Choosing a peach rootstock
Most plantings are still on seedling roots, but growers increasingly have other choices.
Greg Reighard looks over Guardian peach rootstocks being grown in a Tennessee nursery. Guardian was developed by scientists at USDA and Clemson University to provide resistance to nematodes without the use of chemical fumigants.
The easiest way for a peach grower to select a rootstock is to take what the nursery sends—and assume the nursery knows what a grower in that production region needs.
“That’s been the traditional way, and still pretty much is that way,” says Dr. Greg Reighard, horticultural professor at Clemson University in South Carolina. Most peaches are planted on seedling rootstocks chosen to fit regional conditions. But increasingly, growers have other choices.
Reighard is chair of the peach section of the NC-140 project, in which horticulturists at 16 locations across North America evaluate peach rootstocks and annually meet to report and discuss their findings. Most information has come from plantings made in 1994, 2001, 2002, and 2009.
They’ve developed quite a body of information, much of it posted at the Web site www.nc140.org.
Peaches are not like apples, Reighard noted. Peaches bear fruit on one-year-old wood, so there’s a continuing need for vigorous growth. While most growers would like somewhat smaller trees, he said, they can’t afford to give up new growth, nor can they live with lower yields and smaller fruit, both of which have been problems with trees on dwarfing roots. But progress is being made toward smaller trees.
Peaches are naturally somewhat smaller in stature compared to other tree fruits. Putting them on nonpeach roots can either invigorate or dwarf them, depending on the Prunus species or hybrid selected, he said.
While peach breeders may seem behind apple breeders in developing dwarfing rootstocks to manage tree size, peaches are ahead of apples in developing rootstocks to solve problems relating to soils, pests, and diseases. Growers in various regions of the country choose rootstocks based on nematode pressures, soil pH, replant-associated diseases, soil fungal pathogens like Armillaria, peach tree short life, and cold hardiness.
Until about 1930, peach growers used peach seedlings like Tennessee Natural and Bailey as rootstocks. After that, they began to use more Lovell or Halford seedlings raised from pits collected at canneries. After about 1960, when more attention was focused on cold hardiness, Harrow Blood from Ontario, Canada, and Siberian C from China, were introduced. Still, in the northern peach-growing areas, growers are still most likely to plant Bailey or Lovell, which are cold hardy enough for most conditions.
In the Southeast and California, pressure from nematodes led to the search for resistant rootstocks. The USDA Rootstock Breeding Program released Nemaguard in 1959, Nemared in 1983, and Guardian in 1994. University of Florida developed a rootstock, Flordaguard, which resists a new root-knot nematode species that occurs in Florida.
Because peaches are self-fertile and mostly self-pollinated, planting peach pits of an inbred variety will produce a row of very uniform plants that can be used as rootstocks, Reighard said. The rootstock Guardian, for example, which was developed by USDA and Clemson University, is grown from seeds produced in a seed orchard at Clemson.
In apples, all the modern rootstocks are clonal and rated mostly for their dwarfing ability, but more and more of them are being chosen for resistance to diseases like fireblight and the complex called replant disease. Some peach rootstocks are grown from clones, and more of them have been evaluated and are becoming available, especially those from Europe.
There are some conditions under which peach roots are not capable of producing a good tree, Reighard said. In Colorado and Utah, for example, the high pH soils create iron deficiency in all peaches. For these conditions, interspecific crosses of peach with plum or almond make better rootstocks. These tend to be clonal rootstocks, mostly from Europe and California.
In the United States currently, growers have seven seedling rootstocks available—Lovell, Halford, Bailey, Tennessee Natural, Guardian, Nemaguard, and Nemared. Clonal rootstocks developed in the United States include the Controller series, Viking, Atlas, Cornerstone, Brights Hybrids, Sharpe, and MP-29.
Reighard says that peach rootstock testing work is somewhat frustrating. It is not well supported, either by public funding or by private industry. The government provides some funding through Hatch Act funds. “There are few funding sources,” he said, “and research funding is often bootlegged on other grants.”
In peaches, both variety and rootstock breeding is primarily done by private breeders and commercial nurseries, with only a few public breeders.
“Most new rootstocks will be proprietary and protected by patents and licenses,” he said. Many times, new rootstocks will not be entered into the NC-140 testing program because of concerns over proprietary rights, and if they are entered, they come with legal paperwork and nondisclosure agreements.
It is often difficult to get enough rootstocks propagated so that eight to ten trees can be planted at all the locations for good comparison trials. “The work is resource intensive (i.e., field plantings) and long term (5-10 years), thus expensive to undertake and maintain,” he said. “It is very hard to get the trees, the land, and the funds to replicate the trials under all the grower scenarios of spacing, variety, and training systems.”
Asexual propagation (cloning rather than production from seed) techniques continue to improve, Reighard said, which means more efficient and economical production of complex hybrid rootstocks. And the use of biotechnology, with marker-assisted selection, continues to shorten the time required to find rootstocks with desirable traits.