Kiwifruit’s future in protected cropping and hydroponics
Of course, there have been continual incremental changes which have enhanced productivity, such as the use of drip fertigation, improved pollination methods, girdling to improve dry matter, and perhaps most importantly the on-going breeding of new varieties initiated by DSIR in the 1960s, and continuing to this day in New Zealand with Plant and Food Research and private breeders. In the orchard few if any crops are still grown on the original T-bar system. This has been superseded by the pergola training system and its variants because of improved consistency in yields and fruit quality.
The kiwifruit industry has been going through some considerable trauma since the discovery of the bacterial disease Psa-V on a Bay of Plenty orchard in 2010. While the South Island and some areas remote from the main production areas in the North Island are still free of this pathogen, the disease is now widespread in most kiwifruit-producing regions of New Zealand, with huge economic expense and loss of income for many growers. With this in mind, perhaps it is time for a complete re-think about where the industry is heading and what production systems might be appropriate at this time and into the future. There is no question that the search for resistant or more tolerant varieties/rootstocks is a sound strategic approach, and good progress has been made in this respect, as new cultivars with increased Psa tolerance are helping restore industry confidence and production. Other methods can be used in conjunction with breeding and may be equal, or even superior in the long run, especially if adding resistance genes from other Actinidia species is too much of a long shot in the foreseeable future.
Unlike other areas of the fruit industry there have not been any paradigm shifts in how kiwifruit are grown in New Zealand since the industry was first established in the 1940s as a source of vitamin C for use during WWII.
Liquid water on kiwifruit leaves appears to be a major factor influencing plant response to Psa,presumably because it provides conditions favourable for bacterial growth, reproduction and spread. With global warming and more extreme weather events, this is predicted to become an increasing economic constraint that will impact on production. It raises the question of whether a totally new production system might provide the answer, or is at least worth investigating. Controlling leaf wetness markedly reduces the susceptibility of clean vines to Psa attack. A number of growers are using high tunnels to keep rain water from the plants, and this strategy appears to be working extremely well. (see photo 1). Some of these growers report they now only apply copper sprays to “drop the leaves” at the end of the season. High tunnels would keep the plant dry thereby reducing most of the water on the leaves (and reducing Psa establishment), but the training system would have to be greatly modified to get the best out of the new growing environment. Kiwifruit plant training in the past has been limited to two main methods (T-Bar or Pergola), with no viable alternatives developed in recent times. One possible reason being that dwarfing rootstocks have yet to become a commercial reality and there has been no pressure to fix what is not broken. There is potential to look at a horizontal main stem /planar arrangement that might not only allow easy production in a protected (rainfree) environment, but also enhance productivity, both yield and quality without cincturing. The use of “high tunnel” greenhouses for kiwifruit might require the introduction of bumble bee domiciles for pollination. If they can be trained to work under plastic they would become a useful partner with supplementary pollination.
A permanent single layer espalier system with the fruiting wood held upright as in the UFO (upright fruiting offshoots) system for cherries might provide a possible alternative for kiwifruit growers growing under cover. The winter pruning would then simply comprise cutting the fruiting wood back to the permanent espalier (horizontal cordon) framework. Rows could be much closer together, and spraying (if ever necessary) could be by a permanent set of nozzles. Pollination should be improved if bumble bees were effective in the tunnels, and the need for pollen imports could be eliminated. The male plants could be grown in containers and put in the high tunnels for pollination, and then taken outside after fruit set for the rest of the growing season. Compaction of male plants into pots to encourage spur-type growth would reduce the need for pruning cuts and help manage Psa because of reduced points of entry for bacteria.
There is a need for the kiwifruit industry to follow the lead of the pipfruit, grape and citrus industries, and develop multipurpose rootstocks often from interspecific hybrids which could not only help with disease tolerance, fruit quality and vigour management but also play a useful role in development of high tunnel production systems.
Of course, it goes almost without saying that the quality of fruit produced under tunnels would be superior to that from outdoor crops, and this might include higher dry matter at harvest, earlier maturity, improved flesh colour, increased marketable yield and enhanced eating experience.
As productivity pressure in horticulture is predicted to increase due to population growth and marketing, then our restricted land resources will see the need to make more efficient and intensive use of our high quality land or move to an alternative way of growing food plants. In addition to land becoming a scarce resource it is generally acknowledged that soil in its natural state is often NOT a good medium in which to grow high value crops or at the very least is a factor limiting productivity. Normally it is difficult (if not impossible) to provide plants with the optimum balance of aeration, moisture and nutrients in soil, as when the aeration is ideal, then the plants will often be short of water, and if they have adequate moisture then they may have less than ideal aeration. In other words, control of three critical factors (root aeration, water supply and nutrient levels) is much harder in soil than in artificial substrates, whereas when using hydroponics, it is possible to provide a more optimal root environment. This is the major reason why intensive vegetable, cut flower and nursery production have moved towards soil-less culture over the last 50+ years. Perhaps that time is coming for kiwifruit and other fruit crops. The benefits of making this change have been illustrated in a classical paper by Professor Lim Ho (Fig 1) who showed that in the UK over a 30 year period greenhouse tomatoes in soil increased in yield by an average of 1.7% per year, but those producers who
Kiwifruit Journal (2005) Jan/Feb: 21-23. Tukairangi Orchards. Bay Focus Orchard.
Ltd, Hastings, applied as foliar sprays. To date most published “irrigation” studies to determine water requirements of kiwifruit have been carried out in soil. Using containers filled with an ideal growing medium offers the potential to ensure not only that the plant roots have good aeration and moisture conditions, but also the ability to optimise nutrition according to growth stage. Management of plant growth and development through moisture stress has the potential to control plant vigour and increase fruit. Dry matter becomes an option where plants are container grown in soil-less media.
Re-circulating hydroponic systems (as are becoming common in intensive greenhouse cropping) would introduce the prospect for significant water efficiencies, and minimise nutrient leaching and discharge into waterways. The increased water use efficiency achieved using a more controlled production system are illustrated in Figure 2.
Collecting rain water from the greenhouse covers into reservoirs would not only provide a high quality source of irrigation water, but would also make the whole operation much more water efficient. Ideally, this would reduce the need for any water from other sources. The photograph (Photo 3) shows one of two reservoirs at a greenhouse tomato property in Australia, which supply 60% of water to grow the crop for a full 12 months of the year.
Growers have been told many times that the basics of pruning are simple but everything needs to happen more or less concurrently to be successful, namely (1) summer prune to allow adequate leaves to capture the sunlight required to feed and develop the fruit, and (2) winter prune to select the best fruiting wood and develop a framework
that will support the current season’s fruit and develop reserves to repeat the cycle in the following spring. Do we need to consider a different method of growing kiwifruit to achieve these goals? Certainly the current systems of pruning/training are labour intensive and require knowledgeable staff to implement correctly. The key to any horticultural operation is KIS (Keep It Simple). As kiwifruit flower on 1 year (new) wood, a simple pruning method might be developed using a candelabra or similar training system. This involves establishing two permanent branches (about 0.5-1m high) parallel to the ground, and then allowing the cropping growth to grow from these permanent structures every year. If it is considered necessary the “permanent” branches could be replaced (renewed) every year or so as required.
This training system would allow the fruiting shoots to grow upright, with all the fruit being easily harvested by hand or machine on a vertical growth that might be stopped at 2m simplifying fruit thinning and harvesting.
Summer pruning (where necessary) would only involve the topping of the new growth, and winter pruning the removal of all but the bottom bud or two of the new growth.
All growers recognise the importance of good pollination to producing fruit of high quality. Kiwi fruit have a high requirement for pollen grains (10,000 times) more per fruit compared with apple.
Three methods are currently used to ensure the adequate or saturation pollination of kiwifruit.
New Zealand has 28 native and 13 introduced species of bee. Native bees pollinate many native plants, they are also widespread in kiwifruit orchards and may be important pollinators in horticulture.
Honey bees ( Apis mellifera) are probably the most favoured method, but they are not especially fond of kiwifruit flowers as they show a real preference for flowers with more sugar-rich nectar. Honey Brood Pheromone ( HBP) has stimulated bee activity and pollination in some crops in New Zeland and overseas and may be worth further trialling with kiwifruit. Four species of introduced bumble bees ( Bombus spp.) are among the potentially most satisfactory pollinating insects, as they are predominantly pollen foragers, they are also active under cool windy conditions when honey bees have retreated to their hive.
The problem with both honey and bumble bees is that they lose interest in pollination when they become totally disoriented under certain types of plastic film. Lack of UV light might be the problem, so choice of film will be critical and may be problematic
fruit taste. The disease susceptibility of older A. chinensis seedling stocks will see them eventually eliminated except in niche areas due to a weakness in susceptibility to Psa. The recent rootstock introduction of Bounty 71 has been selected for its reduced vigour and its tolerance of heavier wet soils makes a welcome advance. However, it will take a few years to ascertain how well this will really perform in the field.
We are not aware of any rootstock programme that is currently achieving really marked differences in kiwifruit plant performance especially in the control of excessive vigour. As agronomists it makes little sense to grow leaves and stems which are to be pruned away in the winter, when these resources could (and should) be going into marketable fruit.
By using dwarfing rootstocks and a new training system with sweet cherries it has been possible to greatly modify the production systems enabling the “10 m high trees” to be grown effectively in 4 m high plastic tunnels, using a “candelabra” pruning system. As cherries fruit on 2 year wood the training system is more complex than it would be for kiwifruit since they fruit predominantly on the shoots grown in the previous season.
CONTROLLED DEFICIT IRRIGATION
Managing kiwifruit vigour using controlled deficit irrigation in a controlled environment under a rain shelter may be a viable option. The aim would be to reduce plant vigour without significantly reducing yield, while at the same time increasing fruit dry matter. A normal orchard is not an ideal situation to undertake such studies, as the rooting depth is not controlled, and rain fall can interfere with moisture stress at critical times. This concept could much more easily be tested, and put into practise if the plants are grown in a recirculating (or even a non- recirculating) hydroponic system under a rain shelter.
So where will the industry be in 50 years? We would hope the industry will be sustainably producing increased yields of high quality products using improved production techniques including: growth under cover, soilless culture, nutrient recycling in a closed system with a spray free environment and completely different training system to methods in current production. This may mean that the industry might expand from the Bay of Plenty to other sites, possibly even the West Coast of the South Island, whose temperate climate (apart from rainfall, which would not be an issue with protective tunnels) is not dissimilar from BOP and is Psa-free at present. Wherever the industry is developed the main objectives must be sustainability and profitability.
Zespri invests over $15 million in kiwifruit innovation science each year and the inaugural Kiwifruit Innovation Symposium on October 29 in Mt Maunganui gives people a chance to see the latest developments for themselves.