INTRODUCTION

Prunes are firm-fleshed European plums (Prunus domestica) with a high sugar content which allows them to be dried whole without fermenting at the pit, They are widely grown in Germany, Romania, the US and Yugoslavia. Australia, France and Italy also produce significant quantities of prunes Approximately 560 t of dried prunes, worth $2.2 m, were imported into NZ from the US and Australia in 1990.

Fig 1: Fruit of the 'Cacak Fruitful', a promising cultivar introduced from Yugoslavia.

Prunes are typically oval in shape, the skin is pink to dark purple with a grey bloom, and the flesh is yellow (Fig. 1) Fruit of most prune cultivars mature in mid-late February in Hawkes Bay, late February to early March in Central Otago. They lack the bitter skin of Japanese plums (P. salicina) and can be used fresh or dried. In some parts of Europe, prunes are used to make brandy (e.g. "Slivovitz" in Yugoslavia).

This article reports on the characteristics and performance of 21 prune cultivars and prospects for developing the fruit as a new crop in New Zealand.

CULTIVAR TESTING

Twenty two prune cultivars were imported by HortResearch between 1978 and 1983 (Table 1). Trial plantings of 2 or 4 trees per cultivar were established at HortResearch Orchards at Havelock North, Riwaka, and Clyde. Some cultivars were also planted on a larger scale under the New Crops Scheme (Davison 1979), at commercial orchards in Hawkes Bay, Nelson, Marlborough, Canterbury and Central Otago. Few of these plantings remain, although a repository of the best cultivars has been established al the Clyde Research Orchard. Central Otago.

Each cultivar in Table I was evaluated for a some of the desirable characteristics needed by prunes in order to succeed as a commercial crop in NZ:

• Fruit:
  • medium-large size (at least 25g);
  • high soluble solids concentration (at least 20% SSC);
  • even maturity;
  • good flavour, fresh or dried.
• Tree:
  • regular cropping.

RESULTS

General remarks

Fig 2: Some prune cultivars produce regular, heavy crops in New Zealand ('Stanley').

Prunes possess a number of advantages over Japanese plums (see Glucina et al 1990). Trees are hardy and resistant to most bacterial diseases that infect Japanese plums. Prune trees in the trials were vigorous, sturdy, and did not require support structures. They flowered about one month after most Japanese plum cultivars, thereby avoiding severe spring frosts. In general, prunes appear to be a low management fruit crop, although regular thinning (especially 'Stanley'), annual pruning, and fertilising assisted trees to produce regular crops of high-quality fruit.

Fruit of some cultivars was prone to crack, particularly following rain close to harvest. Cracking led lo problems with wasps and birds. Dry, settled weather over the harvest period is required for the production of top-quality prunes. Some prunes developed rots and superficial moulds at harvest, so pre-harvest fungicides may be beneficial. Most of the cultivars studied were very susceptible to mites; however, this problem can be overcome by encouraging predatory mites, such as Typhlodromus occidentalis, or by the selection of mite resistant cultivars, e.g., 'Italian'.

Cultivar performance

Most cultivars were evaluated over at least 4 to 6 fruiting seasons. Some of the data are presented here (Table 2), as a guide to the main characteristics and performance of these various cultivars. Accurately predicting the optimum time of fruit maturity and harvest by changes in background fruit colour proved to be very difficult, particularly with extensively (100%) coloured, blue prunes. As a result, the need for fruit of a cultivar to reach high % SSC over a short period to aid harvesting was a major cause for rejecting certain cultivars from the trials.

Table 2: Characteristics and performance of 21 prune cultivars (in alphabetical order) at the Clyde Research Orchard, Central Otago. Fruit size and % SSC measurements are means of 2-6 years data.

Cultivar	Mean Fruit Weight (g)	% SSC	Yield1	Flavour Ranking2
				1989	1990
'Buhler Fruhzwetsche'	22.9	20.7	light	4	9
'Cacak Best'	67.1	16.1	light
'Cacak Beauty'	29.3	16.7	light	13
'Cacak Early'	43.5	16.4	light
'Cacak Fruitful'	26.7	21.8	medium	4	7
'd'Agen 698'	28.3	22.2	heavy	10	5
'd'Agen 626'	22.9	19.8	medium	8	11
'd'Agen 707'	23.3	18.6	medium	12
'French (USA)'	19.2	17.2	medium	16
'French Petite'	19.9	21.9	medlum	14
'Hauszwetsche'	17.7	21.0	light	7	2
'Imperial'	27.1	20.1	medium	9	10
'Iroquois'	28.0	16.8	medium
'Italian'	33.4	21.1	medium	1	3
'Merton'	31.8	14.6	heavy	3
'Mowhawk'	34.2	26.1	medium
'Oneida'	52.6	18.0	light		1
'Richard's Early Italian'	31.4	20.1	medium	6	5
'Stanley'	37.1	16.8	heavy	2	4
'Wangenheim'	18.8	19.0	light	15
'Weatherspoon'	32.5	22.3	heavy	11	8
1 Yield: Light, no thinning required (uneconomic); medium, moderately economic; heavy, requires thinning (economic). 2 Flavour: Cultivars were ranked by each assessor and the average ranking is shown; 1= best.

Several Yugoslavian (e.g., 'Cacak Best') and French cultivars produced fruit that fell off after reaching 13 to 14% SSC.

The German prune cultivars in this study set light crops of very small fruit. However, the flavour of dried 'Hauszwetsche' fruit was extremely good. 'Weatherspoon' produced heavy yields of fruit with high % SSC, but with poor flavour.

Hand-thinning fruit in early November increased fruit size and % SSC in trials at Clyde. However, significantly reducing the crop load was insufficient to produce good quality fruit with heavy-setting cultivars such as 'Stanley' (Table 3). As 'Stanley' and 'Merton' trees reached maturity, they produced heavier crops of fruit with low SSC. In light-crop years, however, fruit of these two cultivars were very acceptable. 'Stanley' fruit were prone to crack.

Four prune cultivars introduced from New York, USA - 'Iroquois', 'Mohawk', 'Oneida' and 'Seneca' - have only recently started to crop and require further evaluation.

BEST CULTIVARS

The results of growing prunes at Clyde, Central Otago, were similar to those obtained in other districts of NZ. Low % SSC in fruit and cracking were problems common at all test sites. The results illustrate, however, the value of nation-wide cultivar testing of a new fruit crop.

Despite the excellent reputation of many of the cultivars imported, only three can be recommended for NZ. These are 'Italian', 'Richard's Early Italian' and 'Cacak Fruitful'. All three cultivars produced regular crops of high-quality, good flavoured fruit without being thinned. Fruit of 'Italian' and 'Cacak Fruitful' are normally ready to harvest at Clyde about 8-14 March. 'Richard's Early Italian' fruit mature 2 weeks before 'Italian'.

POLLINATION REQUIREMENTS

'Italian' prune is sometimes described as being self-fertile (Thompson and Liu 1972) or self-sterile (Childers 1973). The general consensus is that self-fertile prune cultivars tend to set heavier crops in the presence of pollenizers, so mixed planting of cultivars is recommended. Trials at Clyde with 'Cacak Fruitful' showed this cultivar to be self-fertile, although pollination by other cultivars improved fruit set (Table 4).

EFFECTS OF DIFFERENT ROOTSTOCKS

Trees of different prune cultivars were grown at Clyde on 4 different rootstocks: Myrobolan (P. cerasifera) seedlings, Myrobolan B. 'Golden Queen' peach seedlings and 'Buck' (a natural peach x plum hybrid). Lack of replication prevented statistical analysis of the results, but all 4 rootstocks proved satisfactory with the cultivars tested.

PRODUCTION, MARKETS AND RETURNS

Dried prunes imported into NZ from the US and Australia currently sell for $3.78 to $4.17 per kg (1992). Trees in the trial at Clyde were grown at 6 x 5m spacings (330 trees/ha). At this spacing, 'Italian' produced approximately 75 kg/tree, or 24.7 tonnes of fresh prunes/ha. This is equivalent to 8.2 tonnes dried (3:1 fresh to dried ratio), or $31,100 gross return/ha at $3.78/kg. On this basis, returns for prunes are similar to those for apricots.

Central Otago apricot plantings produce approximately 25 t/ha fresh, or 4.17t /ha of dried fruit (6:1 drying ratio). With current prices for apricots of approximately $9/kg, gross returns arc approximately $37,500/ha. Regular heavy yields and high packouts may, however, be more difficult to sustain with prunes than apricots. The lack of alternative outlets for second-grade prunes may also be a limiting factor with this new crop.

Prunes can be sold fresh on the local market, although growers with trial trees reported low returns. Canning or other processing may be viable alternatives. 'Sugar' plums are reported to sell well as fresh fruit in S.E. Asia. This cultivar was not included in this study but was apparently one of four prune cultivars imported and tested unsuccessfully in Hawkes Bay by Watties in the 1950's. 'Sugar' plum is a Californian selection of 'd'Agen', and the fruit are reputed to reach very high SSC. Trials with other 'd'Agen' selections in NZ, however, suggest that 'Sugar' plum is unlikely to perform well in NZ.

Although only limited market opportunities other than drying, appear to have been identified for prunes, this does not necessarily mean market niches do not exist for the fruit. Italy and Spain for example, are large producers of prunes for fresh use (Sansavini, 1987). The hardiness of the trees, their ease of management, and the range of potential uses of the fruit make prunes worthy of further consideration. Prunes would make useful additions to homegarden orchards in southern parts of New Zealand.

SUMMARY AND CONCLUSIONS

Twenty one prune cultivars, imported from Europe and the US, were evaluated in trials at Havelock North (Hawkes Bay), Riwaka (Nelson) and Clyde (Central Otago), and at 5 commercial orchards in the main fruitgrowing districts of NZ. Three cultivars produced regular crops of good quality fruit and can be recommended. They are: 'Italian ', 'Richard's Early Italian' and 'Cacak Fruitful'. 'Stanley' was highly productive, but needed lo be thinned early and heavily for the fruit to attain satisfactory SSC. Other cultivars tested in the trial were either too small (German and French types) or the fruit failed to reach satisfactory % SSC. Four cultivars ('Seneca', 'Oneida', 'Mohawk' and 'Iroquois') introduced more recently require further evaluation. Prunes do especially well in warm, dry summer and cool, sharp winter climates like Central Otago.

Markets for dried prunes appear to be very competitive, owing to the ready availability of imported fruit for which NZ currently pays $2.2 m per year. Returns per hectare for prunes in NZ are slightly less than they are for apricots. Prices for fresh NZ prunes on the local market have been disappointing, so alternative markets to drying may not be as readily available as they are for apricots. Small-scale plantings of the best prune cultivars identified in this study are warranted in Central Otago and other low rainfall districts. Prunes are a major crop in many countries and it should be possible to develop markets for fresh prunes in SE Asia where fruit with sweet, bland flavours are preferred.

ACKNOWLEDGEMENTS

We thank M van Geldermalsen, R Bristol, G Hosking, R Volz, J Halsted and P Nelson (HortResearch) for technical assistance, also the growers who participated in the New Crops Scheme.

REFERENCES

Childers, N.F. 1973: Culture of plums. [In] Modern Fruit Science, Horticultural Publications, New Jersey.

Davison, R.M. 1979: Grower trials of new fruits - New Crops Scheme. Orchardist of New Zealand 52 (1): 2.

Glucina, P.G.; Hosking, G.; Mills, R. 1990: New Japanese plum cultivars: progress, problems and outlook. Orchardist of New Zealand 63(9): 17-20.

Sansavini, S. 1987: Current trends in stone fruit growing in Europe. Fruit Varieties Journal 41(3): 114-128.

Thompson, M.M.; Liu, L.J. 1972: Pollination and erratic bearing in 'Italian' prune. American Society of Horticultural Science 97(4): 489-491.