International experience with rootstocks
Scientists at the fifth International Cherry Symposium presented numerous papers devoted to cherry rootstock research. I find that it is often difficult and perhaps dangerous to directly transfer rootstock results from one region to another. One reason is that differences in climate and soils can affect tree vigor, survivability, and fruit quality, as evidenced by the variability in vigor of Gisela 6 between the East and West Coasts of the United States. Another reason is that the goals and priorities of growers vary around the world. For example, for certain conditions, many German growers believe that Gisela 5 is too vigorous. They are looking for a rootstock like Gisela 3 that produces a tree small enough to easily fit under rain covers (Stehr).
Nevertheless, it is possible to glean some information from these studies that may be useful to growers in the Pacific Northwest. The Rhine Valley is an important fruit production area located in western Germany. The region is characterized by sandy soils with most orchards nonirrigated. It is important to note these characteristics because they will have a substantial effect on rootstock performance. Nevertheless, rootstock work conducted by Martin Balmer, using Regina and Sylvia, is interesting and potentially pertinent to Pacific Northwest growers.
Maxma Delbard 14 (a patented selection of Maxma 14) is the most planted rootstock in this area. Rootstock tests were conducted at two locales where Gisela 5 had outstanding productivity, as might be anticipated. However, fruit size remained unexpectedly good. Gisela 5 also performed well in replant situations.
Maxma Delbard 14 had below-average productivity and average fruit size. In these soils, tree size was 30 percent less than Mahaleb, similar to the French experience, but dissimilar to observations made in the Pacific Northwest. Interestingly, on replant sites not treated prior to planting, vigor was drastically reduced. Maxma Delbard 14 also showed sensitivity to water-logged soils.
As with Gisela 5, Gisela 6 exhibited high productivity and good fruit size. However a thunderstorm in 2003 selectively uprooted the Gisela 6 trees. Although Pacific Northwest growers have not experienced this dramatic uprooting, there are many reports of trees, grown on Gisela 6 rootstock, beginning to tilt when irrigation precedes a high wind. The German experience is perhaps a caution to Pacific Northwest growers to avoid planting trees on Gisela 6 rootstock in sandy soils.
Balmer, M. Evaluation of Semi-Dwarfing Rootstocks for Sweet Cherry Orchards in the Rhine River Valley.
Stehr, R. Further Experiences with Dwarfing Sweet Cherry Rootstocks in Northern Germany.