Results of multi-site interstem trials with apple trees

An interstem trial was planted in the spring of 1988 with the tender apple cultivars 'Elstar' and Jonagold' on M.9 rootstock with thirteen interstem c ultivars to study their possible effects on tree hardiness and growth vigou r. The interstems used were:'Gloster: 'Golden Delicious', 'Summerred', beca use of their hardiness shown in an earlier Dutch trial, 'Heyer12: 'Antonovk a' 'Hibernal', 'Dolgo', 'Honeygold', 'Yellow Transparent', 'Ottawa 3','Hara lson' and 'Paulared: because of their reputed hardiness elsewhere, and 'Dub bele Zoete Aagt' as the standard interstem used in The Netherlands. Two-yea r-old trees directly on M.9 served as controls. The material used was free of known viruses, except for 'Hibernal' where this was not quite certain. T he trees were made in one nursery and plant ed at one site in Denmark and t hree in The Netherlands in single rows at distances ranging from 3.00 x 1.2 0 to 3.25 x 1.50 m and raised as slender spindles. The trial lasted till wi nter 1994/95 with 'Jonagold' and one year longer with 'Elstar'. No damaging frosts occurred during years that the trees were in the field, so no conclusions could be drawn with regard to the winter hardiness of the interstem cultivars involved. Growth control was obtained, especially with 'Elstar'. With both cultivars, growth reduction occurred most consistently with 'Dolgo', 'Ottawa 3','Paul ared' and 'Summerred'. Little growth control was obtained with 'Gloster' an d 'Golden Delicious' and especially 'Antonovka.' Some, growth-reduced inter stems ('Paulared', -Summerred') had a thin trunk relative to that of the sc ion cultivar, for others ('Dolgo' with both cultivars and 'Ottawa 3' with J onagold') the opposite was true. Productivity per tree was not much affected by the interstems, with the exc eption of 'Hibernal' allowing high productions and 'Ottawa 3' lagging behin d with 'Elstar'. However, compared to the controls, productivity per cm(2) scion trunk-cross sectional area (TCA) was increased in many cases, especia lly with 'Elstar'. With 'Elsrar', interstems of 'Heyer 12" "Hibernal", Paul ared' and 'Summerred' especially were effective. With 'Jonagold', 'Heyer 12 ', 'Dolgo' and 'Ottdwa 3' interstems rated high. Fruit size was not greatly affected by the interstem and if it was there wa s no clear relation with productivity per cm(2) TCA. Where growth was reduc ed fruit colouring was improved. Except for trees on an 'Ottawa 3' interste m, where in some years lower leaf contents of N, P, K, Ca and Mg were found , no clear interstem effects were observed in the mineral content of leaves or fruits. The results are discussed and it is concluded that where both growth contro l and increased hardiness are wanted,'Summerred' is a valuable interstem. W hen vigour control is not the main aim, other hardy interstems are promisin g as well, such as 'Hibernal' with its good effect on productivity. Given the many interactions found, it is stressed that without thorough exp erimentation the use of the before-mentioned interstems for other cultivars under other conditions bears risks.