THE KIWIFRUIT

Introduction

The kiwifruit of international commerce are large-fruited selections of Actinidia chinensis Planch. and Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson, Actinidiaceae. Kiwifruit are among the most recently domesticated of allfruit plants: A. chinensis has been cultivated commercially for little more than 20 years, A. deliciosa for about 70 years. The common name ‘kiwifruit’ is itself very recent, being devised in 1959 originally for the fruit of A. deliciosa but now being increasingly applied to fruit of other Actinidia species, such as Actinidia arguta (Sieb. et Zucc.) Planch. ex Miq., the ‘hardy’ kiwifruit or ‘baby kiwi’. Kiwifruit has completely replaced the older name of ‘Chinese gooseberry’. In China, ‘mihoutao’ (monkey peach) is used for all Actinidia species, but as A. chinensis and A. deliciosa are by far the most important economically of the various Actinidia species in China, they are often simply referred to together as mihoutao. More explicitly, the two species are sometimes referred to together as zhonghua (Chinese) mihoutao.

Although relative newcomers to the fruit bowl, kiwifruit have found ready acceptance among consumers. Externally, kiwifruit are distinctly different from other fruit. Once cut open, they are strikingly beautiful with a strong radiating pattern of lighter-coloured rays interspersed by several rows of small dark brown-black seeds. The core is creamy white and the outer flesh is a bright green or a sharp yellow. It is thus not surprising that kiwifruit are often used for food decoration rather than just being eaten. The fruit flavours are distinctive but very different in yellow and in green kiwifruit and appeal to largely different sectors of the consuming public. Ripe fruit of A. arguta can have a superb, aromatic flavour.

History and origins

Fruit of different Actinidia species have been collected from the wild for many centuries but widespread cultivation was first attempted only during the last 100 years. Actinidia arguta was being grown in gardens in Europe and North America during the last decades of the 19th century but its commercial potential has yet to be realized. Actinidia deliciosa was first grown in orchards in New Zealand in the 1920s, and cultivation of A. chinensis started half a century later.

Seed and plants of A. deliciosa were sent from China to Europe and to the USA from 1898 onwards, but establishment of the plants was restricted by their climatic requirements, the need for both male and female plants for fruiting, and by the onset of World War I. Seed of A. deliciosa also went to New Zealand in 1904 and there the circumstances were better and the plants prospered. The first small orchard was fruiting by the early 1930s but cultivation remained for many years on a very small scale with just enough fruit being produced for the New Zealand market. Exports, starting in 1953, were at first considered as a means of absorbing surplus fruit that could not be sold locally. Promotion and acceptance of kiwifruit on the export markets resulted in much better returns to growers from the mid-1960s so that production expanded rapidly and by 1976 exports of kiwifruit from New Zealand exceeded local consumption. The subsequent development of the New Zealand kiwifruit industry was the development of an industry geared towards export. Growers in other countries were encouraged to start growing kiwifruit and they used the cultivars first developed in New Zealand. These cultivars were all of A. deliciosa and so it was the hairy skinned kiwifruit with green flesh that became known to consumers throughout the world.

Domestication of A. chinensis is even more recent. The first experimental plantings of this species were established in China in 1957. Subsequently, a survey was made of wild kiwifruit resources in China and many superior genotypes of both A. chinensis and A. deliciosa were selected for further evaluation. These became the basis for most commercial plantings in China. Seed of A. chinensis were imported into New Zealand in 1977. The populations raised from these seed are probably the first plants of A. chinensis to have been grown outside of China, and are certainly the first such plants known to have survived. Since then seed of A. chinensis or budwood of superior selections have been introduced into many different countries.

World production

Kiwifruit are still a minor crop compared to other fruit such as apples, bananas, grapes or citrus. Production of kiwifruit amounts to perhaps 0.2% of total world production of fruit. In only a few countries are kiwifruit an important crop and in New Zealand they are uniquely important, now being the single most valuable horticultural export.

Actinidia chinensis and A. deliciosa are widespread in China and appreciable quantities of fruit (100,000–150,000 t) are collected from the wild each year. About two-thirds of this is of A. chinensis, one-third of A. deliciosa and, in addition, there are much smaller quantities of fruit of other Actinidia species. Harvesting of wild fruit often involves pulling vines down out of trees and this, together with general deforestation, means that the natural resource is under serious threat. However, kiwifruit collected from the wild are often very small or are otherwise of poor quality and they are becoming less important as commercial cultivation increases in China.

In 2002, the total area throughout the world planted kiwifruit was about 120,000 ha. China had 60,000 ha of kiwifruit orchards, Italy 20,000 ha, New Zealand 12,000 ha, Chile 8000 ha and all other countries (mainly Greece, France, Japan and the USA in decreasing importance) a total of about 20,000 ha. Considering only cultivated kiwifruit, in 2002, total world production was 1,350,000 t: China produced 350,000 t, about the same as Italy, New Zealand 250,000 t and Chile 150,000 t, so together, these four countries accounted for over 80% of world commercial kiwifruit production.

Approximately 7.5% of current commercial production would be of cultivars of A. chinensis (almost entirely from China and New Zealand) and the remainder of A. deliciosa, of which about 80% would be of ‘Hayward’. This cultivar still dominates international trade but cultivars of A. chinensis, especially ‘Hort16A’, are rapidly becoming more important.

Actinidia arguta remains potentially important but at present world plantings amount to only 100–200 ha.

Uses

Kiwifruit are produced primarily for fresh consumption, either scooped out with a spoon or peeled. Fruit of A. arguta are much smaller, about the size of a grape, and can be readily eaten whole. For all kiwifruit, the premium product is the fresh fruit, particularly if these can be stored for extended periods while still retaining quality attributes such as flavour.

Processing usually results in major changes to the colour, aroma, taste and texture of kiwifruit and most processed products lack the appeal of fresh kiwifruit. In China, fruit of A. chinensis are often preferred for processing because they are sweeter and because their fruit flesh has a clear yellow colour whereas the chlorophyll-based green of A. deliciosa fruit changes to a ‘dismal’ brown on processing. Increased production of yellow-fleshed kiwifruit in countries such as New Zealand may provide the raw material for more successful processed kiwifruit products.

Processing of kiwifruit is more important in China than in other producing countries, partly because its storage facilities and transport systems are still being developed. Even so, most fruit are eaten fresh, and only 20–30% of the Chinese kiwifruit production is processed into a variety of products such as fruit juices, either natural or clarified, juice concentrates, jams, fruit preserved either whole or sliced in syrup, dried fruit, soft drinks and wine and spirits.

Processing is even less important outside of China. The other three major producers of kiwifruit, Italy, New Zealand and Chile, are dependent on export of fresh fruit. New Zealand has a particularly small home market and 85–90% of kiwifruit produced each year are exported as fresh fruit. Chile and Italy each export about 75% of the kiwifruit they produce. Processing in these countries is still largely an attempt to make use of the significant quantities of fruit that do not meet export standards. Successful processing will probably require identification of specific compounds or combinations of compounds that bestow valuable nutritional or textural advantages on processed products.

Health benefits

Kiwifruit are among the most valuable nutritionally of all readily available fruit. Most published information is for A. deliciosa ‘Hayward’ (Table A.1) but the composition of A. chinensis ‘Hort16A’ is similar.

Kiwifruit are a good source of potassium and have a high potassium:sodium ratio. They are also a useful source of magnesium but other minerals are not sufficient to make a significant contribution to the diet.

They contain about 2–3% dietary fibre owing to pectins, oligosaccharides and polysaccharides that are not broken down and absorbed in the small intestine. A 100 g serving of kiwifruit will supply about 10% of recommended daily requirements.

Depending on the individual, kiwifruit can be a strong laxative, both as bulking agents and for stimulation of motility. Fresh kiwifruit, kiwifruit juice or dried products are often used to maintain regularity in bowel movement, especially for older or sedentary people. There is a huge demand throughout the Western world for natural laxative products and the laxative content of kiwifruit may well prove to be, next to their high vitamin C, their single most valuable attribute contributing to health.

Kiwifruit are outstanding for their vitamin C content. ‘Hayward’ kiwifruit when harvested contain about 85 mg ascorbate/100 g fresh weight and very little of this is lost on storage or ripening. Fruit stored for 6 months at 0°C and then ripened will still contain at least 90% of the vitamin C present in the fruit at harvest. Two medium-sized ‘Hayward’ kiwifruit, even after prolonged storage, can therefore easily satisfy recommended daily requirements (USA) for vitamin C, which range from 30 mg for a child to 120 mg for a lactating mother. ‘Hayward’, however, contains only relatively modest amounts of vitamin C compared to many other kiwifruit cultivars: A. chinensis ‘Hort16A’ usually contains 20–30% more vitamin C than ‘Hayward’ and many commonly grown kiwifruit cultivars in China contain at least twice as much. Fruit of other Actinidia species can contain much more vitamin C, up to 1% fresh weight.

Kiwifruit, however, also have some potential disadvantages such as high oxalate content and allergenic activity. ‘Hayward’ kiwifruit contain appreciable amounts of oxalate but these are insufficient to cause a nutritional problem, assuming normal consumption. Nevertheless, eating some processed kiwifruit products such as nectars, dried slices or fruit ‘leathers’ can cause irritation of the mucous membranes of the mouth which is due, at least in part, to mechanical irritation of the membranes by oxalate raphides.

Kiwifruit also contain allergens that can cause allergic responses in susceptible consumers, possibly 2–3% of the total population. The risk should not be exaggerated: a higher proportion of the population in some countries show some adverse reactions to apples, among the most common of all fruit. Fortunately, extreme allergic responses to kiwifruit are not frequent.

‘Hayward’ kiwifruit contain large amounts of the highly active proteolytic enzyme actinidin (E.C. 3.4.22.14). Actinidin has been implicated in both the laxative and the allergenic properties of kiwifruit but the enzyme activity does not appear to be a major health hazard for most people. Actinidin can cause problems if fresh fruit are incorporated into gelatine-based jellies or are mixed with dairy products.


The anticancer and antimutagenic potential of the fruit are now being studied. The antioxidant capacity of kiwifruit likewise requires more study.

Botany

TAXONOMY AND NOMENCLATURE 

Kiwifruit belong to the genus Actinidia Lindl., an Asian genus of some 70 species. The defining characteristics of the genus are:

● all members of the genus are climbers or scramblers;
● all species are dioecious;
● the ovary of female flowers is formed by fusion of the lower parts of numerous carpels but the upper parts of the carpels remain free forming a distinctive circle of radiating styles;
● the fruit are botanically berries with many seeds embedded in a fleshy pericarp.

The familiar green and hairy kiwifruit belongs to A.deliciosa. Until recently, A. deliciosa was treated as a variety of A. chinensis and was raised to species status only in 1984: prior to that date, most references in the literature to A. chinensis actually refer to what is now known as A. deliciosa.

MORPHOLOGY

Kiwifruit are vigorous vines which, in the wild, can grow to the tops of trees, 5–6 m high. Cultivated plants are tightly controlled into a single trunk, usually about 1.8 m high. The main branches form a permanent framework and the younger shoots are replaced every 2 or 3 years. Shoots of the current season come from axillary buds on canes produced in the previous season. Canes with lateral and second-order lateral shoots are the typical fruiting units of kiwifruit vines.

The canopy of a typical orchard vine has a leaf surface area of 30–40 m2 made up of 4000–5000 leaves. Mature leaves are large, up to 20 cm in diameter, and the lower leaf surface has a thick felting of stellate hairs. Vines are deciduous and the overwintering buds of A. chinensis and of A. deliciosa are characteristically different. In A. chinensis, the bud base is small, and the bud is relatively exposed, being protected only by bud scales; in A. deliciosa, the bud base is large and protruding and the bud is almost completely submerged in the bark.

Flowers are borne either singly (as in most female cultivars) or in small inflorescences of five to seven flowers (as in most male cultivars). In general, vines do not flower until 3 or 4 years old, but A. chinensis is noticeably more precocious than A. deliciosa. Flowers are borne in leaf axils towards the base of flowering shoots, never terminally. Pistillate flowers are generally larger than staminate flowers. Flowers are cupshaped, facing downwards, with five or more petals which are white on opening but within a few days become a rather dirty golden. The gynoecium is surrounded by whorls of stamens with bright yellow anthers (almost black in A. arguta). Both pistillate and staminate flowers have a distinct odour. Flowering occurs about 2 months after budbreak: flowers are differentiated in spring and weather conditions during this period can have a marked effect on flower number and development. Female flowers remain receptive for about 4 days after opening.

The fruit is a berry with many small seeds embedded in the juicy flesh (about 250 in a 12 g fruit of A. arguta, at least 500 in an average-sized ‘Hort16A’, more than 1000 in a ‘Hayward’ fruit) (see Plate 5). Fruit shape and hairiness vary greatly but fruit of A. chinensis are usually covered with soft, downy hairs which are often shed early in fruit development; those of A. deliciosa have persistent, long, hard, bristle-like hairs which are only partially removed during grading and packing. Fruit of most commercial cultivars have an elongated ovoidal shape, are the size of a large hen’s egg and weigh, on average, 80–110 g.

The tough hairy skin of fruit of A. deliciosa is certainly unpalatable, while that of A. chinensis fruit may not be as hairy but is still unpalatable and would not normally be eaten. Consumption of these kiwifruit requires a knife and a spoon. Some Actinidia species, such as A. arguta and Actinidia kolomikta (Maxim. et Rupr.) Maxim., have fruit with smooth, hairless, edible skins. They are ‘snack’ fruit, readily eaten without creating a mess.

The bright green colour of the fruit flesh of A. deliciosa and some cultivars of A. chinensis is due to chlorophyll which is largely retained during fruit maturation, storage and ripening. In most A. chinensis cultivars, such as ‘Hort16A’ and ‘Jinfeng’, the fruit flesh is yellowish green to yellow owing to partial or complete loss of the chlorophyll. ‘Hongyang’ (also A. chinensis) is even more striking as the flesh around the seeds is red, making a cross-section of the fruit most attractive.

REPRODUCTIVE BIOLOGY 

Every female kiwifruit flower normally sets to form a fruit. There is little subsequent fruit drop so crop load is largely determined by the number of flowers. Flower numbers are often limiting and much of vine management is aimed at ensuring that sufficient flowering wood of the desired type is retained at pruning.

All Actinidia species are functionally dioecious: flowers of pistillate kiwifruit may look perfect but the pollen produced is non-viable; staminate plants produce viable pollen but have only a vestigial ovary and poorly developed styles. Dioecism is not absolute and different states have been identified: male, inconstant male, hermaphrodite, inconstant female, female and neuter. Gradients of male or female sterility are also found. Among the most easily noticed variants are fruiting males which carry both staminate flowers and bisexual flowers which have small ovaries, a few ovules and limited stylar development: these can produce small fruit.

Most taxa within Actinidia are diploid, x = 29, an unusually high number possibly indicating ancient polyploidy, but some are tetraploid (4x), hexaploid (6x) or octaploid (8x). There may also be intrataxon variation in ploidy: for example, most plants so far studied of A. chinensis are diploid but plants from a restricted part of the total geographic range of the species are tetraploid. These ploidy races cannot be distinguished morphologically and the only consistent difference so far noticed is that tetraploid genotypes of A. chinensis flower about 2 weeks later than diploid genotypes. Most important A. chinensis cultivars in China are tetraploid but ‘Hort16A’ is diploid. All cultivars of A. deliciosa are hexaploid, 2n = 6x = 174. Most cultivars of A. arguta are tetraploid, 2n = 4x = 116, but several, known as ‘Issai’ are hexaploid.

Dioecism and ploidy variation have important practical consequences. A kiwifruit orchard must contain both male and female plants for transfer of pollen and seed set. Each kiwifruit that is of commercial size contains many seeds and there is a strong correlation between seed number and fruit size. Efficiency of pollen transfer is one of the most important factors in determining crop yield. In commercial orchards, about 10% of the canopy area is allocated to male vines set out in a regular array to ensure that male and female flowers are in close proximity. Male vines must coincide in flowering with female vines, they should have an extended flowering period and they should carry heavy flower loads producing large amounts of viable pollen capable of setting seed. Most pollen transfer is effected by honeybees which are brought into the orchard as female vines start flowering. Kiwifruit flowers are not particularly attractive to bees and many aspects of orchard layout and management are designed to keep bees working, thereby ensuring good pollination. Mechanical pollination, using pollen collected from male vines, is sometimes used to supplement natural pollination.

Variation in ploidy level had no practical significance when only cultivars of A. deliciosa were grown. However, the cultivars of A. chinensis now grown are diploid or tetraploid. To ensure fertilization and continued seed growth, female vines and their pollinators must flower at the same time and should preferably be at the same ploidy level. Nevertheless, ploidy levels may not always be critical as A. deliciosa pollen, which had been collected and stored, has been successfully used for mechanical pollination of diploid ‘Hort16A’.

FRUIT GROWTH AND DEVELOPMENT 

Fruit of small-fruited Actinidia species, such as A. arguta, can reach 80% of their final size after less than 6 weeks growth and can be harvested only 100–110 days after pollination, whereas those of A. chinensis and A. deliciosa are typically harvested 180–210 days after pollination, depending on cultivar and climate. During development of the ovary into the mature fruit, linear measurements increase six- to tenfold and fresh weight and volume increase several hundredfold. About two-thirds of this increase in volume or in weight occurs in the first 10 weeks after pollination and kiwifruit growth shows the double sigmoidal curve typical of many fruit: a period of very rapid growth for the first 8 weeks (a period during which fruit of ‘Hort16A’ can increase by 1.6 g/day), a subsequent 3 weeks of slower growth, followed by a second period of more rapid growth. Cell division in the inner and outer pericarp ceases after the first 3 or 4 weeks and subsequent growth is almost entirely due to cell enlargement.

The chemical composition of the fruit changes during growth and maturation. Total solids increase over much of the growing season but the proportion of carbohydrates present as starch or as soluble sugars changes. Starch at its highest can account for half the total dry weight of the fruit but about 140 days after pollination, starch begins to decrease and there is an increase in soluble solids owing to conversion of the starch and to translocation of sugars from the rest of the vine.

During later stages of fruit growth in A. deliciosa, the internal appearance of the fruit changes little apart from seeds changing colour and the loss of starch and softening of the tissues making the fruit seem juicier and greener. Fruit of many A. chinensis cultivars are green during initial growth and development but during fruit maturation chlorophyll is lost so that the fruit flesh becomes yellow.

CLIMATIC REQUIREMENTS 

Wild A. chinensis and A. deliciosa occur mostly on steep hills and mountain slopes. They grow in relatively damp and sheltered areas and are seldom found where there is little shade or moisture or where they are exposed to strong winds. Young plants in particular do best in shade but sun is required for fruiting. Kiwifruit are abundant in gullies, under the tree canopy or on forest edges where they can scramble up through the trees. Winter temperatures can fall well below 0°C but there is need for a long frost-free period and abundant rain during the growing season. These conditions indicate the ideal conditions for successful growth and cropping of kiwifruit, climatic conditions that are generally restricted to between latitudes 25° and 45°. Kiwifruit can be grown extraordinarily well under the temperate conditions in the Bay of Plenty, New Zealand, but they are by no means restricted to such conditions and can be successfully cropped under more rigorous conditions, in California, Chile or southern Italy, if the vine management techniques first devised for New Zealand are modified.

Actinidia species differ in their climatic requirements, as indicated by their natural distributions in China. Actinidia chinensis is found mainly to the east, A. deliciosa more inland in colder regions and where both species occur in the same area, they are separated vertically, with A. deliciosa at the higher, colder altitudes. Therefore A. chinensis is likely to be more susceptible to winter cold and to spring frosts, especially as it breaks bud and flowers about a month ahead of A. deliciosa.

Kiwifruit are temperate plants requiring a period of winter chilling for adequate budbreak and flowering. Sufficient winter chilling condenses the period of budbreak, budbreak is more uniform, there are more flowers and a condensed flowering period which should reduce fruit to fruit variation in growth and maturity. Inadequate winter chilling can be a serious problems in areas with relatively mild winters but cool springs.

Sprays of dormancy-breaking chemicals such as hydrogen cyanamide can then be applied. Winters can, however, be too cold. In such places, species such as A. arguta will grow and crop even if mid-winter temperatures drop to -30°C, conditions that other kiwifruit will seldom survive. This is not surprising as A. arguta occurs naturally at latitudes much further to the north or at much higher altitudes in the south where A. chinensis and A. deliciosa are common.

Kiwifruit are susceptible to spring and autumn frosts. They flower about 2 months after budbreak and fruit are ready for harvest 5–6 months after flowering. A frost-free growing period of 7–8 months is required.

Kiwifruit vines have very large leaves and very high rates of water conductivity and transpiration. Transpiration rates can reach 80–100 l/day. During the growing season, vines need 800–1200 mm of water evenly distributed. Vines are prone to water stress on windy days or hot sunny days and this can result in reduced fruit growth. If water is limiting during early fruit growth, any reduction in fruit size is irreversible.

A characteristic feature of kiwifruit orchards in New Zealand is the shelterbelts. New Zealand is particularly windy but inadequate shelter is a major limitation to successful kiwifruit cultivation in many parts of the world. Young vigorous shoots that eventually form fruiting canes in the following season are easily blown out in spring and windrub of developing fruit is a major cause for rejection at grading. Establishment of young vines can also be affected by wind. However, there must be a compromise: shelter may be required but living shelterbelts compete with vines for water and nutrients and excessive shading can affect vine growth and flower evocation.

Hail can cause severe damage to young shoots and leaves or fruit. Hail nets are used in some regions.

Kiwifruit may appear to be demanding in their climatic requirements but these are largely the conditions for which cultivars were originally selected in New Zealand. These cultivars may not be well adjusted to other climatic conditions but likewise cultivars selected under more extreme conditions in continental China may not be well suited to New Zealand or other temperate countries. Management practices may have to be modified to particular environments.

Horticulture

PROPAGATION 

Kiwifruit propagation is easily achieved by micropropagation, by rooted cuttings or, occasionally, by root cuttings. Such plants are clonal. Many are also produced by grafting onto seedling rootstocks. Under good growing conditions the method of propagation has no important effects on vine vigour or productivity although it might be expected that plants grafted onto seedling rootstocks would be more variable. Very little use has so far been made of the few clonal rootstocks that have been selected.

Mature plants can also be readily reworked. This allows rapid conversion of an established orchard to a new, more profitable cultivar or replacement of males by better pollinators. In New Zealand, many mature ‘Hayward’ orchards have been converted to ‘Hort16A’ by decapitating the plants and grafting onto the stumps. The existing root systems allow rapid development of the new canopy with good commercial yields of the new fruit being achieved in the second or third year after grafting. Such plants usually consist of a seedling rootstock (A. deliciosa), an interstock of ‘Hayward’ (also A.deliciosa) and a canopy of ‘Hort16A’ (A. chinensis).

SUPPORT STRUCTURES 

Kiwifruit vines are large and individual plants can carry 100 kg of fruit. Vines are not selfsupporting and they require support structures that are strong and can last for 50 years or more. It is false economy to skimp on support structures. Fruiting canes must be firmly held in position so that they are not blown out or the fruit damaged by windrub. Two main types of support structure are used: the pergola maintains fruiting canes in a plane about 1.8 m above the ground; with T-bars, the structure of the vine is essentially similar with fruiting arms held in a fixed position but hanging towards the ground. T-bar systems are somewhat cheaper and easier to manage but pergolas give higher yields of good quality fruit as these are less susceptible to wind damage or to sunburn. Pergolas are more common for ‘Hort16A’ whose fruit have thinner skins and are consequently more easily damaged. Generally there are 400–500 plants/ha.

TRAINING AND PRUNING 

The aim is to establish:
● a well-organized framework of permanent branches;
● a balance between vegetative growth and fruit production;
● a canopy that intercepts light efficiently but is open enough to allow sufficient light through for flower evocation and fruit quality;
● a canopy open enough to allow ready access by bees and to reduce the incidence of diseases such as Botrytis but not so open or uneven that wind can cause fruit damage;
● a canopy that is easily managed and harvested and keeps vines to their allocated spaces;
● a canopy that allows the ready production of fruit of the size and quality required by the market.

Flowers are produced only on shoots of the current season and usually only on shoots growing from 1-year-old wood. Fruiting canes should therefore be replaced on a regular 2- or 3-year cycle. New canes should be evenly spaced and at winter pruning sufficient new wood should be left to provide the appropriate fruit load – in Italy, about 15–20 winter buds/m² canopy are recommended for ‘Hayward’.

The number of winter buds retained at winter pruning can be modified with experience: observations over several seasons will indicate the likely percentage budbreak, the number of shoots that will carry flowers, and the number of flowers per flowering shoot. Higher crop loads can reduce average fruit size, but returns are usually better for larger fruit. Much new vegetative growth is removed during summer to ensure that, while replacement canes are retained, the canopy does not become too dense or tangled. The amount of summer pruning will depend on the climate and cultivar as well as the need to protect fruit from sunburn. With ‘Hayward’, typically 60% of the above-ground mass of the vine is removed in prunings, leaves and fruit each season.

Individual cultivars respond differently to management. ‘Hayward’ is one of the least precocious and tends to carry lighter crops and selection of new fruiting wood is therefore important. Differences in phenology are also important. ‘Hort16A’, when grown under the same climatic conditions as ‘Hayward’, breaks bud and flowers a month earlier. It is particularly vigorous, especially when grafted onto mature rootstocks, and vegetative growth continues about a month later in the season with considerable production of secondary shoots. Canes developing late in the season are less productive than those that grew earlier and also tend to flower later with the fruit maturing later. However, very vigorous canes from early in the season are also not ideal. Management techniques devised for ‘Hayward’ cannot be simply transferred to other cultivars. Yields vary greatly according to country and cultivar. New Zealand orchards produce about 25 t of ‘Hayward’ fruit per canopy ha which equates to about 6000 trays of export quality fruit. ‘Hort16A’ under the same conditions typically produces higher yields, 10,000 to 12,000 trays/ha, because of its growth habit and because it is more floriferous. The size of ‘Hort16A’ fruit is routinely increased by use of biostimulants such as Benefit®PZ. This combination of higher yields and large fruit sizes has made ‘Hort16A’ more profitable for growers. Male plants are usually pruned rigorously immediately after flowering so that their vigorous growth does not shade out neighbouring female plants.

THINNING 

If required, thinning is carried out immediately after fruit set with removal of lateral or misshapen fruit and then any surplus fruit. If too many winter canes have been laid down, the weaker canes can be removed.

FERTILIZATION

Considerable amounts of nutrients are removed from orchards but the need for fertilizers should be determined by leaf or soil analysis. Nitrogen deficiency can cause marked reductions in vegetative growth and yields whereas excesses are believed to affect fruit quality and storage. Potassium and calcium also affect fruit quality. In some areas, iron deficiency is common. There has been little work to determine relative requirements of different cultivars or at different crop loads.

SMAIN DISEASES AND PESTS

Similar types of pests occur on kiwifruit in the countries in which they are grown and all tend to be generalists affecting a broad range of plants. Armoured scales are generally the most serious but although the species involved are cosmopolitan, the abundance of a particular species varies according to country. The other main group of pests, the leafrollers, tend to be specific to each country and are therefore a quarantine problem as well as damaging the fruit. Nematodes are a problem in some countries.

Kiwifruit are also susceptible to bacterial and fungal diseases. Pseudomonas species cause bacterial canker, bacterial necrosis and, potentially most serious, bacterial blossom blight. Sclerotinia can also affect fruit on the vine but the other serious fungal diseases are those that develop while the fruit is in storage (mainly Botrytis cinerea) or after the fruit is taken from storage (e.g. Botryosphaeria dothidea).

When kiwifruit were domesticated, they were freed of many of the pests and diseases to which they are prone in China. However, as plantings have increased so too have the problems. Fortunately, the number of pests on kiwifruit is still fairly limited and they can be well controlled by integrated pest management systems. Organic production is realistic.

MATURITY INDICES, HANDLING AND POSTHARVEST STORAGE

Kiwifruit harvested prematurely have a poor colour, an inadequate flavour when ripened, and a shortened storage life. The criteria used to decide when to harvest depends on the cultivar or species. In A. deliciosa, there are no useful visible indications of maturity, but the rapid increase in soluble sugars at the final stages of fruit growth is a useful indicator of physiological maturity. Fruit harvested at a soluble solids content (SSC) of 6.2–6.5% will store well and be acceptable when ripened.

Dry matter is also used as a maturity index. On the basis of measurements such as SSC and firmness, ‘Hort16A’ fruit reach physiological maturity about 1 month ahead of those of ‘Hayward’, but they are not harvested until about the same time as ‘Hayward’ because they are promoted for their yellow flesh colour. Loss of chlorophyll is very slow once ‘Hort16A’ fruit are picked and in storage, so they must remain on the vine until the flesh has a hue angle of 103° or less. At this stage, firmness may be only 4–5 kgf (as compared to about 7 kgf for ‘Hayward’) and SSC more than 10%. Fruit of A. arguta ripen unevenly on the vine and are therefore harvested when they reach a dry matter (DM) threshold (approximately 20% DM) and about 1% of fruit on the vine are soft. Fruit harvested earlier are firmer, and are therefore easier to handle, but do not store as well. Fruit harvested too late are unmanageably soft and susceptible to mechanical injury.

Once maturity has been reached, the whole crop is normally harvested. Although fruit are still firm, they must be handled gently. At harvest, ‘Hort16A’ fruit are softer than those of ‘Hayward’ and they are more vulnerable to damage, especially because they have the added problem of the sharp ‘beak’ at the distal end of the fruit. The fruit are not cooled immediately but left at ambient for several days as this curing helps reduce the incidence of Botrytis stem end rot.

When harvested sufficiently mature, kiwifruit can be stored for very long periods. ‘Hayward’ is exceptional and can be stored for up to 6 months in air under refrigeration and the fruit will still be acceptable. Controlled atmosphere storage can extend the storage life even further. However, there can be a loss of flavour on long-term storage. Other cultivars of A. deliciosa tend to store less well. Cultivars of A. chinensis generally have relatively short storage lives and ‘Hort16A’ is one of the better ones with an expected storage life of 12–16 weeks.

Kiwifruit are stored at or close to 0°C. They are very firm at harvest and soften in store. There is a period of very rapid softening down to about 1 kgf and then a slow and gradual softening. Excessive softening, low temperature disorders such as physiological pitting and low temperature breakdown, shrivelling from water loss and fungal pitting or stem end rots all mark the end of storage life in different cultivars. Low temperature disorders can be avoided by storage at higher temperatures but then the fruit soften more quickly. They are very sensitive to ethylene and fruit softening in coolstore is accelerated if even low levels are present. Shelf life is 3–10 days depending on the preceding storage period. Fruit of A. arguta are more delicate than fruit of other commercial kiwifruit species and their storage life is correspondingly shorter, at most 10–12 weeks.

MAIN CULTIVARS AND BREEDING

In most countries with commercial kiwifruit orchards, ‘Hayward’ (A. deliciosa) is the only fruiting cultivar grown and it has become the kiwifruit in the marketplace (Table A.2). Thus ‘Hayward’ currently accounts for about 75% of world kiwifruit production, 97.5% of kiwifruit production outside of China. It was originally selected because its fruit are large, have a good flavour and can be stored for extended periods while still remaining acceptable to consumers. The relative flowering times of ‘Hayward’ and its pollinators can vary according to climate so, although ‘Hayward’ is grown in many different countries, the accompanying males grown may vary from country to country.

The situation in China is very different. ‘Hayward’ is grown but, although it is the second most widely planted kiwifruit cultivar in China, it accounts for only 13% of the total area planted in kiwifruit. ‘Hayward’ and the eight other most common cultivars account for only 80% of total plantings. There is a strong preference for cultivars selected locally and most are largely restricted to one or two, usually contiguous, provinces. Thus ‘Qinmei’ (A. deliciosa) comprises about 30% of all kiwifruit plantings in China but is predominantly confined to Shaanxi, the province in which it was selected. The Chinese kiwifruit industry will probably consolidate on fewer cultivars of more consistent fruit quality.

The many kiwifruit cultivars in China are selections from the wild. ‘Hayward’ and ‘Bruno’ (also from New Zealand) were selections from small seedling populations only one or two generations removed from the wild. Only one successful cultivar, ‘Hort16A’, has so far resulted from deliberate breeding programmes. With its distinctive appearance, its golden-yellow flesh, and its very different, sweeter, ‘subtropical’ flavour, ‘Hort16A’, commercialized under the name ZESPRITM GOLD Kiwifruit is perceived as giving the New Zealand kiwifruit industry a competitive advantage. It is the first cultivar of A. chinensis to be traded internationally and its success is likely to encourage the development of competitive cultivars, either some of the existing Chinese cultivars or cultivars specifically bred for the purpose.

By Ross Ferguson in "The Encyclopedia of Fruit & Nuts",edited by Jules Janick & Robert E. Paull, CAB international, UK/USA, 2008,  excerpts 22 (1)- 28(7). Adapted and iluustrated to be posted by Leopoldo Costa.

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