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the sweet spot | research and creative discovery | Clemson University

the sweet spot

Neil Caudle

Clemson released its latest peach, the CaroTiger, in August. A late-season cultivar that keeps its good looks and sweet disposition while others are losing theirs, CaroTiger is a yellow-fleshed freestone. The red pigmentation around the pit? Anthocyanin pigments, which are antioxidants and good for your health. Desmond Layne evaluated the peach at Clemson’s Musser Farm as well as in the orchards of commercial producers such as Cooley Farms in Chesnee, S.C.

If South Carolina has a sweet spot, this is it. The peach. Its gush of luscious flavor has made the state a national power in peach production, second only to California. In fact, a single farm in South Carolina, Titan, ships more peaches than the entire state of Georgia.

meet the peach team

Rootstock to the rescue
Greg Reighard, a horticulturalist and rootstock expert, developed and released the Guardian™ rootstock, which helped peach trees resist short-life syndrome and revived South Carolina’s imperiled peach industry. Clemson is still the sole source of the peach pits from which the rootstock grows. Reighard also studies various methods for managing orchard trees, including mechanized thinning to lower costs.

Do peaches need a chill pill?
If peach trees don’t get enough hours of winter chill, some will not flower and fruit normally. Doug Bielenberg, an environmental physiologist, studies how various peach-tree cultivars respond to cold or a lack thereof.

The basic biology of a peach tree dictates its performance and its response to changes in the environment. Bert Abbott, an emeritus professor who still runs a lab at Clemson, is a leading authority on the genomics of Prunus, the genus that includes peaches. Ksenija Gasic, the newest member of the team, breeds peaches specifically for the Southeast, to help growers stay ahead of pests and diseases, and ahead of the competition as well.

The grower’s go-to guys
Greg Henderson and Andy Rollins advise growers, conduct field trials, and ensure that the results of science reach the orchard. But it’s a two-way street. Henderson and Rollins also bring the growers’ questions and problems back to the lab.

The test driver
Desmond Layne evaluates peach varieties by putting them through their paces on Clemson’s Musser Farm and on the farms of cooperating growers in the peach-growing regions of the state. He is also the team’s highly visible advocate, with a peachy website, Everything About Peaches. You can also find him on his Facebook page, the Peach Doctor.

Pest and disease patrol
Guido Schnabel, a pathologist, pursues the elusive brown rot, an especially virulent fungus that quickly develops resistance to fungicides. Schnabel’s field-test kit, which enables growers to identify the fungus quickly and inexpensively, helps reduce unneeded spraying. Dan Horton, from the University of Georgia, is a fruit entomologist. He knows which insects go for peaches and how to limit the damage.

Keeping it clean
A fast route to bad fruit or dead trees is a weedy, buggy, unkempt orchard. Wayne Mitchem, a collaborator from North Carolina State University, works with the Clemson team to recommend weed-control measures for the Carolinas. Simon Scott, a virologist, works through an organization of growers, the Clean Plant Network, to help protect orchards from various infections that afflict budwood, the part of the tree that bears fruit.

Why? South Carolina has favorable land and climate, yes, but mostly it has know-how. South Carolina has been in the peach business for 150 years, with farms enduring several generations. Some 17,000 acres of peach orchards yield an annual harvest of 60,000 tons, valued at $60 million. And Clemson is the only university in the Southeast with a team of experts to help keep peach growers ahead of the curve.

Like most things sweet, the peach is appealing to pests and diseases, its survival so precarious that scientists and producers constantly struggle to keep it alive.

Beginning in the 1970s, a scourge swept through the peach orchards, killing trees and putting farms out of business. The villain: peach tree short-life syndrome, the sudden death of young trees in spring, usually caused when ring nematodes invade the roots and bacterial cankers develop on cold-damaged wood.

With funding from the state and federal governments, Clemson went to work studying the syndrome and ways to combat it. By the early 1990s, growers were hearing about a new rootstock, developed by Clemson’s Greg Reighard and U.S. Department of Agriculture (USDA) colleagues, that resisted the syndrome, and the industry was demanding it even before testing was complete. In 1994 Clemson released the rootstock, called Guardian™. Within a few years, most new peach trees in the Southeast were growing on the new rootstock, and orchards were thriving again.

Case closed? No. Another threat was afoot in the orchard, biding its time.

This new adversary is as old as the hills: oak root rot.

Native to woodlands that once covered the Southeast, the Armillaria fungus is everywhere peaches grow. It persists in the soil and creeps across an orchard underground, as roots from one tree connect with another. Over time, the fungus builds up in a peach orchard, killing tree roots and moving up to the lower trunk where it girdles the tree with a deadly, mushroom-like growth.

“Before we began using the GuardianTM rootstock, trees wouldn’t live long enough to die of oak-root-rot disease,” says Desmond Layne, a tree-fruit specialist and state program team leader for horticulture. “But now that the trees are living longer, we’re seeing more and more Armillaria.”

So far the fungus has defied all attempts to control it. The best solution? Move. Don’t grow peaches in soil with a buildup of Armillaria. Trouble is, the best land for peaches is already growing peaches, which require well-drained soil, sunny slopes, and good air circulation, with no pockets to accumulate cold. Most of the sites that meet those criteria have been growing peaches for generations.

Guido Schnabel, a plant pathologist, may have discovered one solution. In his experimental plots, he found that planting young trees into soil mounded into berms suppressed the fungus by discouraging its movement in above-ground roots and up the trunk. If tests in commercial orchards succeed as well, producers may have a low-cost way to keep the peaches growing on their most productive land.

Big, bad brown rot

Even if we gain ground against root rot, a host of other afflictions can spring nasty surprises. One of these is brown rot, also a fungus, which can rapidly spoil a whole truckload of peaches, especially in damp, humid weather. The threat is so severe that producers typically treat their crops with fungicides just before harvest. But over the last few years, Clemson scientists have detected strains of brown rot that resist fungicides—a nightmare scenario for peach producers.

Schnabel attacks the problem from several directions. First, he developed a test kit that can help extension agents determine whether a strain of brown-rot fungus is present in an orchard and, if it is present, which type of fungicide will kill it.

This eliminates unnecessary spraying, saves the producer money, and reduces the chemical load. Meanwhile, Schnabel uses advanced techniques in genetics and molecular biology to find the exact segment of DNA responsible for the pathogen’s fungicide resistance—a step toward finding a way to combat it. Where will the next threat to peaches appear? Perhaps from a bacterium or a virus. Either would be dire. So in their labs and test plots, the peach team feels the heat. Science, they say, is an ally peaches can’t live without.

Peach research is based in the College of Agriculture, Forestry, and Life Sciences.

Do we dare breed a peach?

This won’t be a one-night stand. Peach breeders have to be patient, in it for the long haul. Here are the basics, simplified:

1. Select parent trees with the traits you want. Collect and dry the anthers from flowers of the male parent to release the pollen.

2. Pull off the male parts of the blossoms on the female parent tree, which will bear the fruit. (This is called emasculation.)

3. Pollinate the female blossoms, hundreds of them, and hope that genetic recombination will yield something better than either parent, with the best characteristics of both. The odds are against this. That’s is why hybridization is slow, painstaking work.

4. If pollination succeeds, a peach will develop some months later. Harvest the ripe fruit, extract the seed from inside the pit, and keep it cold. Sow the seeds in pots and set them in a greenhouse. In the spring, transplant the seedlings into a breeding nursery.

5. When the seedling trees begin to flower in the field, evaluate their characteristics. This is both an art and a science, a test of experience and skill.

6. After two or three years, select the most promising, collect some budwood and produce grafted trees. Test the trees at multiple locations for several years. After five or so good years, you may be ready for a commercial release. Two of Layne’s joint releases with breeder Dick Okie (USDA, retired) Early Augustprince and Augustprince, both released in 2008, were first hybridized in 1995.

Why go to so much trouble? Don’t we have lots of good peach cultivars already? Yes, but we’ll always need more, says Ksenija Gasic, a peach breeder and the newest member of the peach team. New types can resist pests or diseases, ripen late or early in the season, or appeal to new markets. While native Southerners tend to prefer an old-fashioned balance of acidity and sugar, newcomers may want eye-popping sweetness, hold the acid. So a new peach can help producers compete for new markets.

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