International Fruit Tree Association members tour a research plot at Wapato, Washington, where a wide range of apple rootstocks are being compared.
Rootstock development is a huge area of research that goes beyond studying the survival, size, and yield efficiency of rootstocks, says Dr. Terence Robinson, horticulturist at Cornell University, New York.
Other tree fruits lack the rootstocks that have helped make apple production more profitable. And, even in apple, where dwarfing rootstocks are widely used, Robinson sees a need for more research relating to a number of traits:
Fireblight resistance: Disease resistance is an important rootstock trait, with fireblight resistance being the most important. Rootstocks can be a key component of fireblight management, though not the only one. A disease-resistant rootstock will not stop an infection in the top of the tree, but, even if some scaffold limbs are lost, the tree can be quickly regrown.
A new set of Russian rootstocks related to Budagovsky 9 and 118 will be assessed for fireblight resistance in a trial beginning this spring, along with Geneva rootstocks that were developed for fireblight resistance.
Replant tolerance: For some growing areas, resistance to replant disease is an important trait. In Washington Tree Fruit Research Commission trials, Geneva 30, G.16, G.41, and G.210 have all shown tolerance to replant disease, where Malling 26 is susceptible.
Propagation: Some Geneva rootstocks, particularly G.41, have been difficult to propagate because of inadequate rooting in the stoolbed. These problems have been addressed through micropropagation research.
Cold hardiness: Research on cold hardiness is being conducted in controlled freezing chambers at the University of Maine, as well as in NC 140 field trials. Mark, G.30, G.16, B.1, Vineland 3, and Ottawa 3 all survived the severe winter of 2004 in Minnesota while many trees on other rootstocks were lost.
Nutrient uptake: Some rootstocks might be more efficient than others in taking up nutrients, such as calcium or potassium, which might translate to better fruit quality. Several Geneva selections seem efficient at absorbing nutrients, judging by leaf calcium levels, but Robinson said he did not know if high leaf calcium means more calcium in fruit also.
Root architecture: The ability of a root system to explore the soil for nutrients or tolerate drought might be due in part to the root architecture. Some rootstocks have lots of fine roots rather than a few dominant roots. For tolerance of replant disease, rootstocks with many fine roots seem best, Robinson said.
Drought tolerance: The Bud 9 rootstock does well in perfect conditions, but not where it is under drought stress, Robinson pointed out. Some other rootstocks are able to produce a reasonable root system even in drought conditions.
DNA fingerprinting: Scientists are studying the differences between rootstocks at the genetic level. Dr. Gennaro Fazio, U.S. Department of Agriculture plant geneticist, has studied 183 progeny from the Geneva program and constructed a molecular map to identify rootstock-related genes. Three genes seem to be related to root architecture. Scientists hope to identify genes for other traits such as precocity and disease resistance.
Dr. Greg Lang, horticulturist at Michigan State University, said most U.S. cherry growers are using seedling rootstocks, such as Prunus avium or Prunus mahaleb.
The cherry industry needs rootstocks with the same characteristics as apple rootstocks, such as dwarfness and precocity, he said. The Gisela rootstocks are becoming more important, but G.5 is not suited to dry climates, such as California’s, because the trees become stressed. G.6 and G.12 are vigorous rootstocks, though precocious.
Interstems are available, such as the Zeestem, that would allow growers to plant cherry trees on a rootstock of a different species, such as plum.
Dr. Amy Iezzoni, cherry breeder at MSU, has been evaluating a series of tart cherries as potential sweet cherry rootstocks, nine of which are between G.5 and G.6 in size. They are being screened for virus sensitivity and ease of propagation and will be available for small-scale on-farm tests soon. It is too early to know whether any of them have commercial potential, Lang said.
Lang outlined several key questions that need to be answered about cherry rootstocks:
- How does soil health interact with the rootstock and affect the uptake of water and nutrients?
- What are the impacts of various tree-training systems on the root system? What are the implications of moving to intensive systems with greater inter-root competition?
- What are the most appropriate rootstocks for new training systems, such as the Kym Green bush, tall spindle, super slender axe, and UFO (Upright Fruiting Offshoots)?
- What are the impacts of rootstocks on orchard efficiency and sustainability in terms of tree size, labor efficiency, early yields, mechanization, and production systems?
Lang said infrastructure and procedures need to be established for developing, obtaining, or selecting improved cherry rootstocks in order to keep the pipeline flowing to growers. Propagation techniques should be improved so they are more efficient and cost effective in order the keep the price of the new rootstocks affordable.
Rachel Elkins, University of California farm advisor for Lake and Mendocino counties, said there’s a great need for research on rootstocks for pears that will enable growers to have successful orchards.
Generally, the U.S. pear industry has old orchard systems with large trees on Pyrus communis seedling and other nondwarfing rootstocks, which are becoming more difficult to manage as labor costs increase. Growers would like access to rootstocks that produce compact trees with early production and high yields of large, high-quality fruit, she said.
“If you don’t have fruit size of 80 or 90, it’s not adequate. To have to spend hundreds of dollars on growth regulators and thinning to achieve large size makes it less economically sustainable.”
In addition, rootstocks that are cold hardy, tolerant of drought or wet soils, and resistant to fireblight and pear decline would be desirable.
Quince is used in intensive orchards in Europe, but Elkins said it is seldom used in the United States because of its incompatibility with most varieties, susceptibility to fireblight, and questionable cold hardiness.
Greg Reighard, environmental horticulturist at Clemson University, South Carolina, said 95 percent of peach trees are still planted on peach seedling rootstocks, but there are problems associated with them, such as replant disease and restrictions on fumigation. In addition, growers would like to plant more intensive systems.
Reighard said objectives for stone fruit rootstock development include resistance to nematodes, replant disease, bacterial canker, phytophthora, and crown gall. Rootstocks should ideally tolerate waterlogging and drought, be cold hardy, provide enough vigor control for high-density systems, and be compatible with scions.
The discussion took place during the International Fruit Tree Association’s annual conference in Pasco, Washington.