Evaluation of Pyrus and Quince Rootstocks for High Density Pear Orchards

High density planting systems are a prerequisite to economise the use of land and labour costs of orchards. Dwarfing rootstocks controlling the vigour of the scion cultivars form the basis for such orchards (Wertheim and Webster, 2005). In the Netherlands, rootstock research is limited to and focussed on testing rootstocks selected abroad. For the Dutch pear growers the main selection criteria for new rootstocks are: 1) control of tree size; 2) production; 3) fruit size; 4) fruit quality; 5) production efficiency; 6) frost resistance. Additional criteria for Dutch fruit tree nurseries exporting trees to other countries are: 1) compatibility with scion cultivars; 2) suitability for growth in calcareous soils; 3) easy propagation. In all trials rootstock performance is compared to Quince MC, the most commonly used rootstocks for pears in the Netherlands. Recently, a number of Pyrus (Pyrus communis) and Quince (Cydonia oblonga) rootstocks have been tested with ‘Conference’ and ‘Doyenné du Comice’ as the scion cultivars. Generally, the production efficiency of the Pyrus rootstocks was much less than for Quince MC. Another disadvantage of the evaluated Pyrus rootstocks was their high sensitivity towards pear decline. Several rootstocks were rejected after examination of the graft union because of suspected compatibility problems. Of the tested Quince rootstocks C 132 shows promise because of its control of tree growth in combination with good fruit size and Eline because of its reduction of fruit russeting in ‘Conference’. INTRODUCTION High density planting systems are the starting point of modern orchards. Small trees that come into production in the second year after planting are a prerequisite to achieve regular yields of high quality fruits and to economise the use of land and labour costs for pruning and picking. Dwarfing rootstocks, controlling the vigour of the scion cultivars and inducing precocity of cropping, form the basis for such high density orchards. European pears are predominantly grown on rootstocks of Quince (Cydonia oblonga). In the Netherlands, the majority of pears are grown on Quince MC, but Quince MA and Quince Adams are also used. ‘Conference’ is the most important cultivar. Grown in high density planting systems yearly yields of 60 to 70 tons/ha are feasible. To obtain this production level a good control of shoot growth, flower bud development and fruit set is required. Although Quince MC has been used successfully for many decades in the Netherlands, there are several reasons to look for alternative rootstocks. Until 2001, the growth retardant chlormequat (CCC) was used the reduce shoot growth and stimulate flower bud development. The withdrawal of CCC for use by fruit growers has renewed the interest in rootstocks more dwarfing than Quince MC. Besides dwarfing of the scion cultivar other desired traits of new rootstocks for pears are: 1) precocity of cropping, to ensure early yields starting in the second year after planting; 2) fruit size, for ‘Conference’ pears the proportion of pears with a diameter >65 mm should be as large as possible; 3) disease resistance, especially towards fire blight and pear decline; 4) Proc. X IS on Pear Eds.: A.D. Webster and C.M. Oliveira Acta Hort. 800, ISHS 2008 600 compatibility with scion cultivars; 5) frost resistance; 6) ease of propagation; 7) suitability for growth on calcareous soils (absence of lime-induced chlorosis). In addition, new rootstocks may also be used to meet the changes in market demands for ‘Conference’ pears like larger and less russeted fruits. Since there is no breeding program for pear rootstocks in the Netherlands, the research on pear rootstocks is limited to testing rootstocks selected in other countries. The main selection criteria for the Dutch pear industry are: 1) tree size; 2) production; 3) production efficiency; 4) fruit size distribution. Additional criteria for nurseries exporting fruit trees to other countries are: 1) compatibility with as many modern scion cultivars as possible; 2) suitability for growth in different soils; 3) easy propagation. During the past decades a large number of rootstocks were collected by Wertheim (1998). The results of evaluation trials with several of these rootstocks have already been published (Wertheim, 2002; Wertheim and Vercammen, 2000). This paper contains the results of three more recent trials with the cultivars ‘Conference’ and ‘Doyenné du Comice’ grafted on a number of other Quince and Pyrus rootstocks. MATERIALS AND METHODS Plant Materials All trials were planted in the experimental orchard of the fruit research station at Randwijk (5.707° East, 51.937° North) in fresh soil consisting of river clay with 30% silt. Trees were planted at a distance of 3.5 x 1.5 m and were trained as spindle trees according to local commercial practice. Growth of the trees was not regulated by chemical growth regulators, root pruning or girdling of the trunks. No gibberellins were applied to stimulate fruit set. Contrary to common practice the rootstocks were not protected by a layer of mushroom compost during the winter in order to evaluate the frost resistance of the different rootstocks. During a period of at least 5 years the growth and production of the trees was monitored by: Annual or biennial measurement of the girth of the trunk at a fixed point ca. 30 cm above the graft union Annual shoot growth on a scale from 1 (no shoot growth) to 9 (very vigorous growth) Annual flower intensity on a scale from 1 (no flowers) to 9 (very abundant flowering) Annual fruit production (number of fruits and kg fruits per tree) Analysis of mineral content of leaves and fruits (once for each trial 3 to 5 years after planting) Fruit size distribution (at least once for each trial 4 to 5 years after planting) Calculation of production efficiency (number of fruits/cm2 trunk cross sectional area) Trial 1 (code 044-Ra99107) Trees were planted on April 8, 1998. The statistical design of the trial was a split plot with the scion cultivar being the whole plot and the rootstocks the sub plots. Plots were replicated 8 times and contained one tree of each treatment. In this trial the growth of ‘Conference’ and ‘Doyenné du Comice’ budded at 10 or 25 cm height was compared for the Quince rootstocks MC, C132, and the Pyrus rootstock BP10030 (Trajkovski and Andersson, 1990). Rootstocks were obtained from Naktuinbouw (Netherlands Inspection Service for Horticulture) (MC), HRI East Malling, UK (C132), Eliteplantstation Balsgård, Sweden (BP10030) and Fleuren Nurseries, Netherlands (Eline). MC, C132 and BP10030 were virus free. The virus-status of Eline was unknown. Trial 2 (code 044-Ra00105) Trees were planted on March 30, 2000. The statistical design of the trial was a split plot with the scion cultivar being the whole plot and the rootstocks the sub plots. Plots were replicated 10 times and contained one tree of each treatment. The growth of ‘Conference’ and ‘Doyenné du Comice’ grafted on Quince MC was compared with trees