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Preserving Georgia's peach pride.

Whether its flesh be white, yellow, orange, or even red as a beet, a Georgia peach can virtually melt in your mouth.

Most fresh-market peaches are what is known as melting freestones, except for the very earliest ripening types. Canning peaches are usually nonmelting clingstones.

ARS plant breeder William R. Okie says that the weather has a lot to do with a peach's delicate taste.

"Dry, sunny days make for an excellent flavor," he says. "A lot of rain or irrigation means they will be more watery and less tasty."

And Okie knows plenty about what makes a good peach. For 13 years, he's been breeding peaches at the ARS Southeastern Fruit and Tree Nut Research Laboratory at Byron--an area in Georgia that has produced peaches for over 150 years.

Since Georgia is the third-largest peach producer in the United States and the Southeast supplies much of the eastern markets with fresh peaches, ARS scientists at Byron are looking for ways to keep the crop healthy and productive.

U.S. growers produced over 2.7 billion pounds of freestone peaches in the 1992 crop year, valued at around $373 million. The Southeast's share of that production was 345.2 million pounds and about $58.3 million.

Breeding New Varieties

Offering growers new peach varieties has been a primary goal since 1937, when USDA began breeding peaches in the heart of the peach-growing area, first at Fort Valley, Georgia, and later at Byron.

The breeding program has produced over 120,000 seedlings, resulting in 32 new varieties, Okie says.

Summerprince is the latest. "It blooms later than most other early-ripening peaches grown in the southeastern states and sets a heavy crop," he explains. "It's also more likely to make a crop in our area than Dixired or Redcap."

Okie started breeding Summerprince in 1980, bringing it to fruit for the first time in 1983. Every year since then, he says, it has produced large crops in Georgia and has looked promising in field tests in Texas and Alabama.

Springcrest, an older variety released from Byron by USDA researchers, has been one of the most widely grown peaches in the world, Okie says.

Now, additional effort is going into developing white-fleshed peaches as an alternative crop for growers and a new idea for consumers. "Sixty years ago, white-fleshed peaches predominated," Okie says, "but firmer, more attractive yellow-fleshed peaches gradually replaced them because they show bruises less readily."

Many Asians and Europeans still prefer peaches with white flesh. Okie thinks that as new, firmer, white-fleshed peaches are released, this market will reappear in the United States.

Brown Rot and Other Fungal Problems

The major disease problem facing peach growers in the Southeast is brown rot. Caused by the fungus Monilinia fructicola, brown rot appears on peaches late in the season, just before harvest.

Prevalent throughout the world, this rot attacks all stone fruit but can be controlled with fungicides.

Few effective weapons are available to fight such a formidable foe. Because of potential health and environmental risks, many fungicides have been banned by the U.S. Environmental Protection Agency or have been withdrawn from the market. Benlate, once widely used on peaches, was discontinued for postharvest use about 4 years ago.

ARS plant pathologist P. Lawrence Pusey has some innovative ideas, however. He found a strain of the bacterium Bacillus subtilis that controls brown rot on peaches, and he and ARS hold patents on its use against storage rots of various commodities.

ISK Biotech Corporation, Mentor, Ohio, is developing this bacterium for commercial use against rots in peaches, grapes, and bananas.

"B. subtilis is a soilborne bacterium that works by producing antibiotics that kill the fungus," Pusey says. "But we're looking for an organism that can be just as effective without the antibiotic."

He has tried several naturally occurring yeasts, but so far, they have not been as good as the bacterium.

"We're looking for an integrated approach, since no single method is likely to replace chemicals entirely," he says. "Possibly an antagonist could be used along with heat treatments, ultraviolet light, reduced rates of chemicals, or substances generally regarded as safe."

Ultraviolet (UV) light is already giving some positive results. Working with Clauzell Stevens, Department of Agricultural Sciences at Tuskegee University in Alabama, Pusey is using low amounts of UV light to increase the resistance in peaches to postharvest storage rots. The research also involves Charles L. Wilson, an ARS scientist at Kearneysville, West Virginia.

"UV light should not be confused with gamma rays. The UV light that we're using is a component of sunlight that plants are not normally exposed to because of ozone screening," Pusey explains.

Collaborator Stevens and Tuskegee colleagues John Y. Lu and Victor A. Khan have been experimenting with this technique for about 8 years.

In addition to peaches, they have used UV light to reduce black mold and bacterial soft rots of onions; Fusarium and Rhizopus soft rots of sweet potatoes; black mold, gray mold, and Rhizopus rot of tomatoes; and green mold of tangerines.

UV light does not penetrate the peach; it just skims the surface. The positive response to UV light is called radiation hormesis, which is the stimulation caused by low-level exposure to something that would be toxic at higher levels.

"We think the UV hormetic effect triggers a response in the peach that fights off pathogens," Stevens says. "Perhaps the light increases the production of antimicrobial compounds such as phytoalexins."

He says that UV light also seems to delay ripening and increases peach flesh firmness and acidity.

Stevens says that UV light, which leaves no toxic residues, could be used very simply and inexpensively in packinghouse sorting lines.

But Pusey says more research is needed before UV light can be used in commercial systems to prevent storage rots. More study should be given to the effect of UV light in relation to the peach variety, stage of fruit maturity, storage conditions after treatment, and pesticides applied before harvest.

Meanwhile, ARS researchers are searching for an entirely different means of controlling another peach fungal problem. Whereas brown rot attacks the fruit, gummosis attacks the tree itself.

This peach tree disease was first reported in the United States in 1974 in Byron by an ARS scientist. It also occurs in Japan, China, and Australia.

Gummosis causes blisters on the bark of young peach trees in the third year after planting.

"It invades the natural openings in the bark," says Pusey. "Gum then oozes from the resulting lesions."

Overall health of the tree declines; it loses vigor and can die. The causal fungus, Botryosphaeria dothidea, spreads when spores are carried by wind or rainwater.

"A control strategy for this disease must include removing deadwood from the orchard, irrigating during periods of drought, and possibly applying fungicides," Pusey says.

He and plant breeder Okie have developed a way to screen young peach trees for resistance, and they've found differing levels among varieties.

Now that they've learned more about peach tree gummosis, Pusey says growers can limit fungicide sprays to the peak infection period in June and July.

Underground War: Nematodes, PTSL, and Oak Root Rot

No part of the peach tree is safe from pest attack. Rots hit the fruit; diseases assault the tree; and nematodes thrive on the roots.

If a young, apparently healthy peach tree suddenly dies in the spring, the ring nematode could be the cause, says ARS nematologist Andrew P. Nyczepir.

"The bark may crack and separate from the tree trunk. The tree can suddenly wilt, give off a sour-sap odor, and then just die shortly after bloom," he says.

This is Peach Tree Short Life (PTSL), induced, Nyczepir says, by thousands of hungry ring nematodes, Criconemella xenoplax, feeding on the roots.

All of the tree above the soil line dies, though the main root system below ground looks healthy. But a close inspection shows the feeder roots to be destroyed where ring nematodes have been feeding.

"The trees actually die from cold injury or bacterial canker infection, which result from the parasitic action of the nematode," Nyczepir explains.

A recent survey in South Carolina--the number-two peach-producing state--showed that about 1.5 million trees died from PTSL between 1980 and 1990. This amounted to about a $6.3 million loss per year, just in South Carolina.

Although tree loss varies from year to year, Nyczepir says almost every orchard in Georgia and South Carolina is infested with the ring nematode. And PTSL occurs when the nematode population density and environmental conditions in the spring are just right.

"We've shown that if we can manage the ring nematode, then we can control PTSL," Nyczepir says.

His research shows that the disease occurs only when this nematode is present. PTSL attacks the tree when it is from 3 to 6 years old, just when it starts to bear fruit.

Although chemical control is still used, nematicides are very expensive and may not be available in the near future because of environmental concerns. The cost of preplant fumigation can range from about $275 to $1,300 per acre, depending on which nematicide is used. Presently, only one postplant nematicide is available to growers, and it must be applied twice a year to be effective. It costs about $400 per acre annually and must be used throughout the life of the orchard.

But Nyczepir has found a nonchemical way to thwart the ring nematodes. He takes away their tasty peach roots and gives them Stacy, a variety of winter wheat, to chew on.

"Judging from the reduced count in soil taken from a 3-year test plot, they don't like wheat,"he says.

In a PTSL orchard known to be heavily infested with ring nematodes, Nyczepir and cooperators from the University of Georgia removed the peach trees from a large area. Then he used a 1-, 2-, and 3-year rotation system of preplanting wheat/fallow and wheat/sorghum before replanting the plot back to peaches. Rotation decreased the numbers of nematodes significantly. Although some nematodes were still present in the soil, their empty intestines indicated that they had not been feeding.

"Our results show that preplanting Stacy wheat as a nonchemical control before replanting a peach orchard dramatically lowers the nematode population," Nyczepir says.

In another test, peach tree deaths from PTSL were no greater after 2 years in plots previously planted to Stacy than in plots that had been preplant-fumigated with methyl bromide or that were left fallow.

Nyczepir, along with University of Georgia scientists, is also investigating the use of ground-cover grasses as a way to manage the ring nematode.

"We've had good success with two types of bahiagrasses--Pensacola and Tifton 9 that seem to suppress the nematode," he reports.

Peach growers in the Southeast are already following Nyczepir's lead. They are interplanting wheat in their orchards, between the tree rows. Even this, surprisingly, seems to reduce the numbers of nematodes around the tree roots. There is another way to fight the problem, according to Tom Beckman, horticulturist for rootstock development at Byron.

"We've been looking at peach breeding lines for something that can tolerate PTSL," he says.

Working with Okie, Nyczepir, and researchers at Clemson University, Beckman experimented with more that 100 standard and exotic peach and plum lines on nematode-infested sites in Georgia and South Carolina.

After 10 years, they found one breeding line that showed a significantly longer tree life than Lovell, the peach rootstock commercially recommended for planting in sites that are predisposed to the disease.

In a followup commercial trial now in its fourth season, no trees on this experimental line have shown any injury. But 34 percent of the trees on Lovell and 85 percent on Nemaguard (an alternative commercial rootstock) have already died from PTSL.

Beckman plans a series of full-scale commercial trials throughout the region. One in Georgia and one in South Carolina were established last spring.

"We plan to field-test this line in the fall in Alabama, Arkansas, and Louisiana. If all goes well, we'll formally release the rootstock to growers in the next few years," Beckman says. Until then, a limited amount of seed is available through nurseries for growers interested in trying it.

Although it kills fewer trees than PTSL, oak root rot is another sighificant problem that afflicts the roots of peach trees. It causes losses of about $4 million each year in South Carolina.

All commercial peach rootstocks are susceptible to oak root rot, Beckman says. And there is no practical control for this disease, which is caused by a soilborne fungus, Armillaria spp. One reason, says collaborator Pusey, is that by the time symptoms appear on the tree, it is too late to do anything. The tree is doomed.

Since the rot attacks roots, the only treatment would be preplant fumigation of the orchard site. But this is a cost most growers can't afford, especially when they're not sure the rot is present.

"To rid themselves of oak root rot, growers just move their orchards," Beckman says. "But since the patbogen can spread from the roots of hardwood species that are left in the soil after preparation, this practice is only a stopgap measure."

However, Beckman has developed a few plum breeding lines that show some tolerance to the rot. "Our first trials of seedlings of these plums as rootstocks for peaches didn't work because of graft incompatibility," he says. "But we are now testing some new plum/peach hybrids that should offer tolerance to the rot and improved graft compatibility with peaches ."

Beckman has propagated other selections from the plum lines by cuttings rather than seed. These appear to be compatible with peaches, he says, and are being tested to confirm their resistance to oak root rot and their commercial suitability.

Chemicals Are Important Weapons

Use of pesticides to control insects and some diseases and maintain peach quality is necessary. So ARS scientists at Byron are dreaming up some new ideas for applying those pesticides.

Peaches are planted on a grid with 20-foot-wide rows between the trees in each direction.

"We're spraying just the middle of alternate rows instead of spraying the complete orchard," says Byron entomologist Carroll E. Yonce. "And we're getting control of key early- and mid-season disease and insect pests equal to that of complete spraying."

Yonce and collaborators at the University of Georgia have been using this technique, which they call ARM (alternate-row-middle), for 2 years. This technique has a two-fold benefit: It cuts in half the amount of chemicals released into the environment, and it cuts growers' costs considerably.

Orchard pesticide applications are made almost entirely by airblast sprayers on each row, Yonce explains. Since concentrated sprays are more efficient and economical than high-volume dilute sprays, most Georgia peach growers use them exclusively for their cover sprays.

The ARM technique takes advantage of the considerable drift from one row to another of pesticides applied by airblast spraying. Therefore, spraying every other row still gives adequate protection.

Some chemicals that are used are long residual. But, since this method allows more frequent spraying, growers can use more short-residual materials.

They are already successfully using the technique.

"We spray alternate rows for the first 6 weeks of the season and get good insect control," says Billy Davidson, who has peach trees on 2,000 acres in Fort Valley, Georgia. He feels more complete coverage is needed after that time since foliage gets so heavy.

Yonce and his colleagues recommend ARM spraying in an integrated pest management program. ARM has been incorporated into the spray guide for Georgia growers.--By Doris Stanley, ARS.

Scientists in this article can be reached at the USDA-ARS Southeastern Fruit and Tree Nut Research Laboratory, P.O. Box 87, Byron, GA 31008. Phone (912) 956-5656, fax number (912) 956-2929.
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Author:Stanley, Doris
Publication:Agricultural Research
Date:May 1, 1993
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