College of Agriculture, Forestry and Life Sciences; Public Service and Agriculture

Clemson peach experts share peach research knowledge


The peach industry is a major contributor to the South Carolina economy.
Growers learned about Clemson peach research during the 2023 Ridge Peach Meeting in Edgefield, South Carolina.
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January 2023 was the sixth warmest January on record and a Clemson University professor says if temperatures stay warm, it could have an effect on the state’s peach crop.

Peaches need a certain number of chill hours, or temperatures below 45 degrees, to grow. The amount of required chilling temperatures varies according to the variety. Peach trees need cold winter temperatures to break dormancy and blossom.

Two models – the Chill Hours Model and the Modified Chill Hours Model – are commonly used to determine how much winter chill trees receive. Using these models can help growers make cultivar selection decisions when establishing orchards. Two other models, the Dynamic Chill Portions Model and the Utah Chill Units Model, also may be used. Growers attending the 2023 Ridge Peach Meeting in Edgefield, South Carolina, learned which model works best for South Carolina’s temperatures.

“For warm winter climates, such as in South Carolina, chill portions calculated from the Dynamic Model give the best estimate of when peach trees receive enough chilling to begin normal flower and vegetative bud growth in the spring,” said Gregory Reighard, professor emeritus of horticulture at Clemson.

Chill portions are calculated using a 2-step process, which more accurately measures the accumulation of cold required for peach trees. Chill hours do not take in consideration warm temperatures above 45 degrees and subfreezing temperatures, both of which can have small or negative effects on chilling accumulation in the winter.

Modified chill hours – 32 – 45 degrees – remove temperatures below 32 degrees but do not compensate for warm winter temperatures. Chill units, such as those calculated by using the Utah Model, do consider warm temperatures but when calculated in warm climates tend to overestimate chilling. Because of this, these units are often less useful, Reighard said.

For projections of future chill accumulation, see the Climate Mapper on the Climate Toolbox website, Select Agriculture as the “Impact Area.” Under “Variable,” projections from Chill Hours, Chill Units and Chill Portions are shown.

For more farm or crop-specific information, growers can contact their local Clemson Extension professionals

According to the National Oceanic and Atmospheric Administration (NOAA), the Earth is getting warmer. The global temperature has risen at an average rate of about 1/4 degree per decade. Chill hours are different from cold hardiness, which is the lowest temperature trees or plants can tolerate without dying.

Growers also learned about the “ideal peach,” which today for South Carolina is one that includes a moderate chilling requirement, a high heat requirement, as well as tolerance to fruitlet freeze, bacterial spot and brown rot. Ksenija Gasic, peach breeder and geneticist, said three varieties – testing names SC1, SC2 and SC9 – are expected to be released this year.

Gasic also talked about Armillaria root rot (ARR) studies and encouraged growers to participate in a survey to help provide information for a grant study related to helping determine the economic effects of ARR and cost-benefits of using cultural controls.

During the meeting, participants learned what to do now that Lorsban, or chlorpyrifos, is no longer available for control against the lesser peach tree borer and San Jose scale. While chlorpyrifos is less expensive to use, Brett Blaauw, entomologist with both Clemson Extension and the University of Georgia Extension Service, assured growers there are alternatives.

“First, this ban is for use on food products,” Blaauw said. “So, if you have young, non-bearing trees, you can still use chlorpyrifos as long as the trees do not bear fruit within one year of application.”

Alternatives being studied for use against borers are Assail, Asana XL, Altacor, Rimon, Mustang Maxx and Cormoran.

For San Jose Scale, Blaauw said options include using insect growth regulators such as Centaur and Esteem. Venerate and Carbaryl can also be used. For crawlers, Movento can be used. Blaauw is looking at the possibility of using lime sulfur. Mating disruption is a promising alternative. While still in the experimental stage, Blaauw said this option seems to work best in low- to mid-range pressure.

As for controlling weeds, Wayne Mitchem, Extension associate and horticulturist from North Carolina State University, said Oryzalin is no longer being manufactured. Prowl H2O and Satellite HydroCap are alternatives. They are in the same herbicide family as Oryzalin but need rainfall within 10 days to remain 100% effective, he said.

Mitchem also talked about the herbicide Diruon, which will potentially be banned by the Environmental Protection Agency. For bermudagrass and johsongrass control, Mitchem suggests using Poast, Select Max and Fusilade.

In a discussion about peach diseases, Guido Schnabel, Clemson plant pathologist, said an integrated approach is being used to manage brown rot. This involves improving canopy airflow by pruning and thinning; and reducing inoculum by removing tree mummies, blossom cankers, wild plums and keeping a clean or mowed orchard floor. Brown rot also can be managed by disrupting mechanical spore transfer by thinning the fruit early to break up fruit clusters and by using pest control. He also reported about findings related to recent Sour rot outbreaks in stored peaches. Research is ongoing to try to find alternative measures for Sour rot control.

Plans for 2023 include investigating new biocontrol agents for possible bloom and green fruit rot management to reduce the number of conventional products used during the season.

Video about MyIPM – Integrated Pest Management for your smartphone

To help growers identify diseases and pests in fruit crops, Clemson has developed a MyIPM Smartphone App. The content supplements the spray guides and is maintained in collaboration with fruit Extension specialists at the University of Massachusetts, Pennsylvania State University, the University of Maryland, North Carolina State University, the USDA-ARS, Mississippi State University, the University of Florida, the University of Georgia and Virginia Tech.

The fruit crop app – MyIPM – and spin off apps, MyIPM Row Crop and MyIPM Hawaii, are available in the Apple Store and Google Play. The apps feature diagnostics, including descriptions and picture galleries of crop diseases, insect pests and disorders; name and description of the causal agents, including a 2-to-4-minute audio from a regional specialist; chemical, biological and cultural control tactics and more.

For information visit


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