NC_OLD140: ROOTSTOCK AND INTERSTEM EFFECTS ON POME- AND STONE-FRUIT TREES
Statement of Issues and Justification
The NC-140 Regional Research Project is designed to address a number of high-priority areas within the North Central Region as well as other parts of North America. This project seeks to enhance economically and environmentally sustainable practices in temperate fruit production by focusing on rootstocks. The NC-140 project meets all guidelines presented by the North Central Regional Association (NCRA) in Guidelines for Multistate Research Activities (May 2001). Specifically, it addresses high priorities defined by NCRA, within the crosscutting research areas of agricultural production, processing, and distribution, genetic resource development and manipulation, and integrated pest management. The project is multidisciplinary and multistate and directly benefits society. Further, Federal and state dollars leverage significant additional resources. Lastly, outreach is integral to the project and includes electronic information transfer through web sites, much written material for growers and other stakeholder groups, and numerous educational programs in individual states and at national and international grower and scientific meetings.With the increasingly competitive international market, the growing demand for higher quality fruit by consumers, the strong pressure to reduce chemical use, and an ever increasing need to enhance the economic efficiency of production, tree-fruit growers must look to alternative economically and environmentally sustainable management strategies for production. Growers who want to stay profitable must establish high-density plantings with much smaller trees with new scion cultivars. These high-density plantings may cost several times more to establish than low-density plantings, thus greatly enhancing the economic risk. Rapid returns are also vital for providing the ability to change cultivars in response to market or genetic opportunities. The central component of high-density systems is the rootstock. The root system imparts many characteristics to the mature tree such as size, precocity, productivity, fruit quality, pest resistance, stress tolerance, and thus profitability.
As the industry moves from low- to high-density plantings, several rootstock-related problems must be addressed. New pome- and stone-fruit rootstocks cannot be recommended to commercial growers without reservations until there is sustained research as to soil and climatic adaptability, root anchorage, size control, precocity, productivity, pest resistance, and propagation ability. In general, field testing of rootstocks in an orchard setting requires a minimum of ten years to assess accurately the potential for improved profitability, reduction of inputs, and enhancement of production efficiency. With year-to-year variation in weather, this time span is necessary to obtain a true indication of rootstock performance.
The establishment of the NC-140 technical committee in the 1970s enabled researchers to develop a coordinated effort in apple rootstock research through the uniform testing of rootstocks and multiple genetic systems, and to discuss, evaluate, and coordinate other rootstock research. Since NC-140 is the only regional project focusing on apple rootstock problems, researchers from around North America (see Appendix E) have joined the committee. Stone-fruit crops and pears were included in the project in its 1982 revision.
A stable tree-fruit industry based on economically and environmentally sustainable orchard systems is one of the primary goals of NC-140 research. Prior to the organization of NC-140, knowledge of rootstock and multiple genetic system performance and adaptability had to be obtained from a number of unrelated studies. The lack of common planting materials, spacing, and cultural procedures made comparison of the results of these studies difficult, and slowed the accumulation of knowledge that could be applied by orchardists. These difficulties resulted in serious planning and management errors. Also, such unrelated studies have been incomplete or slow in evaluating rootstock tolerances to biological, environmental, and edaphic stresses. Through the uniform cooperative testing undertaken by NC-140, new rootstocks can be exposed quickly and systematically to widely varying soil and climatic conditions to shorten the time necessary for thorough evaluation.
Evaluation and Environmental Effects
Each rootstock may react differently with a particular scion cultivar under a particular training system or in a different environment. This interaction makes it necessary to field test rootstocks that may be profitable for North American growers across a wide range of environments with appropriate orchard-training systems. Orchard systems must be designed to meet the specific needs of each fruit crop. In past years, the free-standing, central-leader system has been very profitable for North America, particularly with the weak, spur-type Delicious cultivar. As labor became more expensive and the list of profitable cultivars started changing rapidly, some North American growers began converting to smaller, high-density orchard systems. Also, tree response under a particular management system in different areas in North America can vary greatly. These problems require that rootstocks be tested under several North American climates and that modifications of training and pruning techniques be developed to match local growing conditions.
Ultimately, any newly developed rootstock must exist as an integral part of a total orchard-management system. Current economic trends make production efficiency of new scion/rootstock combinations under various cultural-management systems one of the most important factors that must be evaluated thoroughly before specific combinations are recommended for large-scale plantings by fruit growers.
The experimental design most commonly used to evaluate rootstocks at a single location is the randomized complete block design (RCBD). In a RCBD, the plot of land that is available for the experiment is usually divided into a number of blocks (often rows), based on orchard location. The assumption is that soil and environmental conditions are more uniform within blocks than between blocks. One tree on each rootstock is assigned randomly within each block. In early NC-140 rootstock trials, fewer than 10 rootstocks were compared, and RCBD was probably appropriate. Recently, the NC-140 rootstock trials have included 20 rootstocks. The RCBD may not be appropriate for these large blocks, because variation within blocks increases as block size increases. An alternative experimental design may be preferable. Our objective is to determine which experimental design is most efficient using data generated from the project.
Rootstock Propagation
Knowledge of the propagation characteristics of newer rootstocks and reasons for incompatibility between cultivars and rootstocks is a continuing need. Some promising pome- and stone-fruit rootstocks presently cannot be considered for commercial use because of difficulty in rooting clonal material using existing techniques. Alternative methods of propagation only recently have begun to offer solutions to these problems. Using different propagation methods such as hardwood or softwood cuttings, which are not commonly used, may be very effective for mass production of some of the rootstocks that do not propagate well by conventional means. Expanded research with micropropagated plant material needs to be done to document genotypic and phenotypic stability, thereby anticipating potential problems before widespread adoption by the fruit industry occurs. These techniques may also allow early screening of plant material for desirable characteristics such as disease and insect resistance.
Genetic Resources
If the pome- and-stone fruit industry is going to continue to change to meet the needs of the consuming public, new genetic material will need to be incorporated into existing material to enhance performance of rootstocks. Rapid means to screen potential rootstock candidates or breeding materials for susceptibility to various biotic stresses is vital. Using both traditional plant breeding and genetic engineering methods, researchers can incorporate insect and disease resistance into existing rootstock material, as well as develop rootstocks with enhanced horticultural performance as a greater understanding of the physiology of multiple genetic systems is elucidated. Obtaining genetic material from research programs from throughout the world and testing these new rootstocks cooperatively has been an integral part of the project. Clonal materials of different rootstocks have been obtained from many countries since the inception of the NC-140 project and will continue. The 1994 apple rootstock trial contains rootstocks from England, Poland, Russia, and the United States. As a result of this and the previous NC-140 apple plantings, one of the Russian rootstocks is being propagated and sold extensively to orchardists in North America. Additionally, the first commercially important dwarfing cherry rootstock originated in Germany and was identified as having commercial value by the 1987 NC-140 Rootstock Trail.
Physiology and Stress
In multiple genetic systems, rootstock can dramatically alter the developmental physiology of the scion. That is, rootstock can alter meristem determination (flower bud formation) and carbon partitioning (tree vigor, fruit quality, and shoot growth). Similarly, rootstock can alter abiotic stress tolerance of the scion, such as cold hardiness, drought tolerance, or adaptability to differences in soil chemistry or physics. One or more of these factors limit the use of existing rootstocks, and potentially will limit the adaptability of some new rootstocks within North America. Understanding how different rootstocks affect developmental and abiotic-stress physiology can lead to recommendation for use or non-use under certain orchard conditions. Such studies must include factors contributing to developmental differences or stress tolerances, development of practical means for controlling various stresses, or development of rapid means for screening potential rootstock candidates for susceptibility to various stresses. A better understanding of the physiological mechanisms behind these responses may allow for development of cultural practices that relieve the detrimental effects of stress.
Cooperative testing of new and existing rootstocks by NC-140 researchers continues to generate interest and support from the fruit and nursery industries. This interest has resulted in industry financial support for the establishment and management of cooperative plantings, grants for state rootstock research, and propagation of trees for several of the NC-140 plantings. It is estimated that over the term of the current project (2002-2007), nearly $2,000,000 will be received to support its research from sources other than universities and Hatch Multi-State Research Funds, and more than one half of this total will come from grower organizations.
A compelling need exists to continue the present coordinated studies and to initiate new studies for both pome- and stone-fruit rootstocks, as new plant materials are made available. Continued testing will provide a thorough evaluation of promising rootstocks, multiple genetic systems, and planting and training system efficiencies. This research project has led and will continue to lead to sound recommendations to growers and nurseries based on widespread knowledge of adaptability and performance of the plant material.
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