Clemson experts share peach research

EDGEFIELD – The peach is a sweet summer treat, but it also has enemies galore: insects, fungi and diseases, ready to attack the South’s favorite fruit.

Peaches are a sweet summer treat and the Clemson Extension Peach Team is researching to determine strategies for helping this fruit fight off diseases and pests.

During the annual Ridge Peach Producers Meeting in Edgefield, the Clemson University Cooperative Extension Service Peach Team briefed peach farmers on the counterattack against these pests, including Armillaria Root Rot, an insidious soil-borne fungus that is a major and costly problem in South Carolina.

Guido Schnabel, a Clemson plant pathologist and Peach Team member, described how exposing infected trees’ roots can cause the fungus to retreat, possibly doubling the return on investment on severely infected sites.

His research on Above Ground Root Collar Excavation calls for trees to be planted shallow on “berms.” Two years later, soil is removed from around each tree’s root crown, or root collar, exposing the primary roots to air and solar heating. While not “bullet proof,” this practice delays Armillaria colonization by about 2 years.

Planting peach trees shallow on berms and excavating the top roots after two years prevents Armillaria root rot cannot move from infected roots to trees.
Planting peach trees shallow on berms and exposing the top roots delays Armillaria colonization by about 2 years.

“Our research involves planting trees shallow on berms,” Schnabel said. “Two years after the trees are planted, the soil is removed to expose just a tiny bit of the trees’ roots. Armillaria Root Rot of peach caused by the fungus A. tabescens does not grow above the soil line, so exposing the roots also exposes the fungus to heat and drying, thereby increasing trees’ longevity.”

Schnabel said research on trees at Clemson’s Musser Fruit Farm shows Root Collar Excavation can reduce tree mortality and not impact fruit quality or yield which, in turn, can result in growers being able to double the returns on their investments in heavily infested sites. Schnabel also said heavily infested sites will benefit dramatically from raking infested roots out of the soil with a chisel plow, for example, prior to establishing the berms.

While planting trees on berms can help better expose the trees’ primary roots, there are challenges involved. These include risks of trapping water, which can result in water stress and wind lean, as well as increased suckering and interfering with field work in orchards.

In addition to Root Collar Excavation, Schnabel also talked about bacterial spot disease. This disease begins as twig cankers in the spring and spreads to leaves and fruit as the season progresses. Symptoms on green fruit include water-soaked lesions, while symptoms on maturing fruit vary from light speckles to deep crater lesions. Leaves also are affected and show yellowing starting at the tip. Severe leaf infections can cause early defoliation resulting in reduced fruit size, sunburn and cracking.

Hehe Wang, a Clemson plant bacteriologist, said this bacterial pathogen is difficult to control because most commercial peaches are susceptible and the limited number of disease-tolerant/resistant cultivars could develop symptoms during high-infection periods. In addition, there are no cultural controls available and chemical control options are limited. Wang is conducting research towards development of novel and more effective control options for integrated management of bacterial spot.

The fight against bacterial spot continues as researchers strive to determine the most effective means of controlling this disease. Brodie Cox, a master’s student in plant and environmental sciences from Aiken, is researching where the bacteria overwinter and how they adapt to chemical exposure.

The scientists also talked about research on bronzing. Bronzing is a peach skin disorder that can lower the value of a crop. It is cosmetic and damages the skin, not the flesh. Still, peaches with this disorder often are difficult to sell.

“Bronzing is a physiological disorder,” said Juan Carlos Melgar, assistant professor of pomology and a Clemson Peach Team member. “We believe this disorder is caused by a nutritional imbalance due to excessive potassium combined with high transpiration rates at final swell causing disruptions in the cell walls of peach skin.”

Transpiration is when plants absorb water through the roots and, then, release the water as vapor through pores on their leaves.

Bronzing most often occurs after rain events during July when temperatures are high. The researchers believe high levels of potassium move into cell walls of the fruit and compete with other nutrients such as calcium. Melgar said researchers are seeing more bronzing on peaches in areas of high water and high potassium levels.

“There have been many speculations around the cause of bronzing of peach skin,” Melgar said. “These include early-season thrips damage, the use of captan too close to harvest, use of oil-based crop protectants, application of certain fungicides too close to harvest, handling of fruit during harvest and pH or chemical reactions with using chilled water to cool produce right after harvest.

“Our recent experiments indicate too much water during periods of high transpiration together with high concentrations of potassium may cause nutrient imbalance at the cell wall level to the fastest growing cells of the skin found in the top, middle and bottom of the fruit, which can lead to cell collapse.”

Brown rot is another peach disease the Clemson team is focused on. Caused by a fungus, brown rot begins when trees start blooming. This fungus infects flower blossoms and shoots. To minimize damage, Schnabel said growers should remove all fruit mummies and cankerous parts of the tree. Strategically spraying peach trees at the correct time with the correct material is another step that can be taken to fight brown rot.

In addition to diseases, insects, including San Jose Scale and thrips, also can cause problems for peaches. Brett Blaauw, a peach entomologist with a joint appointment between Clemson and the University of Georgia, said problems from San Jose scale include branch dieback and eventually tree death if left untreated. Pruning peach trees to improve spray coverage and applying horticultural oil twice during the dormant season can help control this insect.

For thrips, Blaauw said to monitor “early in the season” and treat orchards if adults infest more than 10% of blooms, or if larvae are present.

“Use a hand lens to inspect fruit,” Blaauw said. “The critical period is when fruit ripens. If you find five adults per 50 fruit, apply insecticides.”

To help control thrips, Blaauw said to limit weeds in and around orchards.

Paraquat is one herbicide used to fight weeds. Wayne Mitchem of North Carolina State University said new paraquat regulations require certified applicators must successfully complete online training every three years before they can mix, load and/or apply paraquat. Information and a link to the training module can be found at

Other topics discussed during the meeting included an economics update from Nathan Smith, Clemson Extension economist, and an update from Clemson peach breeder Ksenija Gasic on a study involving peach heat requirements. Will Lever of the South Carolina Department of Agriculture talked about the Produce Safety Rule of the Food Safety Modernization Act.

More peach research information will be shared during the South Carolina Peach Growers Meeting from 1 p.m. to 5 p.m. March 11 at the Madren Conference Center, 230 Madren Center Drive, Clemson, S.C. 29634. For more information, contact Guido Schnabel at (864) 656-6705 or

Peaches are a driving force in the South Carolina economy and Josh Yonce of Big Smile Peaches in Johnston said meetings between growers and researchers are important.

“We can attend these meetings and get updates, as well as learn about other key issues involving our industry,” Yonce said. “We learn about new regulations, research and other information beneficial to us as growers. These meetings are very important.”

According to the United States Department of Agriculture, South Carolina is second only to California in peach production. In 2018, South Carolina growers produced 65,800 tons of peaches for a production value of about $72 million. California produced 479,000 tons of peaches and Georgia came in third with 26,000 tons of peaches produced.



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