Apr 7, 2007

High-density Orchards, Dwarfing Rootstocks Gaining Popularity

High-density apple orchards on dwarfing rootstocks have become common in many apple growing regions of the world. This has allowed apple growers to achieve earlier production, higher production and better fruit quality than previously. However, for many apple growers in North America, New Zealand and many locations in Europe, the bacterial disease fire blight is a serious threat to dwarf apple orchards. M.9 and M.26, the most common dwarfing apple rootstocks, are very susceptible to this disease and in some locations this disease limits the planting of dwarfing rootstocks. Outbreaks of the disease in the Eastern United States have decimated many dwarf apple orchards. There is a great need to develop new, highly productive apple rootstocks that are resistant to the biotic and climatic stresses common in North America.

Fire blight infections usually occur to the blossoms during bloom and result in diseased branches that die and give the appearance of burned leaves and twigs thus the name fire blight. Recent research has shown that the bacteria once inside the tree can travel symptomless down the trunk of the tree to the graft union and the rootstock below. With highly susceptible rootstocks like M.9 and M.26 the rootstock cambium, xylem and phloem at the graft union are killed causing the death of the tree. Although the problem of fire blight has been a serious problem for many years in North America losses of whole orchards were not experienced until dwarfing rootstocks came into use. With seedling or semi-dwarf rootstocks that were not as susceptible to fire blight as M.9 and M.26, the scion could become infected but the tree usually survived if the infected parts of the scion were pruned away. Resistant dwarfing rootstocks similarly would not prevent the scion from becoming infected, however, since the rootstock would survive an infection the tree would usually survive. This would allow growers to prune out the infected parts of the scion and regrow the lost canopy quickly and restore full production much more rapidly than replanting an entire infected dwarf orchard.

Cornell University has had an apple rootstock breeding project located at Geneva, N.Y., since 1968. The project was led by Drs. James Cummins and Herb Aldwinckle until Dr. Cummins’ retirement in 1993. Since then, the program was continued by Drs. Terence Robinson and Aldwinckle. In 1998 the USDA’s Agricultural Research Service joined the project and provided a new lead scientist (Dr. Bill Johnson from 1998 through 2000 and Dr. Gennaro Fazio from 2001-present). The project continues to be joint program between the USDA and Cornell University with Drs. Robinson and Aldwinckle continuing to contribute significant efforts to the program. The objectives of the project are to develop rootstocks with improved nursery and orchard characteristics and that are better adapted to the problems of fire blight, crown rot and replant disease which are common in the United States. Additionally, some selections from this project show resistance to woolly apple aphid, which is a problem in warmer apple growing areas of the world. New rootstock selections were developed by crossing desirable parents and then subjecting the resulting seedlings to greenhouse screening procedures to select for tolerance to fire blight and crown rot. This was followed by field evaluations of their propagation characteristics in the nursery, and productivity and dwarfing in the orchards at the New York State Agricultural Experiment Station in Geneva. By the mid 1990’s, Jim Cummins had selected several promising fire blight resistant apple rootstocks with a range of dwarfing. These were then tested in second level orchard trials that included on-farm trials within New York state and national trials in the United States, Canada, France and New Zealand. In this paper, we report on composite results from these trials.

CG rootstock characteristics

To date, five rootstocks from this project have been patented and released to several licensed nurseries for commercial propagation and sale. Additionally, two more rootstocks were released in December of 2004 and licensed nurseries will be authorized to begin commercial sales.

• Geneva 65: The most dwarfing CG rootstocks is G.65. It is significantly more dwarfing than M.9 (about 60 percent the size of M.9) and is similar in vigor as M.27. It is a cross of M.27 and Beauty Crab. It has proven to be highly resistant to infection of fire blight and phytophthora, but it is not resistant to woolly apple aphids. It has very high yield efficiency but fruit size from trees on G.65 has been only about 90 percent that of trees on M.9 (Table 1). It is somewhat difficult to propagate in stoolbeds, which has limited its commercial production but it is available from several nurseries in the United States in limited amounts. G.65 may be too dwarfing for most situations but we believe it has a place in very high planting densities (Super Spindle) with large fruited varieties such as Jonagold and Mutsu. With these two varieties and other similar large fruited varieties fruit size often is too large for optimum commercial packouts. G.65 used as a rootstock would help hold down the fruit size. Under these conditions, G.65 has a significant advantage over M.27 due to its fire blight resistance. Orchards with this rootstock should be planted at very high densities of 4,000 trees per ha to 6,000 trees per ha.

• Geneva 16: G.16 is a fully dwarfing rootstock with tree growth and vigor similar to vigorous clones of M.9 (i.e. Nic29 or Pajam2). It is a cross of Ottawa 3 and Malus floribunda. It is essentially immune to fire blight and highly tolerant to phytophthora, but it is not resistant to woolly apple aphids. G.16 has excellent performance in the stoolbed and produces a large tree in the nursery. Tree growth in the first years in the orchard is vigorous, but with the onset of cropping, tree vigor is moderated giving a final tree similar in size to M.9. Precocity and cumulative yield efficiency has been similar or slightly better than M.9. G.16 appears to have wide soil adaptability and some tolerance to replant disease. It has very good mid-winter hardiness and survived a recent winter freeze event in Northern New York state that killed many M.9 and M.26, M7 and MM.111 trees. However, it does appear have some susceptibility to very early winter freeze events. This is likely due to its vigorous growth characteristics in the nursery and in the orchard during the first few years where it grows late. Its greatest known deficiency it that it is sensitive to one or more latent viruses in scion wood. Infected scion wood results in death of the trees in the nursery or the first year in the orchard. This requires absolute use of virus-free scion wood. If virus-free wood is used it appears that G.16 is currently one of the best alternatives to M.9 in high fire blight areas. Most commercial nurseries in the United States produce trees on G.16 but some scion varieties are not available on this stock because virus-free scion wood is not available. Orchards with this rootstock should be planted at high densities of 2,000 to 4,000 trees/hectare.

• Geneva 3041 A second fully dwarfing stock with vigor similar to M.9 and G.16 is CG.3041. It is a cross of ‘Malling 27’ and ‘Robusta 5’. It is scheduled for commercial release and propagation in December 2004 and will be named Geneva 41. It is highly resistant to fire blight and phytophthora, but it is not resistant to woolly apple aphids. In the stoolbed G.41 is a shy rooter and will require higher planting densities in the stoolbed or tissue culture to improve its rooting. It also produces some side shoots in the stoolbed. In the orchard its precocity and productivity have been exceptional surpassing M.9. It also has excellent fruit size and induces wide branch angles. It has very good winter hardiness. Although it is similar in tree size and yield efficiency to G.16 and M.9, it does not have the virus sensitivity of G.16. It has similar graft union strength to M.9 and will require trellis or individual tree stakes. G.41 also has been tested in France where it was shown to be smaller in tree size than M.9Pajam2 but more productive while producing similar fruit size as M.9. Several commercial nurseries in the United States have limited amounts of G.41 for sale this year. Orchards planted with this rootstock should be planted at densities of 2,000 trees per ha to 4,000 trees per ha. It appears that G.41 will become one of the best alternatives to M.9 in high fire blight areas.

• Geneva 11: G.11 is a dwarf rootstock that produces a tree that is similar in size to M.26. It is a cross of Malling 26 and Robusta 5. It has good propagation characteristics in the stoolbed and in the nursery. In the orchard it has higher yield efficiency than M.26 (similar to M.9) and produces fruit size similar to M.26 (Table 4). It has moderately high resistance to fire blight (similar to M.7) and good resistance to phytophthora root rot, but it is not resistant to woolly apple aphids. It has also been tested in France where trees on G.11 were 15 percent smaller than M.9Pajam2 but 14 percent greater productivity and similar fruit size as M.9Pajam2. Orchards planted with this rootstock should be planted at densities of 1,500 trees per ha to 2,500 trees per ha. Presently G.11 is available only in North America and is just beginning commercial sales. Its fire blight tolerance should make it an excellent replacement for M.26 in fire blight prone areas.

• Geneva 202: G.202 is a semi-dwarfing rootstock that produces a tree slightly larger than M.26. It is a cross of Malling 27 and Robusta 5. It is fire blight and phytophthora resistant, but also has good resistance to woolly apple aphid, which is important in many warmer climates where woolly apple aphid is a rootstock pest. G.202 performs very moderately well in the stoolbed and produces good quality nursery trees. G.202 has been tested mostly in New York state and New Zealand. In New York it produces a tree 50 percent larger than M.9 and with slightly lower yield efficiency. In New Zealand, it has been found to be much more productive than M.26 and is one of the best stocks available. It appears that G.202 will be a useful alternative to M.26 in climates that have problems with woolly apple aphid. This rootstock should be planted at densities of 1,500 trees per ha to 2,500 trees per ha. It was released for commercial propagation in New Zealand by Cornell University in May 2002 and in the United States in 2004. Presently it is only available in New Zealand but rootstock nurseries in the United States are beginning production of this stock.

•Geneva 5935: A second semi-dwarfing stock that produces a tree slightly larger than M.26 is G.5935. It is a cross of Ottawa 3 and Robusta 5. It has good propagability in the stoolbed and produces a large tree in the nursery. G.5935 is the most precocious and productive semi-dwarf CG rootstock available. It has similar efficiency to M.9 along with excellent fruit size and wide crotch angles. In addition, it is very winter hardy. It is highly resistant to fire blight but is not resistant to woolly apple aphid. In U.S. trials, it produces a tree about 40 percent to 100 percent larger than M.9 but with similar or higher yield efficiency and fruit size as M.9. Orchards planted with this rootstock should be planted at densities of 1,500 trees per ha to 2,500 trees per ha. It is scheduled for commercial release and propagation in December 2004 and will be named Geneva 935. It appears that G.935 will be an excellent replacement for M.26.

• Geneva 30 G.30 is a very productive semi-dwarf rootstock with large fruit size that has proven to be widely adaptable. It is not only highly resistant to fire blight and phytophthora, but also is resistant to apple replant disease and is very winter hardy. It is a cross of Robusta 5 and Malling 9. Although it is highly productive in the orchard it is difficult to manage in the nursery. It produces numerous side shoots (spines) on each shoot in the propagation bed. This requires manual trimming of these shoots either before or after harvest from the stoolbed. The removal of the lateral shoots on the liner also removes essentially all of the lateral buds so that new growth the next year in the nursery row must depend on the development of adventitious buds. This is a slow process, which allows 10 percent to 30 percent of the plants to dry out and die before they begin to grow. A solution to this problem is to remove only the side shoots on the lower 25 cm of the liner leaving 5 cm to 10 cm at the top of the liner untrimmed with live buds for next year.

In the early years in the orchard, tree growth of G.30 is vigorous and very similar to M.7. However, branch angles have been wider than M.7 and the heavy crops on G.30 starting in the third year limit tree growth and vigor in later years so that by year 10 it is usually significantly smaller than M.7 and often closer to the size of M.26. G.30 has a relatively weak graft union when it is young. With Gala, R.I. Greening, Honeycrisp, Jonagold and JoBurn, there have been significant tree losses from breakage at the graft union following strong winds. Work by Johnson and Robinson has shown that the graft union of Gala and G.30 is more brittle than M.26 and the union of Empire and G.30 is more brittle than M.7. This means that although G.30 is a semi-dwarf tree, it requires a multi-wire trellis to support the tree. Despite its problems G.30 has found a niche in the apple industry due to its high productivity and wide soil and climate adaptability. Cumulative yield efficiency has been three times to five times better than M.7 and is very similar to M.9. In France, it has produced a tree slightly larger than M.9Pajam2 but with similar productivity as M.9. Orchards planted with this rootstock should be planted at moderate densities of 1,000 trees per ha and 1,500 trees per hectare, but it will require tree support in all situations.

Summary

Fire blight is an important limitation to dwarf apple trees in eastern North America due to the extreme susceptibility of M.9 and M.26 to this disease. Fire blight resistant rootstocks have been developed through the rootstock breeding project of Cornell University and the federal USDA-Agricultural Research Service at the Geneva Experiment Station. To date, five rootstocks from this project have been patented and released to several licensed nurseries for commercial propagation and sale. Additionally two more rootstocks were in December of 2004 and licensed nurseries will be authorized to begin commercial sales. None of the new rootstocks are perfect; each has strengths and weaknesses. All have good fire blight resistance and are quite productive. However, each has deficiencies that must be understood by commercial nurserymen and apple growers before their adoption. Currently we suggest G.30 as an excellent alternative semi-dwarfing stock for spur type varieties, replant sites and for northern climates with short cool growing seasons, however, trees on G.30 must be trellised to prevent graft union breakage. In the near future G.935 may replace G.30 for orchards planted with densities of 1,000 trees per ha to 1,500 trees per ha. G.11 and G.202 are possible replacements for M.26 and should be used for dwarf orchards planted at densities from 1,500 trees per ha to 2,000 trees per ha. G.16 and G.41 are possible replacements for M.9 and should be used for orchards planted at densities from 2,000 trees per ha to 4,000 trees per ha. For excessively large fruited varieties planted at very high densities of 4,000 trees per ha to 6,000 trees per ha, G.65 is better than M.27.

Watch for a second article on the Geneva rootstocks in an upcoming issue of The Fruit Growers News. Ron Perry will discuss the new Geneva rootstocks and how they work in Michigan’s growing conditions.