Growing Magazine - March, 2012

COLUMNS

Seed Research: A Peach of a Weapon

By Rebekah L. Fraser


Horticulturist Tom Beckman checks greenhouse-grown Guardian hybrids.
PHOTO BY ROB FLYNN.
The name MP-29 sounds like a weapon, and in a sense it is one. This plum-peach hybrid, which was released for commercial trial in 2011, is the one rootstock for peach with resistance to the eastern peach industry's three biggest predators: peach tree short life (PTSL), root-knot nematodes and Armillaria root rot (ARR). If you're growing peaches east of the Rockies, you know the challenges are great. In addition to these three predators, the presence of plum curculio and brown rot plus a host of other diseases and pests necessitate the use of a spectrum of fumigants and pesticides, dashing any large-scale commercial grower's hopes of going organic. Fertile, sloped land that is near packing and processing plants is hard to come by. Yet growing on the same land year after year only increases disease and pest pressures. As the cost of petroleum rises, so do your petroleum-based fertilizer and pesticide costs. It may at times feel like you and your orchards are under siege.

Peach tree short life (PTSL) is so named because it kills mostly young trees. The vast majority of losses occur between years three and seven. An older orchard is comparatively safe from PTSL. Root-knot nematodes are soilborne parasites that infect the roots of many different plant species. They are responsible for 5 percent of crop losses worldwide. Since its release in 1993, Guardian rootstock has held the defensive line against PTSL and root-knot nematodes. Today, Guardian is the most widely utilized rootstock in the southeastern U.S. peach industry.

However, a peach tree without resistance to Armillaria root rot is a sitting duck. Armillaria is a fungus that spreads through the soil and attacks a wide range of tree species. There are no known viable treatments, and until recently there were no peach rootstocks available in the U.S. that could resist the fungus. Armillaria can act so quickly that orchards are often devastated within five to seven years after planting.

MP-29, Guardian and numerous other peach varieties were developed at the USDA Agricultural Research Station in Byron, Ga. Research Horticulturist Tom Beckman co-developed Guardian and took the lead on MP-29. He joined Byron's ARS research team in 1988.

"When I started here years ago, we didn't know where we'd find sources of resistance to some of the diseases we were faced with," he explains. "We knew there was resistance to peach tree short life, then the number one problem, in a small number of materials. Armillaria root rot was the number two problem at the time. There was very little known about relative susceptibility in much, if any, of the rootstock material that was available."

Beckman and his colleagues searched peaches for resistance to PTSL and found a few useful sources. That resistance was developed into the rootstock called Guardian, a pure peach seedling line. Despite its resistance to PTSL and several species of root-knot nematode, Guardian had no resistance at all to Armillaria root rot (ARR). Beckman could not find any pure peach material with resistance to ARR, so he began screening plums, apricots, almonds and cherries. He found what he needed in plums, but then faced the challenge of creating an interspecific cross to overcome the graft incompatibility the plum lines displayed when budded with commercial peach varieties. At this point, you're probably wondering about Beckman's methods. Crossing two different species of fruit seems like a job for genetic engineering. In some cases, that would be the only possible way to create an interspecific cross. However, Beckman and his colleagues have used only conventional breeding technologies.

A unique marriage

Because plums are generally obligate outcrossers, every seedling from a plum tree will be a bit different, which can yield very unpredictable results if these seedlings are then used as rootstocks for peach varieties. Genetic engineering would enable the researcher to manipulate the peach seeds, integrating plum DNA to create the desired hybrid. Uninterested in using genetic engineering, Beckman relied on bee-mediated crosses of selected plum lines with advanced peach selections to produce the desired interspecific hybrids.

Eventually, they developed successful first-generation crosses. Not only were the F1 crosses graft-compatible with peach varieties, but they also brought the best properties of the plum and peach parents. The plum parent contributed Armillaria resistance, which complements the PTSL and root-knot nematode resistance of the peach parent. "MP-29 is the very first rootstock to make it out the door from our breeding program," reports Beckman. "We have quite a few others coming behind it."

Being an interspecific hybrid, MP-29 doesn't produce seed. Therefore, the rootstock must be clonally propagated. After cloning, Beckman grew the hybrid for a year or two to verify its health and vigor. He followed this up with a graft compatibility test, propagating the rootstock and grafting some peaches on it.

After a successful graft compatibility test, resistance testing began in earnest. It only took one season to verify the new hybrid's resistance to important root-knot nematode species. PTSL screens take seven years to get results. ARR screens used to take 10 years, but Beckman and his team developed innovative inoculation techniques that lowered the screening time to five years. Since that went well, MP-29 was grafted with a commercial peach variety for horticultural productivity tests. In a replicated trial, Beckman tested the line against standard rootstock lines for other important attributes including anchorage, vigor, flower production, yield, and fruit size.

After receiving the initial results from MP-29's Armillaria and short life trials, Beckman conducted a second round of PTSL and ARR tests at another location to verify the initial results, while continuing the long-term horticultural trial. The long-term trial is now at year 12. Fifteen years is a full lifetime for an orchard, and conducting a test that spans the expected life of a peach tree gives Beckman confidence that the rootstock is as good as he hoped it would be.

MP-29 rootstock is also in small-scale commercial trials at several different locations. With at least two species of Armillaria in the southeast U.S., Beckman wants to ensure that his rootstock is exposed to, and can resist, all the fungus' species and their local variants. "It would be reasonable to expect that some of the isolates would vary in their pathogenicity," he explains. "Even the isolate that was here on station when I started in 1988, we've done a good job of spreading it around our trial blocks, but you can't be certain you have covered all the variables. So you put it out in the hands of commercial growers in different parts of the production area, and then watch and listen to what they have to say."

Good for the grower; good for the consumer?

From the consumers' point of view, MP-29 is a more sustainable rootstock for peach production. It lessens the need for growers to use toxic pesticides in their orchards and extends the life span of their trees. From a marketability standpoint there is no downside. Fruit from peach varieties grafted onto MP-29 are comparable in size, sugar content and flavor to those from trees grafted onto competing rootstocks. If trials continue their current positive trajectory, MP-29 may turn out to be a weapon that is both sweet and profitable.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.