NOVEL TRELLISING AND PLANT MANAGEMENT TECHNIQUES FOR DRAGON FRUIT IN VIETNAM

  • Vietnam
  • Crop Management
  • 2018.04.23
  • 51,095
PDF 333

Author

Le Thi Hoang Truc, Nguyen Van Son, Nguyen Ngoc Long, Nguyen Van Hoa J.M. Campbell and R.A. Fullerton

Abstract

The conventional dragon fruit production system in Vietnam involves plants growing up widely spaced concrete posts (‘mop top’ system). While it has low establishment costs, this system leads to a mass of cladodes of different ages at the top of the post. The plants are difficult to prune and there is poor spray penetration for control of pests and diseases, particularly canker. Over time the weight of the plant can break the support post. An alternative trellis-based production system (the T-bar system) is being introduced in Vietnam. The trellis uses a single (double) top wire and two spreaders down each post with each end supporting wires running the full length of the row. This system has many advantages over the conventional system. Cladodes from each leader can be trained, parallel to each other, down either side of the trellis resulting in a simple open, controlled canopy. Canker is the single greatest threat to the sustainability of dragon fruit production in Vietnam. A canker control protocol developed and verified by SOFRI calls for the removal of all infected cladode material from the canopy and the precision application of appropriate fungicides to the most susceptible parts of the plant (the young growing tips). The open canopy of the T-bar system allows excellent spray coverage for pest and disease control. Although the establishment costs of the T-bar system are higher than for the conventional system, field trials show that, with the precision canopy management possible with the T-bar system, yields per unit are of up to twice that of the conventional system. It is also possible that the fruit size and quality are consistently better. The T-bar system is superior to single wire trellis systems which will develop into single walls of dense overlying cladodes with disadvantages similar to those of the conventional system.
 

Keywords

Dragon fruit, pitaya, production systems, T-Bar, canker.
INTRODUCTION 
Dragon fruit or Pitaya (Hyalocereus undatus) is one of the most important fruit crops in Vietnam. It now accounts for approximately 50% of fruit exports reaching over 40 countries worldwide and represents 36.1% of Vietnam’s total fruit and vegetable exports with sales worth US$895.7M in 2016. The area in production has expanded from 13,400 ha to 45,500 ha in the past eight years and is continuing to grow. Total production is now approaching 1,000,000 tons per year.
While postharvest handling and cool-chain technologies have advanced in response to the increased volumes being exported, production methods are still anchored in conventional plant management systems which present many challenges to achieving the yields of quality fruit required for the discriminating export markets. Production systems in other horticultural crops such as apples, pears and kiwifruit have all evolved from widely spaced individual trees to high density plantings often incorporating various types of wire trellises to support the fruit bearing branches (Mollah, 1989; Mussachi, 2014; Strick, 2005). In order to sustain the current rate of expansion and the ever-increasing demands on available land and labor, for dragon fruit in Vietnam significant increases in production efficiency will be required. This paper describes a novel, trellis based production system that is currently being introduced to Vietnam. 
CONVENTIONAL PRODUCTION SYSTEM IN VIET NAM
In the conventional production system, often referred to as ‘Mop Top’ because of the shape of the plants, the productive unit consists of 4-6 individual plants trained up a single concrete post.  The posts are arranged on a square or rectangular system commonly 2.8 to 3.0m apart. When the plants reach the post top, they each produce multiple cladodes. These cladodes, initially upright, later bend down under their own weight and commence flowering and fruiting. At each growth flush new cladodes are formed at the crown subsequently overlying those produced in previous seasons. Although this conventional production system has low establishment costs there are a number of serious disadvantages in terms of production efficiency, crop management and disease control.
The continuous production of new cladodes that overlie the older cladodes results in an impenetrable mass of cladodes of different ages at the top of the post. Most orchards are not pruned and a single post can be carrying over 350 cladodes weighing up to 750gms each. Only the top 20-30 outer cladodes are productive (Fig. 1). The remaining cladodes are redundant for production but still use nutrients and water, significantly reducing nutrient and water use efficiency. The weight of plant material can become so great that it breaks the reinforced concrete support post completely eliminating that productive unit.

Fig. 1. Conventional production system: only the 20-30 outer cladodes are productive.
The removal of the old cladodes is difficult, safety risk to pruner and labor intensive. They can be reached only from below the skirt of the canopy and behind the younger cladodes. They cannot be cut back to their bases because they are not accessible through the mass of intertwined cladodes on the top of the post and the height of the plant continues to increase in height over the years. This region of the canopy is a refuge for pests and diseases. (Fig. 2).
Fig. 2.  Conventional system: The removal of the old cladodes is difficult and the plant continues to increase in height over the years
The most serious of the disease of the crop is canker caused by the fungus Neoscytalidium dimidiatum.  This pathogen infects cladodes and is often most severe at the base of the cladode when it is just forming. It causes large areas of dead tissue in which the fungus can survive and reproduce for several years.  Because it is not possible to remove the old infected cladode bases, and as they cannot be reached by fungicide sprays these old infections are a continuous source of infection for new cladodes and fruit. 
In order to maximize the number of productive units on a block of land the planting distances are reduced.  As the canopies increase in diameter they encroach on one another making access difficult for routine management operations such as weed control, mulching and fertilizing, for the application of growth hormones to fruit, and for harvest and removal of fruit.
Because of the size and structure of the ‘mature’ Conventional production a significant length of each cladode lies horizontally as it extends over the mass of older cladodes. These portions of the cladode are susceptible to severe sun burn with large areas of cladode being killed and colonized by opportunistic parasitic and saprophytic fungi. The damage not only affects the vigor of the plant but some of the species colonizing the damaged tissue are also those organisms that can cause postharvest rots. Thus severely sun damaged canopies can be a major source of inoculum for postharvest rots.
To save costs, the land is often not leveled before planting. The uneven land surface with ridges, and sometimes with tree stumps creates further difficulties for management operations and prevents the option for any kind of mechanization. Many conventional plantings are purposely planted on raised mounds to reduce flooding effects.
THE T-BAR PRODUCTION SYSTEM
Modern horticultural practice for many crops is characterized by high density row planting, the use of wire trellises to separate and support productive branches and the mechanization of routine orchard operations such as sward management to reduce labor costs. The T-bar system being introduced to Vietnam applies these principles to dragon fruit.
The system uses rows of concrete posts 5.0m apart and 1.1 to 1.3m high. A top wire extends the length of the row and fixed to the end posts. The post end assemblies are stayed (supported) by screw anchors in the soil. The screw anchor appears to be the most effective trellis end support for the soft soils of the Mekong Delta. Each post carries a wire at the top and two horizontal and parallel steel spreaders, a top spreader 600mm long attached 100mm below the top wire and a lower spreader of 800mm long attached 800mm below the top wire. Support wires (spreader wires) are threaded through holes in, or attached to, each end of the spreaders and tied off at the end posts. All trellis wires are kept at medium to high tension to prevent sagging and this provides the support structure for the plants.
Dragon fruit are planted 600mm apart in the row. Plants are supported by stakes or twine until they reach just over 600 mm above the top wire. The topmost cladode should be from a single, uninterrupted growth flush. The cladode is then bent over, tied to the top wire and cut off just short of the next plant in the row to form a short ‘leader’. All leaders lie in the same direction along the row. This is the basic structure of each plant. The original leader and the bases of subsequently pruned cladodes form the ‘head’ of the plant (Fig. 3).

Fig. 3.  Basic structure of T – bar system
Up to 12 cladodes (six on each side approximately 100 mm apart) are established and maintained on each head. All others are removed as they emerge. The selected cladodes are allowed to grow upright until approximately 1.0 m long then slowly brought down on either side of the trellis by attaching a light weight to each. This method allows the cladodes to be brought down and into a nearly vertical flowering configuration relatively quickly without the risk of them breaking. The cladodes are aligned parallel to each other approximately 100mm apart on each side of the trellis and tied to the spreader wires to hold them securely in place. The upper spreader wires carry the shoulder of the canopy and the weight of the plant. The lower spreader wires anchor the cladodes to prevent cladode movement and fruit rub. (Fig. 4).

Fig. 4.  The cladodes are aligned parallel to each other approximately 100mm
This system divides the canopy into a vegetative zone, with new replacement cladodes being selected and grown at the top of the plant and a productive zone of mature cladodes fruiting down each side of the row. Fruit which form at the top of the cladodes in the vegetative zone are typically of poor quality and are removed as they set to allow plant resources to be diverted to the higher quality fruit lower in the productive zone.  
This plant structure allows for the continuous production of new healthy cladodes for the replacement of older spent cladodes in the fruiting zone.  For each old cladode observed to be approaching the end of its productive life a new cladode in the correct position on the head is selected in advance and allowed to grow.  When the old spent cladode is removed it can be replaced immediately by young cladode from above. In this way the total number of cladodes on each plant remains relatively constant while maintaining a canopy of productive young cladodes. When old cladodes are removed a short length should be left attached to the leader. This stub, carrying a number of buds, ensures a supply of buds in the head from which new cladodes can be selected and grown on.  In this way the cladode productivity of the head can be maintained. 
Because the fruiting cladodes on each side of the row are held apart from each other on the carrier wires the canopy remains open with every part of each plant being visible and accessible. This canopy structure for allows for a level of precision of management not possible with the conventional production system.
With the T-bar system only currently productive and replacement cladodes are retained on the plant. The open canopy allows easy access for removing spent cladodes completely preventing any build-up of redundant, non-productive cladodes on the plants. All fruits are produced on the outer wall of each canopy providing for easy access for hormone treatments or other potential operations such as covering of fruit to protect from fruit fly and access for fruit harvest and removal from the orchard. (Fig. 5).
Fig. 5.  The open canopy allows easy management and all fruit are produced on the outer wall of the canopy
By leveling the land before installation of the trellises, the inter-row areas become available for mechanization such as mechanical sward mowing or, in larger orchards, machine spraying of pesticides, etc. It also allows for vehicle access for removal of fruit avoiding multiple handling operations which can damage fruit and reduce quality.
The greatest continuous threat to profitable dragon fruit production in Vietnam is dragon fruit canker (Fig. 6). This disease has proven to be almost impossible to control in conventional orchards because of the retention of diseased cladodes in the canopies. SOFRI have developed a canker control protocol which involves three critical steps: orchard hygiene, which requires the complete removal of all disease symptoms from the canopy; regular application of the recommended fungicides; ensuring the fungicides are applied to the most susceptible parts of the plant – the tender tips of rapidly growing cladodes. The open accessible canopy of the T-bar system is ideally suited to the implantation of this protocol. Every part of the canopy is visible and any symptoms can be easily seen and removed before the pathogen reproduces and spreads. If a cladode has to be removed completely it can quickly be replaced by an existing cladode from the vegetative zone. All sides of the cladodes are accessible to sprays either from the outside, or through the inside of the open canopy. (Fig. 7). Thus all sides of the three-sided cladode tips can receive sprays. With regular scouting and management it is possible to keep T-bar canopies completely free from canker, ensuring 100% of fruit are free from canker spots.

Fig. 6.  Typical symptoms of dragon fruit canker (Neoscytalidium dimidiatum) on cladodes
Fig. 7.  Tissue most susceptible to canker (Neoscytalidium dimidiatum) are tips of rapidly growing cladodes and all sides of the cladodes are accessible to sprays either from the outside, or through the inside of the open canopy
Experimental evidence (unpublished data) to date indicates substantially higher yields per unit of land area with the T-bar canopy management system than with the conventional system. Yields over a three year period ranged from 1.5 – 2 times greater in the T-bar system. The T-bar system is currently undergoing evaluation under commercial production conditions.
OTHER TRELLISING SYSTEMS
Various forms of trellising for dragon fruit are being used globally and also by some Vietnamese growers. These usually involve high density planting on a single wire with a ‘single wall’ structure of the canopy. This system carries with it the key disadvantage of the conventional system in that the younger cladodes overlie the older cladodes.  Over time it becomes a dense wall of old and young cladodes and is becomes inaccessible and unmanageable for pruning, hygiene and application of pesticides.  
CONCLUSION  
Although the SOFRI T-bar production system has higher establishment costs than the conventional or single wire trellis systems, the costs are quickly recouped by the higher yields, the higher proportion of first quality fruit resulting from the separation of fruiting cladodes and less physical damage. The system also allows for savings in labor costs through faster and easier access to plants and, in larger orchards, the potential for mechanization of sward management and spray application. The T-bar system introduces to dragon fruit production the concepts of high density planting and precision canopy control, crop management practices that are now a standard feature of all commercially successful horticultural crops worldwide.
REFERENCES
Mollah,  M.R.1989. Review of trellis structures for horticultural crops.  Technical Report Series No 177. Department of Agriculture and Rural Affairs, Victoria, Australia. (https://www.driedfruitsaustralia.org.au/library/files/o_19vuedrgnhld1msergunp4psl7.pdf)
Mussachi, S. 2014. Horticultural systems and practices to facilitate mechanism in apples and pears. Washington State University. (http://jenny.tfrec.wsu.edu/wsha2014/Precision_Horticulture/MusacchiPrecisionMechanization.pdf)
Strik. B. 2005. Growing Kiwifruit. A Pacific Northwest Extension Publication. Oregon State University, University of Idaho. Washington State University.
(http://jenny.tfrec.wsu.edu/wsha2014/Precision_Horticulture/MusacchiPrecisionMechanization.pdf)

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