Taxonomically, Prunus spinosa is in the Rosaceae (Rose) family. In countries where the plant grows indigenously, it is phytosociologically classified within the ‘thorny bush’ plant community (Rhamno-Prunetea). The natural habitat range of Prunus spinosa is the Palearctic region. This region covers all of Europe, North Africa, the Middle East, and North, Central and East Asia.
The range of suitable, weak-growing rootstocks for intensive plum cultivation is limited worldwide. This also limits the possibility to further intensify the cultivation of plums and other stone fruit crops for which the Prunus spinosa rootstock selections may be suitable, such as apricot, peach and almond.
Prunus spinosa was also tested to a limited degree as a rootstock in the 19th century. One publication lists Prunus spinosa as being used for the slow growth of mirabelle plums (Roville, 1876).
Experimental Station for Fruit Cultivation
A century after this publication, Dr H.J. van Oosten was looking for a suitable weak-growing rootstock for plum and tested a small population of blackthorn seedlings as a rootstock for Victoria at the former Experimental Station for Fruit Cultivation in Wilhelminadorp (Van Oosten, 1986). The seedlings used showed an extremely wide range of vigour. A number of them showed weak growth. Due to considerable thorniness, no further work was done on these selections. Incompatibility problems were not expected, and did not arise, due to the close genetic relationship between plums and blackthorn.
At the initiative of Dr S.J. Wertheim, blackthorn (Prunus spinosa) seedlings were cultivated at the former Experimental Station for Fruit Cultivation in Wilhelminadorp starting in 1987 in order to identify one or more selections that would be suitable as weak-growing rootstocks for plums. The reason for this search for weak rootstocks for plum was because no viable rootstock had been cultivated to date. VVA-1 was not yet in the picture and Wertheim saw possibilities based on Dr van Oosten’s experiences. A good weak-growing rootstock was necessary for the further intensification of plum cultivation. Close to a thousand Opal seedlings were oculated in 1990. The year-old Opal trees were evaluated, described and selected in 1991. Considerations included the extent of growth, healthy leaf condition, straight main trunk and, if relevant, horizontal side branch implantation. All selections were assigned a code consisting of a letter (S, M or Z) and a serial number. The letter S meant that the oculation for that selection showed relatively strong growth, while M represented moderate growth and Z weak growth. A total of 113 trees were selected and planted in Wilhelminadorp in early 1992.
From 1992 to 1999, the growth, health and fertility of these 113 Opal trees were monitored at the Experimental Station in Wilhelminadorp. Based on the results, a selection was made in 1999 of the 24 most promising trees, which demonstrated a clear growth weakness compared to St. Juliën A and positive fertility and fruit size. The others were excluded due to high vigour or lower productivity.
During the first years in Randwijk, the research was oriented towards ways to propagate the blackthorn selections. Various propagation methods were tested using both winter and summer cuttings. Propagation using winter cuttings turned out to be less viable, while the summer cuttings fared better. The use of mid-summer shoots proved to be most successful.
The rooted rootstocks obtained were crossed with Victoria, cultivated for three tests and planted in 2005, 2006 and 2008. Since two selections resulted in insufficiently rooted cuttings, fifteen of the seventeen selections were included in the test. The goal of the study was to further test the rootstocks in order to arrive at a selection of one or several of the most suitable types with the best characteristics for practical introduction.
The vigour, production, fruit size and number of root sprouts of the blackthorn rootstocks were monitored through the year 2011. WUR S766 showed the best combination of properties: slow growth, high production efficiency, large fruit, relatively few root sprouts and moderate thorniness.
Starting in 2014, Wageningen UR PRO Fruit began testing and demonstrating the WUR S766 in practice on a larger scale, with several varieties and in different types of soil. This was done both in the Netherlands for plums and internationally for plums and other crops, such as apricot, peach and almond, for which the rootstock appeared to be suitable in terms of genetic compatibility. Since sep 2016, the PRO Fruit is called business unit Fruit of Wageningen Plant Research.