In today's tough economic times it makes sense to lower orchard costs as much as possible. One way to achieve this is to use vigour-reducing rootstocks combined with growing trees at closer spacings. The profitability of apple orchards has for example been improved considerably by growing trees in high density plantings and using rootstocks that reduce tree vigour. Until recently, however, such opportunities were not available for summerfruit growers in New Zealand. Summer pruning and growth regulators such as `Cultar' can be used to control tree vigour in summerfruit, but these practices are costly and the results are not long term and may restrict export market opportunities.

In most New Zealand fruit growing districts, seedlings of `Golden Queen' peach (Prunus persica) and/or `Myrobalan' plum (P. cerasifera) are normally used as rootstocks for apricot (P. armeniaca), peach and nectarine (Glucina et al 1992). Apricot trees grown on `Golden Queen' seedlings are usually vigorous and healthy and do not produce suckers. Few instances of incompatibility between stock and scion have also been reported (Glucina 1988). However, large apricot trees are not ideal for high density plantings and may be sensitive to wet soil conditions. In addition, they may not necessarily produce the best export quality fruit.

In 1984, a rootstock trial began at the Clyde Research Centre with `Sundrop' apricot planted on 24 different plum, peach, plum x peach hybrid and apricot rootstocks. Trees were planted at 6 m x 3 m spacing. The soil was a Galloway-mottled, moderately deep, fine sandy loam. Trees received standard commercial spray programmes and other practises.

The main aim of the trial was to identify rootstocks which produced smaller, more efficient and compact apricot trees under Central Otago conditions.

Rootstocks used

The 23 rootstocks used in the trial (see Table 1) were divided into 2 groups: Group A included 15 rootstocks which were fully tested: Group B included rootstocks which were planted later (in 1986) or which experienced very high numbers of tree deaths.

Table 1: Description of Trial Rootstocks

Group A: Rootstock	Origin/parentage	Source
`Buck'	Possibly a cross of P. cerasifera and peach	a chance seedling discovered by Mr R Shepherd, Rooty Hill near Sydney, Australia
`P 1609'	Natural hybrid of `Myrobalan' plum and peach	INRA, Pont-De-La-Maye, France
`Damas P 1869' (`Damas de Toulouse')	Natural hybrid of P. domestica and P. spinosa.	INRA, Pont-De-La-Maye, France
`Golden Queen'	Peach seedlings	New Zealand
`Ferciana Ishtara'®	Selected from the F1(P322 x S1058) family. P322 is the plum cultivar `Belsiana' (P. cerasifera); S1058 is the `Yunnan' peach (P. persica)	INRA, Pont-De-La-Maye, France
`Marianna 6.64'	Open pollinated seedling of Marianna (P.cerasifera and P. munsoniana)	INRA, Pont-De-La-Maye, France
`Marianna 9.52'	Open pollinated seedling of Marianna	Fruit and Fruit Technology Institute, Stellenbosch, South Africa
`Marianna 2624'	Hybrid of P. cerasifera and P. munsoniana	University of California, USA
`Marianna GF 8/1'	Natural hybrid of Marianna P. cerasifera and P. munsoniana	INRA, Pont-De-La Maye, France
`Yumir Myran'®	Selected from the F1(P332 x P871) family. P332 is the plum cultivar `Belsiana; (P. cerasifera); P871 is a natural P. cerasifera x P. persica hybrid	INRA, Pont-De-La-Maye, France
`Pixy'	Open pollinated P. insititia, selected from a progeny of St Julien d' Orleans seed.	Seed collected from France and raised at East Malling Research Station, England
`Prunus zailisky Altai'	P. armeniaca	collected from Altai mountains by Dr Bruce Taylor
`P.S.A 5'	selected from a population of wild peach seedlings (P. persica)	selected by the Istituto di Coltivazioni Arboree (I.C.A), Pisa University, Italy
`INRA St Julien GF 655/2'	Seedling selected from `St Julien d'Orleans' (P. insititia)	INRA, Pont-De-La-Maye, France
`St Julien X'	Seedling of P. domestica	selected by C. Woodhead, DSIR, Auckland, New Zealand
Group B: Rootstock	Origin/parentage	Source
`B1 Cacak'	Plum `Beloslijiva'	Yugoslavia
`Citation'	Hybrid between `Red Beaut' and peach	Floyd Zaiger, California, USA
`INRA GF 43'	Open pollinated seedling of French prune (P. domestica)	INRA, Pont-De-La-Maye, France
`Marianna 6.46'	Open pollinated seedling of Marianna	Fruit and Fruit Technology Institute, Stellenbosch, South Africa
`Marianna 8.6'	Open pollinated seedling of Marianna	Fruit and Fruit Technology Institute, Stellenbosch, South Africa
`Myrest Myrabi'® clone P2032	Chance seedling of `Myrobalan	INRA, Pont-De-La Maye, France
`Ferlenain Plumina'® clone P2038'	Hybrid of P. besseyi and P. cerasifera	INRA, Pont-De-La Maye, France
`SJ 53/7'	Pentaploid seedling of St Julien d'Orleans	INRA, Pont-De-La Maye, France

Rootstocks in Group B were excluded from the trial analysis because of high tree losses or later planting. Although we have insufficient data to make firm statements about the performance of these 8 stocks, `B1 Cacak', Myrest Myrabi® clone P2032, and `Citation' produced smaller trees than those on `Golden Queen'. `Ferlenain Plumina® clone P2038' and `Citation' were the only 2 rootstocks to produce yields comparable with trees on `Golden Queen'. `B1 Cacak', `Myrest Myrabi® clone P2032', and `Citation' all showed 100% survival at the conclusion of the trial which suggests further work is warranted with these 3 stocks.

RESULTS

The cumulative yield efficiency can be used to estimate the amount of energy trees put into fruit production. Generally, a tree which has a small to medium trunk cross-sectional area and produces heavy crops will have a high cumulative yield efficiency. A large tree producing small crops will be quite inefficient because most of the tree's energy is going into leaf production; and therefore the tree will have a low cumulative yield efficiency. Cumulative efficiency was one of the most important characteristics we used to distinguish between different rootstocks in the trial, along with tree survival, cumulative yield to 1993, mean fruit weight (grams), bacterial marking scores on fruit, and levels of tree suckering and general tree health.

Of the 15 rootstocks in Group A which were compared using statistical analysis, 6 were identified as the most promising (see Table 4). These could be grouped into 3 tree size categories: large, intermediate and small.

Figure 1: Trunk cross-sectional area (cm²) of 15 rootstocks ranked according to tree size, compared with the mean yield efficiency (kg/cm²) of each rootstock.
Large trees

The 2 best performing rootstocks in the large tree size (ie. vigorous) category were: `Marianna 6.64' and `Marianna GF 8/1'. `Marianna 6.64' was in fact the largest tree in the block at the end of the trial in 1993 (Fig 1). It produced the largest yield in 1992 and had a comparable cumulative yield with `Golden Queen'. Trees on both `Marianna 6.64' and `Marianna GF 8/1' rootstocks also showed almost 100% survival by 1993 (Table 2). They also produced very high cumulative yields. Although the trees were larger than trees grown on `Golden Queen', they had extremely high cumulative yield efficiencies.

Fruit from `Marianna 6.64' and `Marianna GF 8/1' were relatively free of bacterial markings and had good fruit size most years when fruit records were collected (Table 3). However, the fruit were slightly smaller than from trees grown on `Golden Queen'. The main problem with both these rootstocks was they produced large trees and there may be difficulties controlling tree vigour and hence precocity. The use of `Marianna GF 8/1' may also be limited by its tendency to excessively sucker. `Marianna GF 8/1' had a suckering score just outside the acceptable range, but this could probably be managed if necessary.

Intermediate sized trees

`Golden Queen', `Prunus zailisky' and `Pixy' were the most promising rootstocks which produced intermediate-sized trees (Table 3). Of the three `P. zailisky' was the only rootstock to have 100% tree survival at the end of the trial (Table 2). Tree health of trees on `P. zailisky' was also moderately better than those on either `Golden Queen' or `Pixy'. Both `Pixy' and `Golden Queen' produced the highest cumulative yield efficiency scores (Fig 1). However, of the two, `Golden Queen' produced slightly larger fruit with less bacterial markings. Bacterial marking on fruit from trees grown on `Pixy' was unacceptably high in both 1991 and 1993. Suckering with `Pixy' also exceeded acceptable levels. However, `Pixy' performed well in most parameters including exceptional fruit size.

In this trial `Golden Queen', `Marianna 6.64' and `Marianna GF 8/1' produced between 20 t/ha and 25 t/ha of fruit between 1990 and 1992 when grown at 6m x 3m spacing. Yields were lighter in 1993 for all rootstocks (Table 5). Both `Pixy' and `P. zailisky' also produced excellent yields in 1991 and 1992. Although these yields are quite acceptable we do not recommend reducing the spacing of these trees as fruit quality will possibly be sacrificed.

Small trees

`Marianna 9.52' and `INRA St Julien GF 655/2' were the only 2 promising rootstocks that offered some reduction in tree vigour. Both rootstocks had excellent tree survival and very high cumulative yield efficiencies (Fig 1). `INRA St Julien GF 655/2' however, produced unmanageable suckering and small fruit. The fruit were "clean" and the trees scored well in all other parameters tested (Table 4). The commercial use of this rootstock would nevertheless be severely limited by its tendency for extensive suckering.

`Marianna 9.52' was the only rootstock which produced a small tree that scored highly in all parameters (Table 4), including an exceptional cumulative yield efficiency (Figure 1). `Marianna 9.52' has also been reported to be an outstanding semi-dwarfing rootstock under South African conditions (Hurter 1978). Although fruit size was smaller than fruit from trees on `Golden Queen', it was still acceptable for export. The bacterial marking scores on fruit from `Marianna 9.52' were significantly lower than fruit from `Golden Queen, which suggests it may also provide some adaptability to heavy, wet soils. This is highly likely owing to its plum parentage. The productivity of `Marianna 9.52' could be increased if the trees were planted at closer spacings. In this trial 6m x 3 m spacings were used. The effect of closer tree spacing on fruit quality was not investigated in this trial. However, the between-row width could easily be reduced to 5.0 m spacing and within row spacings of 2m to 2.5 m used. Table 5 illustrates the increase in production which could possibly be achieved if `Marianna 9.52' were planted at 5m x 2.5m spacing with 800 trees/ha. The yields at the closer spacing would be very comparable to the larger trees planted at 6m x 3m spacing. Based on the results of this trial we consider that the reduction of tree spacing could only be applied to the trees which were grouped in the small category in Table 4. At higher densities (above 800 trees/ha) increased management costs due to increased summer pruning requirement, higher outlays for pest and disease control and costly harvesting can often cancel the effects of increased production. Fruit quality and set may also be affected by the reduction of light into the tree.

CONCLUSIONS

1. `Golden Queen' performed well in all categories tested except tree survival (67 %). The result confirms that `Golden Queen' is still one of the most suitable rootstocks for `Sundrop' apricot in Central Otago.

2. `Marianna 6.64' and `Marianna GF 8/1' were the 2 most promising rootstocks which produced a tree larger than `Golden Queen'. `Marianna 6.64' would be preferred because of its lack of suckering.

3. `Golden Queen', `P. zailisky' and `Pixy' were the most promising rootstocks which produced intermediate sized trees.

4. `Marianna 9.52' was the only rootstock tested which offered some reduction of tree size and still maintained good fruit quality. Trees on this stock could possibly be planted at closer spacing to increase productivity.

Table 2: Survival of `Sundrop' apricot trees on 23 different rootstocks

Rootstock	Year planted	Number of trees at planting	Number of trees alive in 1993	Percentage tree survival
`B1 Cacak'	1985	4	4	100
`SJ 53/7'	1986	6	4	67
`Golden Queen'	1984	15	10	67
`Ferciana Ishtara® clone P2038'	1986	5	1	20
`Myrest Myrabi® clone P2032'	1986	6	6	100
`Citation'	1986	6	6	100
`INRA St Julien GF 655/2'	1984	15	15	100
`Marianna GF 8/1'	1984	15	14	93
`Marianna 6.46'	1984	15	3	20
`Buck'	1985	15	13	87
`P. zailisky'	1984	7	7	100
`Yumir Myran®'	1984	20	20	100
`Marianna 6.64'	1984	15	15	100
`P 1609'	1985	15	15	100
`Marianna 8.6'	1984	15	1	7
`Pixy'	1984	15	9	60
`Marianna 9.52'	1985	15	14	93
`P.S.A 5'	1984	10	10	100
`Damas P 1896'	1984	15	14	93
`Marianna 2624'	1984	15	12	80
`INRA GF 43'	1984	15	8	53
`Ferciana'	1984	16	15	94
`St Julien X'	1984	20	20	100

Table 3: Mean fruit weight over 3 years to 1993, annual yield for 1992, cumulative yield to 1993, cumulative yield efficiency 1992 and 1993.

Rootstock	Trunk cross- sectional area (cm2)	Mean fruit weight over 3 years (g)	Annual yield 1992 (kg/tree)	Cumulative Yield to 1993 (kg/tree)	Cumulative yield efficiency (kg/cm3)
					1992	1993
`Marianna 6.64'	331.3	69.3	62.35	162.1	.48	.47
`Marianna GF 8/1'	315.5	66.8	43.99	146.3	.45	.48
`Golden Queen'	276.6	74.3	44.23	165.6	.55	.60
`Pixy'	256.5	70.3	49.79	142.1	.54	.58
`P. zailisky'	286.9	66.4	48.19	133.7	.43	.47
`Marianna 9.52'	171.2	68.1	36.67	94.5	.52	.55
`INRA St Julien GF 655/2'	173.2	65.6	31.79	93.8	.54	.56

Table 4: Summary of the performance of `Sundrop' apricot on 15 different rootstocks in the most important categories

Rootstock	TS	Tree size	CY	CYE	Fruit weight (g)			Bacterial marking			Suckering	Tree health	Best Overall
					91	92	93	91	92	93
`Marianna 6.64'	*	L	*	*	*	*	*	*	*	*	*	*	*
`Yumir Myran®'	*	L	*	U	*	*	*	U	*	U	*	*
`Marianna GF 8/1'	*	L	*	*	*	U	*	*	U	*	*	*	*
`Damas P 1869'	*	L	*	*	U	U	U	*	*	*	U	*
`Marianna 2624'	*	I	*	*	*	U	*	U	U	U	U	*
`Golden Queen'	*	I	*	*	*	*	*	*	*	*	*	*	*
`P. zailisky'	*	I	*	*	*	*	U	U	*	*	*	*	*
`St Julien X'	*	I	U	U	U	U	U	U	*	U	U	*
`Pixy'	*	I	*	*	*	*	*	U	*	U	*	*	*
`Ferciana Ishtara®'	*	I	U	U	*	*	*	U	U	U	*	*
`Buck'	*	I	U	*	U	U	U	U	U	U	*	*
`Marianna 9.52'	*	S	*	*	*	*	*	*	*	*	*	*	*
`INRA St Julien GF 655/2'	*	S	U	*	*	U	U	*	*	*	U	*
`P1609'	*	S	U	U	U	U	*	*	U	U	*	U
`P.S.A 5'	*	S	U	U	*	U	U	*	*	U	U	U
Key: * = acceptable U = unacceptable (TS = tree survival, tree size; L=large, I=intermediate, S=small, CY = cumulative yield, CYE = cumulative yield efficiency (kg/cm2). A star is awarded where performance is greater than the acceptable.

Table 5: Yields of the 6 most promising rootstocks expressed as tons per hectare

Rootstock	Yield per hectare at 6m x 3m tree spacing (tons/hectare)
	1990	1991	1992	1993
`Marianna 6.64'	21.8	19.3	34.6	10.1
`Marianna GF 8/1'	20.1	19.9	24.4	12.8
`Golden Queen'	20.5	25.4	24.6	13.9
`Pixy'	10.6	21.8	27.6	12.4
`P. zailisky'	11.3	21.5	26.7	11.2
`Marianna 9.52'	8.29	14.9	20.4	8.0
Rootstock	Yield per hectare at 5m x 2.5m tree spacing (tons/hectare)
`Marianna 9.52'	11.9	21.46	29.33	11.55

ACKNOWLEDGEMENTS

We thank Andrew King, Alastair Currie, Martin Van Galdermelson and Ross Bristol for assistance with data collected in the trial.

REFERENCES

Glucina, P.G. 1988: Peach and Nectarine culture in New Zealand. [In] The peach - World Cultivars to Marketing, New Jersey, Somervillee, USA. 251-254.

Glucina, P.G.; Mills, R.S.; Manson, P.J. 1992: Comparison of the growth, yield, fruit size and survival of `Golden Queen' peach on seven rootstocks. New Zealand Journal of Crop and Horticultural Science 20: 297-303.

Hurter, N.; Van Tonder M.J.; Calitz, F.J. 1978: New plum rootstocks - Real economic advantages. The Deciduous Fruit Grower July: 249-256.