Rootstock Breeding for Stone Fruits

Over the last 20 years stone fruit rootstock development has begun shifting from seedling to clonal types, many of interspecific origin. Publicly funded breeding programs have produced most of these rootstocks due to the time, cost, and risk associated with their development; however, private industry is emerging as a significant contributor of many of the newer rootstocks. Particularly noteworthy among recent releases has been the incorporation of resistance to soilborne diseases, nematodes, waterlogging and vigor control, the last most notably in recent cherry rootstocks. Nevertheless, despite the remarkable progress in the development of clonal stocks, seedling rootstocks still dominate most stone fruit industries around the world, if only because of their relatively low cost, ease of propagation, and proven utility. Many opportunities and challenges remain to be addressed in the areas of disease and insect resistance, adaptation to biotic and abiotic stresses, graft compatibility, and rootstock influence on scion performance and fruit quality. Biotechnology is beginning to show potential in accelerating rootstock development. With the development of markers to assist selection for difficult to evaluate traits, new rootstocks with resistance to multiple diseases are feasible. Future prospects for breeding are presented. INTRODUCTION Several surveys have been undertaken to determine the relative importance of the various “problems” facing stone fruit industries around the world. The peach (Prunus persica) industry has received the most attention, if only because of its large size, hence economic importance, relative to other stone fruit crop industries. Results of two of these surveys are summarized in Table 1. Relative order of importance changed little between 1982 and 1997. At this time it would appear that the need for waterlogging tolerance has been alleviated somewhat by recent releases. This factor was the only one that moved down significantly in importance since the earlier survey. Given the number of rootstock releases that offer some relative promise for most of the problem areas listed, this might seem surprising at first. However, given that orchard life expectations for stone fruits range from 15 to 25 years or longer, adoption is an inherently slow process. This may also be due in part to the fact that many interspecific rootstocks change other “non-target” characteristics of the finished tree, for example, vigor or bloom date which require changes in management that growers may not wish to change. Additionally, many “problem” sites have more than one limitation and require that a new rootstock incorporate resistance to multiple problems for successful adaptation. In many cases, new rootstocks are probably best suited for regional or prescription/niche planting rather than broad use over a large industry. Regional testing is the only way to determine each rootstock’s best adaptation. Priorities vary from one stone fruit crop to another. Rom (1991) has summarized priorities for apricots (P. armeniaca) which generally agreed with those cited for peach. Nematodes were of less importance and one particularly vexing issue with apricot was that of scion/rootstock graft compatibility. However, Rom noted that priorities also varied Proc. XXVI IHC – Genetics and Breeding of Tree Fruits and Nuts