Rootstock Development in Temperate Nut Crops

The driving force behind the development of rootstocks for nut crops has been the solution of specific problems, often related to the presence of pests or the need for adaptation to particular sites. The use of interspecific hybrids as rootstocks has lead to increased appreciation for the contribution genetic diversity can make to orchard profitability in the face of site specific challenges. The past decade has seen dramatic increases in access to germplasm resources, while technological developments in molecular genetics have contributed to genetic characterization in some species. The focus on clonal propagation through tissue culture has been somewhat abated by the slow pace of nursery incorporation of available techniques, by challenges with anchorage of some clonal rootstocks, and by the recognition of the vulnerability inherent to a monoculture susceptible to an unsuspected root pest. Increased application of spatial analysis systems (GIS) may contribute to prescription use of site-specific rootstocks. INTRODUCTION Nut crops are relatively long-lived species whose performance reveals the integration over time of the plant’s genetic composition (both of the scion and the rootstock in grafted plants) with the effects of the site (composed of edaphic, climatic and other biotic variables), under the cultural system used for management. The development of improved rootstocks for nut crops requires an understanding, appreciation, and control of all of those potential sources of variation. Rootstock influences are more obscure than scion effects. Systematic rootstock development through breeding requires the same commitment of time and resources needed for scion breeding while the demonstration of rootstock efficacy requires additional care in test establishment and long-term monitoring. Furthermore, various sitespecific challenges within otherwise homogenous regions of cultivar adaptation introduce additional complexity while possibly limiting broad deployment. As a result there are few programs focused specifically on rootstock breeding for nut crops. The historic pattern of rootstock development across nut crops has been one of dynamic interaction between a knowledgeable grower community comprised of nurserymen, traditional farmers (using local materials in seedling culture) and orchardists (using selected genetic materials in grafted configurations under intensive management), an active plant introduction program, and an observant scientific community, all riding a mounting wave of developing technology. In recent decades, the ingredients of that mix have been richly supplemented: the nursery industry has incorporated new techniques of propagation with an abundance of ever “improved” cultivars and rootstocks; traditional farmers have been the focus of unusual and increasingly complimentary attention; orchardists have had the expert consultation of multidisciplinary teams, organized by the wonders of computer technology to address management goals; the plant introduction program has provided increased access to international germplasm; and the scientific community has added the tools of molecular genetic characterization and spatial analysis to its arsenal. There is potential for great improvement in rootstock development for most nut crops. At the beginning of the 21 century we find ourselves with powerful tools to map new territory. Proc. XXVI IHC – Genetics and Breeding of Tree Fruits and Nuts