Evaluation and Selection of New Cultivars
To some people, plant breeding may simply mean crossing two flowers in the hope of producing a new cultivar. In some cases, this may in fact give the desired result. Genetic improvement of plants is a key activity of many research institutes and a priority for most of our fruit industries. This is the third in a series of three articles which aim to give growers a better understanding of what is involved in plant breeding programmes .

Introduction
The first article (1) explained what is involved in developing and maintaining a germplasm collection. The germplasm collections provide a crucial foundation for the plant improvement programmes undertaken by HortResearch. The second article in this series (2) described how plant breeders gain information on the inheritance of characters of interest, and then use this information to assess alternative breeding strategies and to select parents of high breeding value. This article discusses some of the methods and selection criteria used in the evaluation and selection of new cultivars (cultivated varieties).

The main aim of HortResearch's breeding programmes is to enhance New Zealand's competitive advantage in horticulture by producing new, improved and novel fruits. The criteria for breeding new cultivars are developed in consultation with industry and regularly reviewed. These criteria are discussed below.

Testing and cultivar evaluation
Breeding is a long term project and priorities may change over time so HortResearch's breeding programmes aim to create variation and options. The various fruit industries can then help choose new cultivars which fit their market needs. These choices are made in collaboration with the New Product Development Committees of both ENZA and the NZKMB, and trials are carried out in conjunction with FIPIA.

A new selection may well need to be tested in a range of environments and to be grown by several cultural techniques, if its value is to be fully assessed. The purpose of a testing programme must be to discover the full potential of a new selection or introduction and ensure that new cultivars meet the needs of growers, packing and storing cooperatives and consumers. Rigorous testing of a new selection is critical before it is released and there is extensive orchard planting - particularly testing for susceptibility to physiological disorders and storage diseases. Some of this testing is now being undertaken by growers.

In most breeding programmes there are successive stages of testing: seedlings are initially screened for basic agronomic characteristics, and only the most promising lines go on to further testing. Evaluation of storage ability and eating quality such as flavour, soluble solids, pH, texture and juiciness is complex and time-consuming. These are only evaluated on a few selected families that show outstanding agronomic traits. For example in apricots, each seedling tree is assessed over 2 to 3 fruiting seasons. The main culling occurs during the second fruiting year when seedlings with unacceptable levels of disease are removed. Over the next 2 years the evaluation process is extended to include fruit flavour, texture and level of cropping. Fruit shape, colour and size are not screened too critically at this early stage. Normally less than 10% of the population is retained for further evaluation - storage, sensory evaluation and trialling at several research sites.

Assessment of consumer reaction is, undoubtably, the most difficult part of the testing of new lines of any fruit. Full testing by taste panels selected at random from the fruit-eating population are possible only with the most promising selections. Fortunately, the content of sugars and acidity can be measured by relatively simple tests, and these are used as a first screen to pick out types likely to appeal to taste panels. New evaluation methods are being developed which will also assist in this process, such as equipment which can measure the texture of a fruit or can measure the firmness of fruit by "bouncing" it on a sensor.

Fruit Quality
Improving fruit quality is a major objective of all fruit breeding programmes. While many plant characteristics are important in the successful acceptance of new cultivars, fruit quality must be considered the premier factor. Initially field characteristics, such as earliness, yield and disease resistance, play a primary role in selecting plant type. However, without acceptable quality, a new cultivar will fail regardless of the combined superiority of field characteristics.

Fruit quality is likely to assume even greater importance in the future. Consumers are becoming more aware of quality, and will become even more selective in future with fruit purchases. New processing procedures, and new processed products will require fruits with specific quality attributes. Cultivars specific for certain uses will be demanded. One aspect of quality that will require much more attention in future is food value: higher vitamin content; better sugar:acid ratios; fruit which are perhaps high in particular minerals (e.g., calcium). As food additives become less available for use by processors, natural colour, flavour and firmness will also become more important.

Market Requirements
NZ has a reputation for innovation in fruit breeding. This theme is reflected in several HortResearch breeding objectives which aim to produce novel variations on well known fruits.

Winning new Asian markets has created new breeding objectives. We have to be aware of ethnic preferences for colour, flavour and texture. For example, the Asian market prefers peaches that are low in acid but which have high soluble solids, are white-fleshed and firm, with attractive appearance and good storage life. The development of a low-acid, white-fleshed nectarine will give New Zealand the opportunity to export to Japan. We also have to emphasize long shelf life to endure hot, humid climates when fruit comes out of cool storage.

Maturity Season
For New Zealand growers, it is important to develop cultivars that exploit particular "windows" in export markets, especially northern hemisphere markets. For example, there is a lucrative market in the US for blueberries during the Thanksgiving-Christmas period. Development of early maturing cultivars has allowed New Zealand growers to take advantage of this market. Similarly, the development of late maturing cultivars has enabled the previously very short blueberry season to be extended over a four month period.

Competitors, such as Chile, have growing seasons that may be a few weeks in advance of New Zealand. Although our fruit is generally of higher quality, many markets don't want to buy Chilean fruit for the first two weeks and then swap to New Zealand suppliers. If we are able to produce our fruit earlier, we should be able to obtain market dominance.

In stonefruit, both local and export markets require early maturity apricot, peach and nectarine cultivars. The pre-Christmas season is a particularly important target for producers. However, the development of early-maturing cultivars is difficult because of high levels of seed abortion. Embryo rescue techniques are being investigated which may make these cultivars commercially viable.

Storage and Shelf-life
Good storage and shelf life are important since we are mostly dealing with fresh fruit - a perishable product destined for distant export markets. Collaboration with colleagues in postharvest and sensory science is vital. For fruits where cold storage or controlled atmosphere (CA) storage is commercial practice, records of the storage behaviour of cultivars are particularly valuable. These may include penetrometer tests for firmness, pH tests on the juice as a measure of acidity, and refractometer tests on juice as a measure of sugar content. Curves of these three factors during the storage period give an indication of how well the fruit are storing. Records are also made of any storage disorders such as scald, bitter pit, low temperature injury, and senescent breakdown. Such disorders can be considerably reduced by correct nutritional treatment of the trees, but a longer term solution lies in the selection of non-susceptible cultivars.

Fruitfulness and Productivity
Seedling populations are evaluated for their cropping ability, including yield and size of fruit. In some crops, such as kiwifruit, other factors are involved in determining orchard productivity: Currently about 14% of the canopy in kiwifruit orchards consists of unfruitful male pollenizer vines. Hermaphrodite cultivars (containing both male and female flowers) should, therefore, significantly increase productivity. Good pollination and seed set is a major factor in achieving good fruit size in kiwifruit. Pollination should be much improved with hermaphrodite cultivars because pollen would no longer have to travel between vines, and it should be released at exactly the right time in each and every bisexual flower. Bees might no longer be needed in kiwifruit orchards.

Pest and Disease Resistance
Pest and disease resistance objectives are also common to many plant breeding programmes. The increasing requirements for residue-free fruit and the decreasing number of suitable chemicals for use in disease management are driving research in HortResearch to identify alternative ways of control. As well as leading to a better quality product and a healthier environment for growers and the public, pest- and disease-resistant cultivars which require less spraying will be needed to maintain access to increasingly restrictive export markets. They will also help to reinforce NZ's clean, green image overseas and should also result in reductions in the costs of production because of the need for fewer sprays.

A major part of many breeding programmes is screening seedlings for pest and disease resistance. For example, apple breeding populations are regularly screened for resistance to black spot, powdery mildew, woolly apple aphid and fireblight. Resistance to pests and diseases already exists naturally within most fruit crops. However, these resistances usually occur in "wild" species that have no commercial value, such as crab apples, and generally these resistances don't occur together. For example, our apple cultivar breeding lines now incorporate resistance to black spot, powdery mildew, and woolly apple aphid. HortResearch scientists intend making crosses to combine all three resistances with high commercial quality characters.

Adaptation
Since none of our commercial fruit species is native to New Zealand, an important aspect of the work is to breed or select cultivars adapted to New Zealand conditions: our specific climatic and soil environments.

Rootstocks
Rootstock selection can have a marked effect on tree or vine performance and this aspect is also included in most of our breeding programmes. Rootstocks are evaluated on the basis of induction of early and sustained heavy production (precocity, fruit size and set, quality, appearance); control of the growth and ultimate size of the tree or vine; and their compatibility with different scion cultivars.

In apples, control of tree size is an essential element in the new higher density planting systems now being used in NZ. The size-controlling rootstocks suitable for these systems are all susceptible to woolly apple aphid (WAA). WAA is a major problem as our mild winters enable the insect to overwinter on trees roots. As well as debilitating the tree, fruit carrying the insect is banned from all our export markets. There is no insecticide available to control this pest at its overwintering sites. The problem has in the past been successfully controlled by using rootstocks carrying the resistance gene. Our breeding programme involves crossing sources of resistance with dwarfing rootstocks to produce a population of resistant seedlings from which new rootstocks adapted to New Zealand conditions can be selected.

Shortening Juvenility
Although it can take as long as six years for seedlings of some species to commence fruiting, methods are being developed to shorten the juvenility periods, to reduce the time from germination of the seed to the production of fruit. This will also reduce the time required to produce a new cultivar.

Mechanized Harvest
In some fruit crops, such as berryfruit, it is important to develop firm-fleshed cultivars that are suitable for machine-harvesting.

Conclusion
All of these aspects are taken into account when discussing with the various fruit industries the priorities for breeding new cultivars in their particular crop. Some of the specific breeding objectives for the different crops have been detailed previously (3). The techniques and methods discussed in each of the three articles in this series are used by HortResearch in its fruit breeding programmes which focus on enhancing the competitive advantage of New Zealand fruit growers.

References
(1) Muggleston S. 1995. What is involved in Plant Breeding (Part 1). The Orchardist of NZ. 68(9): 49

(2) Muggleston S. 1995. What is involved in Plant Breeding (Part II). The Orchardist of NZ. 68(10): 48

(3) Muggleston S and Glucina P. 1994. HortResearch fashioning future fruits. The Orchardist of NZ. 67(3): 35-39.

Source:
The Orchardist, December 1995, Vol: 68, Number: (11):40