FINAL DISCUSSION

A.R. Ferguson, G. Costa
For us, kiwifruit are an important crop but the total annual production of about 2 million t is still only 0.2-0.3% of the world’s production of all fresh fruit. The main kiwifruit producers are currently Chile, China, Iran, Italy and New Zealand and together these countries are responsible for more than 80% of commercial production. China now probably produces more kiwifruit than Italy, and the statistics quoted by Ebrahimi et al. (2011) indicate that Iran has replaced Chile as the fourth largest producer of kiwifruit. Chile and New Zealand grow kiwifruit primarily for export, at least 90% of their crop, as does Italy (about 75%) and, to a lesser extent, Iran (about 60%). China exports perhaps 1-2% of its crop and it seems that this is not likely to change during the next few years as the intention is to increase local consumption rather than export (H.-W. Huang, pers. commun.). The requirements of the main producing countries therefore differ: Chile and New Zealand require their kiwifruit to withstand long sea voyages and extended storage whereas the markets for Iranian and Italian fruit are generally much closer. It is not only environmental conditions or consumer preferences that may determine the cultivars that are suitable.
The world kiwifruit industry is facing probable changes in production systems, cultivars and world markets and increasing competition from new and exotic fruit (O’Rourke, 2011). Although international trade in kiwifruit is still dominated by the green-fleshed kiwifruit, ‘Hayward’, growers and retailers are looking for new types of kiwifruit that will give them a competitive advantage. The remarkable success of the yellow fleshed Actinidia chinensis ‘Hort16A’, with its much better (and much needed) returns to growers has encouraged development of other new cultivars. So, to have the “planned scarcity” (O’Rourke, 2011) and exclusive vertical marketing arrangements associated with many of the new cultivars. This has, however, encouraged the development of competing cultivars that are available to those growers who are unable or unwilling to grow the proprietary cultivars under plant protection and exclusive marketing arrangements (Kwack et al., 2011). As in previous Symposia, there were many papers and posters dealing with kiwifruit breeding and genetics or describing promising new selections. Many of these new selections will fail. O’Rourke (2011) considers that “the success of any new cultivar will depend as much on the marketing and promotional expertise of its sponsors as on the innate quality attributes of the cultivar”. The experience with ‘Hort16A’ is also sobering. Under New Zealand conditions it is more vigorous and carries higher crop loads than ‘Hayward’ and vine management had to be changed. Such changes are likely to be needed with other new cultivars; selection of a good-fruited form is just the first step in the successful commercialization of a new cultivar. The management practices that are suitable for ‘Hayward’ (or for ‘Hort16A’) will not necessarily be suitable for new cultivars and their fruit may require different handling and storage conditions. It is also a challenge to develop new cultivars with a storage life approaching that of ‘Hayward’. Marketers should consider how they might handle new types of kiwifruit with many desirable or novel attributes but with significantly shorter storage lives.
It is important to understand the principles of kiwifruit vine management and to learn how to regulate the relative distribution of carbohydrate resources between vegetative growth and fruit production. Comparisons of the management practices of more successful and less successful growers – we hesitate to say “good” and “bad” growers – can be illuminating, especially when combined with replicated trials and physiological studies. Patterson and Currie (2011) give an account of the work that has led to the revolution in vine management practices in New Zealand over recent years. The three main aims were to combine high crop loads and adequate fruit size and high fruit dry matter content. Consistently achieving these three aims in concert required studies of how different kiwifruit cultivars respond to vine manipulations such as girdling practices, canopy design, control of vegetative growth control, crop loading, and fruit growth stimulation by growth regulators. The yields that can be achieved are much higher than what would have been thought possible only a few years ago, 43 t/canopy ha of Class I export ‘Hayward’ fruit (as well as fruit removed in thinning or rejected because they do not meet grade standards), even more for ‘Hort16A orchards, c. 60 t of Class I export fruit (and possibly another 30% not achieving grade standards). It is important to stress that these are the yields of Class I fruit, not total yields, and that they are fruit of high dry matter content because the New Zealand industry believes that dry matter content is a useful indicator of fruit quality as judged by consumers, and growers are paid accordingly. It is good to see the concept receiving support from other countries (Crisosto, 2011). If other quality standards are not jeopardized, much higher yields of fruit should increase the returns to growers or at least reduce costs; conversely, those growers who do not achieve such crop yields may become uncompetitive. However, Burdon et al. (2011) warn that such different management practices that alter the relative allocations of carbohydrates to fruit and leaves may affect the reliability of the 6.2 °Brix standard as an indicator of fruit maturity and appropriate harvest time.
For many years it seemed that once established and if properly maintained, kiwifruit vines were invincible. For sure, there were problems with bud rot of flowers or fruit rots in coolstore but, apart from some losses due to Armillaria, mature vines generally did not die. It now seems that A. deliciosa - or possibly ‘Hayward’ in particular - should be considered as being unusually resilient. Actinidia chinensis, or at least those cultivars grown, seems much more susceptible to disease. ‘Hort16A’ is vigorous, perhaps too vigorous in New Zealand, but it struggled to establish in California and it seems to have failed in Chile because of the devastation caused by Verticillium (Auger et al., 2011). In Italy, ‘Hort16A’ plantings are now suffering from outbreaks of the bacterial disease caused by Pseudomonas syringae pv. actinidiae (Balestra et al., 2011; Vanneste et al., 2011). Experience with fireblight suggests that such bacterial diseases of fruit trees can be managed, albeit with difficulty. Much stricter orchard hygiene and altered management may be effective in limiting the spread of this new disease of kiwifruit. At least some of the other cultivars of A. chinensis grown in Italy seem susceptible, even ‘Hayward’ can be affected. Breeding for resistance is an obvious solution, but is certainly not a “quick fix”. It would require a change in thinking because up to now, the emphasis in selection by breeders has been mainly on fruit quality, crop yield and fruit storage life. We need to know much more about the basic biology of the disease (Spinelli et al., 2011) and how the causal organism is transmitted. This could be an ideal subject for international cooperation because it is a problem that threatens all of us.
The number of viruses identified in various kiwifruit accessions continues to increase (Pearson et al., 2011). Most of these have been identified in germplasm plants, not from vines in commercial production, but it is possible that some of the latter could be infected. Fortunately there is no indication that virus infection is having any marked effect on vine growth or productivity but the possibility of infection should be considered when plant material is being exchanged or propagated, especially for commercial release. One of the most pleasing features of this Symposium was the attendance of so many research workers from China, many more than have attended any previous Kiwifruit Symposium except that held in Wuhan in 2002. Even better, many of those from China are young scientists who are developing their careers in kiwifruit research. This augurs well for the long-term future of the Chinese kiwifruit industry. China’s other great asset is, of course, its matchless Actinidia germplasm resources. China is the centre of evolution of the genus and almost all Actinidia species are restricted to China. The species that occur in neighboring countries are best considered as outliers and most of them also occur within China. Although China has some important ex situ germplasm collections, an ideal study of the biology of the genus requires examination of the wild, in situ resources. Li et al. (2011) point out that for taxonomic revisions of the genus to be useful, herbarium specimens need to be collected throughout the entire distribution range of each taxon. Too many descriptions of Actinidia taxa rely on an inadequate number of specimens, sometimes at worst, a single specimen. Likewise, they comment that many of the phylogenetic studies on the genus by both Chinese workers and those outside China may suffer because of the use of only a limited number of taxa within the genus, not necessarily representative of the genus as a whole, and usually only a limited number of individuals from a taxon, sometimes even only a single representative. Frequently phylogeneticists have used the same genotypes which have been exchanged between germplasm collections outside China.
The benefits of comprehensive studies of wild resources are demonstrated by Kataoka (2011) who made a detailed study of Japanese populations of A. rufa and of A. arguta and the closely related species A. hypoleuca, sometimes included within A. arguta sensu lato. His study was possible only because flow cytometry makes it possible to study the ploidy of many individuals within a population. He reports for the first time tetraploid as well as diploid races of A. rufa, another indication of just how common ploidy variance is within Actinidia. The discovery of tetraploid plants of A. rufa may facilitate its crossing with tetraploid A. chinensis and the transfer of the desirable trait in A. rufa of low chilling requirements. His study of A. arguta shows a fascinating geographical localization of diploid, tetraploid, hexaploid, heptaploid and octoploid ploidy races of A. hypoleuca and A. arguta. His findings may help resolve some of the taxonomic complexities of A. arguta within Japan.
Chinese, New Zealand and Italian colleagues described the results of their breeding programs. New selections of A. chinensis and A. deliciosa with yellow, green or red flesh as well as of species such as A. arguta and A. erianth could soon be released raising the possibility that kiwifruit cultivation could extend to new areas. However it was emphasised that new cultivars should be carefully evaluated in each different environmental condition before introduction. Furthermore, it is clear that breeding for disease resistance is necessary when new diseases (e.g., bacterial canker or PSA) are becoming increasingly widespread and jeopardising cultivation.
Ferguson, A.R. and Costa, G. (2011). FINAL DISCUSSION. Acta Hortic. 913, 687-689
DOI: 10.17660/ActaHortic.2011.913.96
https://doi.org/10.17660/ActaHortic.2011.913.96
English

Acta Horticulturae