Cultivars, rootstocks and tree spacing

Rootstocks
Trifoliate orange (Poncirus trifoliata) is the predominant rootstock for all citrus cultivars in Japan. Yuzu and natsudaidai are only used to a very limited extent.

Experimentation in Shizuoka with Flying Dragon (Hiryu) trifoliata has proven very successful for controlling tree size. Flying Dragon reduces tree height and canopy volume, but only markedly so after trees are allowed to fruit in their fourth season. Although yields per tree are higher when on standard trifoliata, the yield per canopy volume is much greater for trees on Flying Dragon. In their first three years of cropping, trees on Flying Dragon had 60% more cumulative yield per canopy volume than trees on standard trifoliata. Closer planting of the dwarfed trees would be needed to capitalise on this effect.

Fruit quality on Flying Dragon has been excellent. Fruit size was only marginally smaller in one season, but Brix levels appeared to improve as trees aged. Six year old Aoshima trees on Flying Dragon produced fruit with Brix levels of 11.7, compared to 10.9 in fruit from trees on standard trifoliata. Acid levels were not altered by rootstock, but better fruit colouring was evident on trees on Flying Dragon.

There is now much commercial interest in Flying Dragon, but because of the problem of high off-type numbers in seedling populations, experimentation has begun on tissue culturing of this rootstock.

Tree spacing and yields
Most of the citrus orchards in Japan are on steep, terraced hillsides. Land area is the major limiting factor, and high density planting is carried out. Young plantings are usually spaced 2 x 2 metres (2,500 trees per hectare), and then progressively thinned as the trees begin to crowd. At 15 years, the density has been reduced to 600 trees per hectare (4 x 4 metres). The commercial life of an orchard is considered to be 30-40 years.

In the Mikkabi district which we visited, yields for satsuma mandarins average 15 tonnes per hectare. Only in plantings older than 10 years are yields of 30 tonnes per hectare achieved.

Standard orchard practices

Leaf to fruit ratios of between 25 and 35: 1 are used. Positioning of fruit around the canopy is important - fruit which grow on vigorous upright shoots will have thick stalks and will have poor quality. Fruit hanging on thin, drooping shoots around the sides and skirts of the canopy will produce the highest quality fruit. Trees generally do not begin producing high quality fruit until they are eight years old.

Figure 1: Fruit thinning has been carried out three times to achieve this crop load on a wase satsuma tree. The first thin is to remove excessive fruit, using a 25-35:1 leaf:fruit ratio; the second and third thins are to remove fruit which will not be top quality

Chemical thinning agents have been extensively tested. Figaron (ethychlozate) appeared to hold promise in experiments, but has not been adopted commercially because of unreliable performance (we reached the same conclusion from trials in Kerikeri.) So all commercial thinning is done by hand the labour requirement is estimated at 300 hours per hectare.

Fertilisers
Applications are based on leaf analysis results. Typical annual quantities of macro-elements are: nitrogen 200 300 kg ha; phosphorus 150 - 200 kg ha; potassium 150 - 250 kg/ha. Fertiliser is applied in three to four equal doses between spring and autumn. Micro-elements are applied as foliar sprays. Organic residues (rice straw etc) are commonly applied to improve soil conditions. Dolomite is applied in winter where soil pH is low.

Pruning
Pruning is carried out in February-March (August-September in New Zealand). The aim of pruning is to prevent shading of branches, and to accentuate those shoots which will carry high quality fruit. Horizontal growth is left - these produce fruit that hang down on thin stalks, and have the highest quality. On young trees, vigorous summer shoots are cut off. Pruning also helps alleviate alternate bearing.

Techniques to improve satsuma fruit quality

Some of these techniques are very labour intensive, and under circumstances other than the highly subsidised Japanese agricultural sectors may not be economical. However, this trend is unlikely to change, and New Zealand growers of export satsuma mandarins are thus faced with having to produce very high quality fruit.

According to the head of the prefectural fruit extension service at Mikkabi, mikan growers can improve their fruit quality (and hence income) through three techniques: mounding, mulching and covering. Various root restriction experiments have also shown promise but are not yet commercially used.

Mounding
Moisture control in satsuma orchard soils in the months prior to harvest is essential for improving Brix levels and preventing 'floating skin' (puffiness). Most satsuma orchards in Japan are on steep terraced hillsides which provide excellent natural drainage, but where orchards are situated on flatter terrain, mounding has become a common practice.

Mounds are steep walled and 500 - 750 mm high, with surface drainage afforded by the central ditches. In some cases, growers were using second hand plastic off greenhouses for lining the central ditches between the mounded rows.

Mulching
Use of reflective mulch began in Shizuoka three years ago, and is still considered an unproven technique for quality enhancement. Tyvek is the only reflective mulch being used at Mikkabi. The manufacturers (Du Pont) claim that use of their product will increase Brix levels by two degrees, but the extension officers were sceptical of this claim. Of the 1700 hectares in mikan production, Tyvek is used on only 35 hectares. Cost was quoted as a major impediment, although the number of growers using mulch was gradually increasing.

The best mulching effects have been achieved on shallow soils; its use on deep soils has been disappointing. Use of Tyvek on flat deep soils has actually resulted in poorer fruit quality. In these cases, the presence of mulch without adequate drainage has been counterproductive. Mulch is usually applied in August/September before the autumn rains begin. This would equate to early-late February in New Zealand. Earlier application of mulch increases Brix but also increases acidity - this is acceptable only if the fruit is then stored to allow acidity to drop.

Tyvek mulch increases the colour of fruit in the lower canopy; this does not bring the harvest date forward, but it does reduce the need for several picks. Uniform ripening and colouring thus results in a labour reduction. Concluding remarks from the extension service were that the cost benefits of Tyvek use are difficult to gauge, and it is premature to judge the technique as successful.

Root restriction
We observed several experimental techniques for mikan root restriction at the Seien Branch Station of the Shizuoka Prefectural Citrus Research Centre. Trees were either grown in above ground pots, in above ground boxes or in the ground but with the rootzone enclosed in woven fibre soil bags. A later experiment consisted of growing trees on mounds above an impervious layer of fibre.

The prime aim of these trials was to prevent 'floating skin' by controlling soil moisture prior to harvest.

Much of the 2300 mm of annual rainfall can occur during the September typhoon season. The box planted orchard also was designed to allow access for radio-controlled robot sprayers. The 150 - 300 litre box sizes had proven best for fruit quality, and also had tree size control effects.

An extreme example of root restriction occurs where some mikan growers wrench their trees out of the ground, prune the roots and then replant the trees. This is apparently done in some cases in a 10-year cycle. The growers carrying out this practice claimed it was successful.

Covering/glasshouse production
The production of glasshouse satsuma mandarins, or house mikan, is increasing gradually, although the number of hectares undercover has been static for some years. The Tokyo markets were supplied with 9000 tonnes in 1993, up from 7 million tonnes in 1989. Mikan growers receive government subsidies to erect covered structures, and these, together with the higher prices received for house mikan, make this intensive growing practice possible.

Houses are either heated or simply vinyl covered. For each district, and for specific markets, there is a schedule of operations for house mikan producers. We visited a heated house in Mikkabi where covering of the structure and oil heating was begun on 20 November and continued July. Heating costs were quoted as V12 million (NZ$ 200,000 pa).

The mikan cultivar grown in houses in this district is almost exclusively Takabayashi - it is one of the ultra-early group of cultivars known as goko wase, although it is considered a later maturing cultivar within this group. Trees were planted in the ground at 1.5 x 2 m spacing, although some trees were grown in large pots to give very early production. A heavier crop loading of 15-20 leaves per fruit is possible under cover. Typical yield was 20 kg per tree.

Postharvest handling

After packing, fruit is held in low humidity (80 %) coolstores at S - 8 C. Traditionally this has been done in small storage chambers (20 m2) in 18 kg boxes, but larger stores which allow forklift access are now used. Tyvek is used to cover boxes in storage as it does not create the excessive humidity that vinyl covers do. The doors of the storage chambers are only closed after the fruit has lost 3 % of its original weight. This curing process makes rinds thinner and less waste prone.

Packing procedures
Most growers belong to a cooperative growers association, which operates a packhouse. Although individual properties are small by New Zealand standards less than 1.5 hectares), the cooperatives are large - the one we visited in Mikkabi had 1,015 grower members producing off 1,700 hectares.

The fruit is delivered to holding bins in the packhouse, where it is sampled for quality and packout. This sample determines the grower return, and overcomes the problem of separately packing large numbers of small fruit lots. Fruit is not washed or waxed, but passes over brushes which gently buff the rind to give a natural glossy appearance.

Grade standards
The fruit is graded into five or six size classes - 3L, 2L, L, M, S, 2S. The most profitable sizes are M and L; in most seasons there is no market for 2S fruit, but the drought experienced in most districts in 1994 reduced fruit size, and 2S has been acceptable (the dry conditions also improved sugar levels.)

Three external quality grades are applied - Excellent, Superior and Good. All fruit is packed into 10 kg corrugated cardboard cartons. Reject fruit (usually less than 15%) is mostly juiced, and some is processed into canned segments or marmalade. Because of continuing oversupply of satsumas, some cooperatives are trying to promote super-quality brands; the Mikkabi cooperative was developing a line called Special Aoshima Mikan Ace, which had a guaranteed 12 degree Brix and improved colour.

Quality requirements for the Japanese market

What problems do these requirements present for New Zealand satsuma growers? Fruit size can be overcome with adequate thinning, and rind blemish is usually not a problem where basic spray programmes are followed. Rind colour in most orchards has generally been good. The main quality issues then are rind puffing and internal quality (sugar and acid).

Sugar
Brix needs to be at a minimum of 10, and preferably at 11. These figures refer to minimums above which the majority of fruit should be. When testing samples of fruit, the variability of individual fruit should be noted.

For most New Zealand orchards, variability is high - this means that for 80% plus of individual fruit to be above 10 Brix, the average of a 20 fruit sample will probably have to be 11 Brix. These values need to be calculated for each exporting orchard each season. Although many individual fruit will reach 10 Brix, a further challenge will be to determine where these fruit occur on the tree canopy, and aim through pruning and thinning to promote and select these sites.

Acidity
Acidity at point of sale in Japan should be 0.8%. This means that fruit should not be harvested before acid levels drop to 1.2%, and should be cured prior to shipping to drop acidity to 1.0%. We were told at a press interview with journalists from Japanese fresh produce trade journals that satsumas imported from New Zealand in 1994 were 'very bitter'. High acidity is thus our main quality problem, but in conjunction with rind puffing.

Rind puffing
Hanging fruit on the trees to allow acid reduction carries the risk of rinds becoming excessively puffy. 'Floating skin' is not acceptable on the Japanese market, and also makes the fruit very sensitive to wastage during picking and packing. Control of soil moisture in autumn will be of prime importance to prevent puffing.

Conclusions for New Zealand conditions

Mounding
All new plantings should be on mounded soil to allow soil moisture control. Mounded rows should be oriented to allow rapid outflow of rainwater. Older plantings on flat land should be modified to allow better drainage and egress of rain water. This can be achieved by digging central ditches between existing tree rows, in effect leaving the trees standing on mounds.

Mulching
Without soil mounding, the effects of reflective mulches such as Tyvek will be minimised and may even be detrimental. However, in conjunction with mounding, the use of mulching will further reduce the risk of poor fruit quality in wet seasons.

Root restriction
The use of a restrictive layer, such as weedmat, under row mounds would allow further control of moisture in the root zone, because the root system is then restricted to soil raised above the autumn winter water table.

Glasshouses/covering
These are the most expensive options to take, and most growers should attempt the other techniques first. Production of house mikan is very capital intensive and highly specialised. It would be advisable to gain extensive training on production techniques used in Japan before embarking on large scale covering.

Postharvest acidity reduction
Fruit needs to be held under low humidity storage conditions after harvest and packing to allow acid levels to drop.

Common NZ quality standards
Volumes of citrus exported from New Zealand to Japan will always be small compared to those from other countries. Regardless of packhouse or brand, our fruit will always be judged in Japan simply as 'from New Zealand'. It is therefore very important that all exporters adhere to a set of common quality standards. This will be especially important for internal quality of satsuma mandarins, to which initial reactions have not been good.

Acknowledgements

Source: The Orchardist, April 1995