit maturity was indicated by color change from green to reddish brown (81-85). The pericarp of the fruit turned dark reddish brown (87-89) within four to six weeks of the first color change (81). Kishore [14] reported identifying the correct time of harvest is essential to ensure fruit yield and quality. The best time to harvest jujube fruit for consumption is fully mature stage (87-89). After full maturation, fruits dried and wrinkled. Overall, in our location (Portales, New Mexico), jujube fruit reached the optimum harvesting stage during the mid to late September, about 10 to 12 weeks after fruit set. Delaying in harvesting causes fruits to drop and quality to deteriorate. After harvesting, leaves dropped down and plant entered into the winter dormancy (95-97). During late spring to early summer the trees started to produce suckers.
Effective management of fruit orchard includes several horticultural practices such as pruning, fertilizer application, irrigation scheduling, disease pest management, flower and fruit thinning, and timely harvest. All these activities require proper phenological knowledge [14]. In jujube orchard, pruning improves the passage of air and light. Furthermore, pruning encourages proper vegetative growth, thus increase fruit yield [24]. Pruning should be started at the beginning of spring season and can continue throughout the dormant season (stage 00). Liu [1] and Liu and Zhao [3] stated that pruning operations in jujube trees could be performed when tree is in growing or dormant phase. The dormant pruning involves