By Richard Lehnert
Growing peaches is getting easier in some ways, harder in others, a rather normal description of many orchardists’ efforts. Take two steps forward, then one back, and call that progress.
Clemson University professor of horticulture Dr. Greg Reighard talked about the challenges peach growers face during the International Fruit Tree Association’s 56th annual conference in Boston last winter. The list was quite a long one.
First on his list of challenges was the changing climate. With winters getting warmer, more of the Southeast’s peach production regions are getting fewer chill hours. Most growers there would like to get 1,250 chill hours a year for the varieties they grow. Chill hours are necessary for all deciduous trees if they are to emerge from winter dormancy and flower and grow normally.
In the last 13 years at Byron, Georgia, five winters have failed to deliver the required numberof hours by the end of March. “It’s a growing concern,” Reighard said.
One solution is for growers to spray with Dormex or BudPro, both of which contain the plant growth regulator hydrogen cyanamide. It has been used for 20 years on fruit crops worldwide to replace winter chilling, but Reighard thinks planting varieties that require fewer chilling hours would be a better approach.
Breeders are developing low-chill varieties, but often these varieties come from the dry areas of California and are not suited to the Southeast, where growers need varieties resistant to bacterial spot.
While winters are warmer, spring frosts are just as frequent, and trees can be exposed to more freezing weather after they emerge earlier from dormancy. Growers are looking mostly at wind machines for protection. Reighard is looking at other solutions.
“When I started here in the 1980s, we lost our crop in three of the first five years, so I started working with bloom delay,” Reighard said.
Research showed that applications of ethephon can delay bloom for a week or more, but it is not labeled for that use.
Reighard did some research on peach trees infected with prunus latent mosaic viroid, because infected trees bloom five to ten days later, and the viroid also delays fruit maturity two to four weeks.
“There are some quality issues with fruit from infected trees, so it’s not a useful tool as it now stands,” he said. But the research did find that dormancy was related to the fatty acid content of trees at the end of dormancy. Dormancy ends as linoleic acid is transformed to linolenic acid. Reighard is looking at ways to manipulate that. Delaying the transition delays the bloom, he said.
More extreme weather events have also exposed peaches to greater threat of hail damage, and hail netting can be seen more frequently over peach orchards.
While it is not clear what the future holds for rainfall, large areas of the Southeast have been under severe drought stress in the last several years. The hydrology of the area is such that water is not available from wells, but only from surface water.
Competition for water with urban areas has been increasing, and there are questions about how much water will be available for agricultural uses, he said.
Growers are shifting to drip irrigation, which is more efficient in use of water, and studies are under way to detemine how much water peach trees really need to grow and produce a crop.
Growers were losing 300,000 trees a year to peach tree short life when Reighard started working with the industry in the 1980s. The disease has multiple causes, including pressure from nematodes.
Fumigation reduced those losses greatly, extending the life of peach orchards, but fumigants are themselves under pressure. Methyl bromide can no longer be used, and while Telone II and Telone C-17 work quite well, fumigation isn’t the best answer in the long run, he says.
Cover crops and rotations are being explored as ways to rehabilitate peach sites for new plantings.
Growing wheat and sorghum on rotation for two to three years can suppress ring nematode in Georgia and South Carolina. Brassica green manure crops are being used for nematode control in the Mid-Atlantic states, and it has been shown to be as effective as chemical fumigation, Reighard said.
Growers can select rootstocks that are tolerant to bacterial canker. Reighard rates Guardian, Viking, and MP-29 as excellent in that department. Guardian was developed at Clemson for its resistance to root knot nematodes, and several others are immune or tolerant.
“Trees are living longer now that we’ve controlled bacterial canker, so Armillaria is becoming a larger problem,” he said. There are two approaches to control of Armillaria, or oak root rot.
Sharpe and MP-29 rootstocks offer some tolerance. “We’re not there yet with rootstocks,” he said, “so in the meantime we have to use other means to combat Armillaria.
Some growers are now using the “walking tree” method, developed by pathologist Dr. Guido Schnabel at Clemson. It involves planting trees on berms, then removing the soil around the base of the tree after two years of growth. This exposes the major roots to the air; Armillaria doesn’t move above the soil line to infect the tree trunk.
The method was described in Good Fruit Grower in “Peaches on ridges” published in July 2011.
Reighard is a major figure in the world of rootstock evaluation for peaches. Growers in the Southwest have problems with iron chlorosis on high pH soils, and there are a dozen rootstocks that address that problem.
Several rootstocks are resistant to waterlogging, in areas where that is a problem.
Peach orchard management is very labor intensive in terms of pruning, thinning, and harvesting, Reighard said. To grow the large peaches required by the market means thinning as early as possible—bloom thinning being preferred.
Several chemicals are effective for bloom thinning, but they are not consistent from year to year, he said, so that means returning to the orchard to thin by hand. For many years, growers have used the Phil Brown Welding rope thinner.
In the last few years, growers have adopted the Darwin string thinner to remove blossoms. While it works very well on trees grown in upright systems, Reighard says it’s “been a disaster” on open vase trees. “It’s not going to work in our situation,” he said. Most peach orchards in South Carolina are open vase trees.
Growers have been finding other tools for blossom thinning, including hand-held devices like The Cinch developed by Phil Miller of Manistee, Michigan. A hand-held battery-powered drill powers a whirling wand containing “strings” made of soft rubber tubing. One South Carolina grower has purchased 200 of them, and even more battery packs, and uses them on 5,000 acres of peaches, reducing his hand thinning costs by $200 an acre, Reighard said.
Early thinning can increase fruit size and yield. Research in California has shown that high spring temperatures strongly influence the rate of fruit development and can result in smaller fruit sizes and less yield, Reighard said. Large fruit size has been hard to achieve in hot climates.
“Factors that slow down rates of fruit development extend the period of time that the plant has to supply potential fruit growth demands (carbohydrates, water, and minerals) and thus can lead to larger fruit size and yield,” Reighard said. Growers can use that information to decide how hard they need to thin so the remaining peaches will size well.
“In the Southeast, our traditional vase-shaped geometry is not conducive to mechanization,” he said. As in the Mid-Atlantic states, growers are looking at taller trees, more upright trees, and at working from platforms to prune, thin, and harvest.
In his talk, Reighard ran short of time long before he ran out of challenges to talk about.
Brown rot is a problem in the hot, humid Southeast, and the causative organism quickly becomes resistant to fungicides. Clemson researchers have developed a quick test so growers can determine which fungicides will still do the job.
Reighard and his Clemson associate, peach breeder Dr. Ksenija Gasic, are both participants in the RosBREED project, which is focusing on finding genetic markers for important peach traits. Bacterial spot resistance is one of them, and Reighard believes the work will accelerate breeding for resistant varieties.
New discoveries in the way genes function promise to help peach breeders develop varieties that have interesting new flavors and aromas as well as the traditional quality traits such as color, sweetness, and firmness.