Presented is the multi-stage process used to create rootstock genotypes and then evaluate them to identify those most promising for commercial release. Our strategy is to create large, highly variable populations by crossing successful, traditional rootstock cultivars with other wild or cultivated apple accessions that have desirable attributes (such as resistance to pests, diseases, or difficult environmental conditions). At each new stage of the apple rootstock breeding and evaluation program we decrease the number of genotypes to be evaluated and increase both the level of replication and the number of types of data collected. Our final goal is to identify a group of rootstock genotypes that are superior to the commercially available lines.

The stages, plant numbers, and time frames presented describe how we expect the project to work ideally, without unanticipated setbacks. In reality, genotypes may sometimes require more time than is scheduled to move from stage to stage, and any particular genotype could move more slowly through the breeding program. It would be difficult, however, to move a genotype through the program more rapidly than is described and still obtain adequate levels of testing. We have resumed making hybridizations in 1999 on a small scale, and in 2000 on a larger scale (we are presently collecting and growing the parents necessary for the crosses). We also have approximately 5000 rootstock genotypes presently in stages 2-9, and these will continue to be evaluated by the program and to move through the appropriate stages.

Stage 1. Parental Selection, hybridization, disease screenings, stool plant establishment, Years 1-3 / 5000 seedlings.
The first stage of the breeding program is parental selection. Pairs of parents are carefully chosen for hybridization that have complementary characteristics (for example, an easily propagated dwarfing parent might be crossed with an exceptionally disease resistant parent). Seeds are collected from the fruit of these crosses, and the seed are stratified (cold treated to break dormancy) and germinated. We then attempt to infect thousands of seedlings with fire blight bacteria (Erwinia amylovora) and crown rot fungus (Phythophthora spp.), the rootstock diseases that cause the most difficulty for US apple producers. We expect that only 20% of our seedlings will survive these screens, and we plant these populations in our fields to establish single plant stool tree populations with enhanced disease resistance.

Stage 2. Stool plant selection, nursery liner establishment, nursery tree growth, Years 4-6 / 1000 stool trees.
Genotypes are propagated as single tree stool plants, and nursery liners are harvested from genotypes that show adequate rooting and do not have brittle wood. Liners are moved to a nursery for years 5 and 6, where finished trees are produced. In years 5 and 6 stool trees are again evaluated for resistance to fire blight and for infestation levels with woolly apple aphids, and susceptible genotypes are discarded from the nursery and from the stool tree fields. Only approximately 100 genotypes per year move on to actual orchard tests.

Stage 3. Initial test orchard establishment, precocity evaluation and selection , Years 7-9 /100 rootstock genotypes.
Two to four finished trees on each rootstock genotype are planted in an initial test orchard. In addition to the test genotypes, size standard varieties are included (M.27, M.9, M.26, MM.106, MM.111). Trees are trained to develop an open branching pattern, but pruning is minimized to allow an accurate assessment of size control. Data is collected annually for yield, yield efficiency, suckering, and response to any unique stress events.

Stage 4. Initial test orchard evaluation and selection, elite stoolbed establishment, Years 10-12 / 50 rootstock genotypes.
Rootstock genotypes that exhibit precocity, some size control (< MM.111 in size), and adequate yield efficiency by year 9 are propagated to increase plant material for an elite stoolbed. Stoolbeds are developed from liners retained from stage 2 or from root cuttings of older orchard trees. Elite stoolbeds are stooled in year 12, and liners of rootstock genotypes that continue to perform well in initial test orchards are collected. Initial test orchards are maintained, and data collection continues. The most promising genotypes evaluated from other breeding programs may join the evaluation portion of the Cornell/USDA program as stage 4 genotypes if they have not yet been commercially successful internationally.

Stage 5. Liner production, stoolbed evaluation, nursery tree growth, Years 13-15 / 25 rootstock genotypes.
Liners in nursery are budded with Golden Delicious (an easy to grow, familiar cultivar with wide adaptation), to produce 30 high quality finished trees. Initial test orchards are removed after harvest in year 15 (after 9th leaf). After 30 trees are produced in the nursery, liners are collected from elite stoolbeds and subjected to evaluations of disease resistance and stress tolerance (extreme temperature soil tests, replant soil tests, fireblight tests, crown rot tests, virus resistance / hypersensitivity tests, graft union strength tests) while in stages 5-7.

Stage 6. Intermediate stage orchard establishment and early evaluation, Years 16-18 / 10 rootstock genotypes.
Intermediate stage orchards are planted beginning in year 16 at three sites representing a cross-section of domestic apple production environments. Each year's planting includes commercial standard genotypes (M.9, M.26, MM.106) and 5 to 10 elite rootstock genotypes that have shown promise in elite stoolbed liner production, initial test orchard performance, and biotic and abiotic stress resistance screens. These orchard trees are evaluated for precocity in their early years.

Stage 7. Intermediate stage orchard evaluation, commercial stoolbed trials, Years 19-21 / 5 rootstock genotypes.
Intermediate stage orchard trial data collection continues. Biotic and abiotic stress screenings of rootstock liner trees is completed. The most promising rootstock genotypes (a maximum of 5 per year) from the Cornell/USDA program are distributed to cooperating nurseries for commercial stoolbed trials (50 liners to each of 2 cooperating rootstock nurseries) beginning in year 19. The most promising Geneva rootstock genotypes are submitted for phytosanitary certification (NRSP5, Prosser WA) to enable international distribution.

Stage 8. NC-140 and on-farm trials, distribution to all cooperators, Years 22-24 / 2 rootstock genotypes.
Intermediate stage orchard trial data collection continues. For outstanding rootstock genotypes from the intermediate stage orchard trials and commercial nursery stoolbed trials, liner production from cooperating nurseries is used to propagate trees for NC-140 and/or on-farm trials. NC-140 trials and on-farm trials are established. Best rootstock genotypes are distributed to domestic cooperating nurseries for propagation, and to international cooperating nurseries and institutions for propagation and local evaluation trials. Rootstock genotypes that are commercially successful internationally join the USDA/Cornell rootstock evaluation program as stage 8 materials following evaluation in biotic and abiotic stress screenings.

Stage 9. Final evaluations and selections, commercial ramp-up, patent applications, Years 25-27 / <1 rootstock genotype.
Plant material for rootstock genotypes demonstrating marked improvement over commercially available varieties is increased in commercial stoolbeds. Intermediate stage evaluation orchards are removed after 11th leaf. Patent and UPOV protection applications are filed on commercially viable rootstock genotypes.

Stage 10. First commercial sale of Geneva rootstocks, elimination of all unreleased genotypes from trials, Years 28-30.
NC-140 and on-farm trial data collection continues. Unreleased genotypes that showed promise but were not demonstrably superior to commercially available rootstocks are eliminated from the program.

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