Selection of Prunus spinosa as a dwarfing rootstock for high density plum orchards

In 1987 a selection program was started to find a dwarfing rootstock for European plums. Prunus spinosa (blackthorn) was the chosen species due to its winter hardiness, drought tolerance and previously shown compatibility with plum. In 1990 about 1000 blackthorn seedlings were budded with ‘Opal’ plum. Based on the vigour of the trees during the first year in the nursery, 113 selections were selected for further testing for their effect on tree development and fruit production. The amount of root suckers and thorns were used as additional selection criteria. In 1999 the number of selections was reduced to 24. In several trials carried out between 2006 and 2011, the vigour and yield of ‘Victoria’ on these selections was shown to vary between that on the commonly used rootstock ‘Saint Julien A’ and ‘Krymsk1’ (VVA-1), a dwarfing and highly productive rootstock that had been introduced in the Netherlands concurrently with the trials. Although ‘Krymsk11’ gives very good tree-size control and precocity, it is too dwarfing for some soils and cultivars and sensitive to bacterial canker caused by Pseudomomas syringae. Recently, the five most promising selections were planted in higher numbers. The best selection will be released as soon as it has been registered, and virus free material becomes available. Additional tests will be needed to determine the suitability of these blackthorn selections for other stone fruit species like peach and apricot and for other soils and climatic conditions.