Satsumas in New Zealand
By Nick Sakovich
The Satsuma mandarin grows quite well in New Zealand. The ingredients are all there - a good export and domestic market, consumer acceptance, and low disease and pest problems, which translate into low chemical inputs. The few disadvantages include overcropping and marginal internal quality, which can be overcome by appropriate management techniques.
An Owari selection, Silverhill, has been the traditional variety for New Zealand. However, there are extensive plantings of Miyagawa, Hiho and Okitsu. The cultivar Kawano is also grown in significant numbers.
The New Zealand citrus industry is dependent entirely upon one rootstock, trifoliate orange. This rootstock insures the production of high quality fruit with a good tolerance to many diseases. However, the shortcomings are small sized fruit, high acidity and the potential for incompatibilities with certain scion cultivars. Many rootstocks, including our popular California varieties are presently being research in field trials.
Satsuma on trifoliate rootstock makes for a small tree, and is therefore ideally suited for high density plantings. In the far north, of the northern island, trees are planted at 1518 per acre, called the triple row system. Essentially this is a 3' x 6' pattern, with every forth row open to allow for machinery access. Tree crowding often occurs at 6-7 years. At this time, complete rows will be removed and sometimes trees within the remaining rows also removed. This will results in either a 12' x 3' or 12' x 6' spacing ( 1,012 or 506 trees per acre). Growers will use these various patterns depending upon which variety of Satsuma they are planting.
Because of the cooler climate in New Zealand, fruit must often hang on the tree until late fall to allow for acidity reduction. This however carries the risk of excessively puffy fruit. According to researchers, controlling soil moisture in the late summer and fall is essential in preventing this puffiness. In Japan, orchards are planted on steep, terraced hillsides, which provide excellent natural drainage. When trees are located on flatter ground, they are planted on 20 to 30 inch mounds. This has now become the recommendation in New Zealand in order to maximize drainage and keep the root zone above the water table.
Requirements for the major nutritional elements are relatively low. Young trees need small amounts of nitrogen with regular applications of foliar trace elements (at least 3 per season). After the second season, fertilization is based on leaf analysis. Typically, light nitrogen applications (90 lbs/A) are needed annually. Foliarly applied urea is often used. Potassium is rarely needed, while phosphate and magnesium is often required. Magnesium can be applied foliarly, to the ground, or through the irrigation system. Calcium is sometimes needed, either by the addition of lime or gypsum, depending upon soil pH. Foliar calcium sprays are also an option. Foliar sprays of zinc and manganese are needed each season, and sometimes molybdenum.
Alternate bearing is a serious problem with Satsumas. Fruit thinning is therefore the most vital operation in producing a steady stream of good size fruit. The trees are not allowed to crop for the first two seasons. Researchers state that even a few fruit left on a young tree will severely retard its growth.
In New Zealand, fruit thinning is done by hand. In addition, a second light thinning may be required to remove missed fruit. The first thinning is done just after the natural fruit drop - here characteristically known as June drop. How much fruit is thinned? For early maturing cultivars, their research has shown the need for a ratio of 20/25 leaves per fruit (of export size). For first time growers, slow and careful thinning to this ratio is performed. When the trees are finished, they can then be observed to see what the crop (and this ratio) looks like. Subsequent thinning will hopefully be done more quickly, as experience is gained.
In Japan, positioning of the fruit around the canopy is of prime importance. The fruit which grows on vigorous, upright shoots, will have poor fruit quality. Fruit hanging on thin, drooping shoots around the skirts of the canopy will produce the highest quality fruit. Internal, shaded fruit will have lower Brix levels.
Pruning is carried out in Aug/Sept. in New Zealand, which would correlate to our Feb/Mar. The intention is to prevent shading of branches and to accentuate those shoots which will carry high quality fruit. On young trees, vigorous summer shoots are cut off. Pruning is also an important factor in helping to alleviate alternate bearing.
Disease and Insect Pests
Satsumas have been selected over the centuries in Japan for their tolerance to wet weather diseases, such as Alternaria and Botrytis. In New Zealand, Melanose is the primary rind disease, but only during excessively wet springs; and with a good pruning program, the removal of dead wood and twigs, even the melanose infections are dramatically reduced. Fruit may also be susceptible to brown rot, especially when ample rainfall occurs.
As with our export fruit, thrips is a major problem for the New Zealand exported fruit. At present they are using products like Diazinon, but are looking for more IPM friendly chemicals.
And just as we have had our bouts with Japan over the Fuller rose weevil, so is New Zealand - with the fruit vigorously inspected for FRW eggs.
New Zealand is cursed with the lemon tree borer, a pest which has not invaded our country yet. Larvae bore into branches which then must be removed and burned. This is a very laborious practice. Several scale insects also infest satsumas. The most common being soft wax, Chinese wax and black scale. A properly timed oil spray can effectively control the scale. However, excess oil sprays have been shown to decrease Brix levels and delay fruit coloring. The greatest risk is during the second half of fruit development. In New Zealand, it is recommended not to apply oil after January (July for us) and that the total oil used per season should not exceed 2% (i.e. two 1% sprays, or four 0.5% sprays).
Citrus red mite can also be a problem, often the result of broad spectrum pesticide sprays.Katydids are suspected for causing deep, silver-gray scarring on the exposed surfaces of the fruit.
For harvesting, a Brix:acid ratio of at least 7:1 is recommended for the local market. For the Japanese market it is 10, and preferably 11.
Satsuma rinds are very sensitive to rough handling. Picking should be done only when fruit is dry. The fruit needs to be clipped flush at the button, preferably in two actions, clipping the stalk first and then trimming at the button. Picking bags and containers need to be free of grit and stalks, with fruit bins being smooth walled. Transporting fruit to the packinghouse over rough roads can severely damage the fruit.
In Japan, fruit is stored by the grower for 5 (early season cultivars) to 20 (`midseason) days before packing. During this time, acidity levels will decline. In New Zealand, this may also become the standard practice.