Useful differences in growth vigor between subclones of the apple rootstock M.9

In a national trial on five sites, performance of the apple cultivar 'Red Elstar' on three subclones of the rootstock M.9 was compared between 1990 to 1995. On each site, one additional scion cultivar was evaluated as well. Subclones involved were: T 337 (the standard M.9 from the Dutch Inspection Service for Woody Nursery Stock (NAKB), Fl 56 (a selection from the nursery of H. Fleuren at Baarlo, The Netherlands), and RN 29 (a selection from the nursery of R. Nicolai at Alken, Belgium). All scions and rootstocks were free of known viruses. On all sites, 'Red Elstar' on Fl 56 showed significantly lower vigor than on the two other sub-clones, which were equal in vigor. 'Red Boskoop', 'Cox's Orange Pippin', and 'Jonagored' were also least vigorous on Fl 56, but for 'Alkmene' and 'Gloster' no differences were observed. For 'Alkmene', the trees on all subclones were extremely weak, masking possible differences. In flowering intensity, productivity, fruit size and color, mineral composition of leaves and fruits, degree of root suckering, and number of burrknots on the rootstock shanks no differences of importance were observed. In a few cases, fruit size on Fl 56 was slightly smaller than on the other two subclones, but this could be explained by higher fruit number per m(3) tree volume. It is concluded that when less vigor is wanted than provided by standard M.9 types, Fl 56 offers an attractive possibility. It means that fruit growers can remain within the well-known M.9 features.