Breeding improved apple rootstocks has been a priority research area in New York State since Dr. Jim Cummins and Dr. Herb Aldwinkle initiated crosses in 1970. In 1998, the rootstock program became a joint Cornell/U.S. Department of Agriculture program and is now headed by Dr. Gennaro Fazio. The focus of the program has been to create rootstocks that are resistant to the major apple diseases, fireblight (Erwinia amylovora) and crown rot (Phytophthora cactorum), tolerant to a wide range of soils, and provide a wide range of vigor classes for all orchard situations. New rootstocks need to be precocious, productive, and easy to propagate, yet burr knot-free.
Crosses were made from candidate plant material that was known to be pest-resistant and dwarfing. More than 100 genotypes have been used as parents. The most common successful parents include the hardy and disease- resistant Malus robusta (Robusta No.5), Malus floribunda, Beauty crab apple, Malling 9, M.26, M.27, and Ottawa 3 (Robusta 5 x Antonovka) from the Ontario rootstock breeding program.
After successful crosses were made, progeny were subjected to a harsh selection process that weeded out those that did not show disease resistance. Then, surviving candidate rootstocks were grafted to a number of varieties for field testing, first at experiment stations across the United States. Then, the best candidates were evaluated in replicated trials on grower-cooperator farms. Northern Spy, Red Delicious, Golden Delicious, McIntosh, Empire, Liberty, Jonagold, and Crispin were all used for these evaluations.
To date, seven rootstocks have been released to cooperating nurseries and are available for planting as a result of successful field testing.
Geneva 65 is an extra-dwarfing rootstock approximately 60 percent the size of M.9 and is a product of an M.27 x Beauty Crab cross. It is immune to fireblight and crown rot with extremely high yield efficiency. Our experience with Empire has been that fruit size has been smaller than desired. This rootstock is too dwarfing for most commercial orchard situations except with varieties that are vigorous and large fruited and at the highest planting densities.
Geneva 30 is most useful in northern growing areas where it shows wide soil adaptation, good winter hardiness, and high yields. It is resistant to fireblight and crown rot, and has very high yield efficiency for its size. This rootstock produces a tree slightly larger than M.26. It has performed well in apple replant situations. Despite this rootstock’s vigor, tree support is required because of early heavy cropping and weaker graft unions in the early years with brittle varieties. In New York, we have minimally supported blocks with Rome that have done very well. In a nearby commercial orchard, a small percentage of graft unions have snapped with Rhode Island Greening and Golden Delicious in the third and fourth years with heavy crops and stiff winds. Care should also be taken to properly support trees when Gala, Jonagold, Braeburn, and Honeycrisp are budded to G.30.
Geneva 16 has performed well in a broad range of locations. It had good survival in a severe winter cold snap in northern New York in 2004 where many of the Malling stocks and Budagovsky 9 had poor survival. It did show some cambium damage from an early winter freeze in Washington State. However, nursery trees tended to recover later in the orchard. It is resistant to fireblight and crown rot. It has very high yield efficiency and produces a tree similar in size to M.9. A grower test with Gala demonstrated that trees on G.16 survived an intense fireblight infection that completely killed trees on M.9. This rootstock is sensitive to latent viruses and requires virus-free bud wood.
Geneva 202 was patented and released in the United States in 2004. It has achieved significant commercial acceptance in New Zealand but is not yet being sold by U.S. nurseries. It is resistant to fireblight, crown rot, and woolly apple aphid. It has also shown moderate tolerance to the apple replant disease complex. It would be an excellent rootstock where woolly apple aphids are severe. It has very high yield efficiency and produces a tree between M.26 and M.7 in size.
Geneva 11 is being produced by several U.S. nurseries, and finished trees should be available this winter. It produces a tree about the size of M.26. It is crown rot resistant and shows good tolerance to fireblight. It has been the best performer in European trials with exceptional fruit size. This rootstock is our choice where woolly aphid is not a problem, since its yield efficiency is better than G.202
Geneva 41 (tested as CG.304l) was released for commercial sale in February 2005. Nurseries are beginning to bulk up this stock for commercial sale. Only a few rootstocks will be available for sale during the winter of 2005-2006, however. G.41 is a fireblight-resistant rootstock that dwarfs the tree similar to M.9 with equal productivity. It is very winter hardy. Branch angles tend to be naturally flatter using this rootstock. It has been the best performer in every New York trial.
Geneva 935 was also released for commercial sale in February 2005, and nurseries are beginning to bulk it up for commercial sale. It is a fireblight-resistant rootstock about the size of M.26 with productivity on par with M.9. It has been the best semidwarf stock in New York trials and appears to have good graft union strength. However, because of heavy early cropping, we do recommend tree support as for all Cornell Geneva rootstocks.
It has taken 30 years and more than 300,000 hybrid seedlings to get these rootstocks ready for commercial acceptance. The next stage of testing now is up to the grower with on-farm commercial-scale plantings. It will take more time to determine fully all the benefits and disadvantages. However, we are confident that fireblight, crown rot, and replant disease resistance, as well as improved productivity and fruit size are advantage enough to warrant investment in these stocks.
The Cornell/USDA program still has many rootstocks remaining to be evaluated. We are confident there are more winners among these stocks and hope to soon discover a “world beater.”