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Some peach cultivars struggled to leaf out this year in South Carolina due to lack of chilling this winter, including these trees seen at Clemson University’s Musser Fruit Research Center in April. Lower chill varieties bloomed well and early, but were then hit with a hard frost. <b>(Courtesy of the Clemson Peach Breeding Program)</b>

Some peach cultivars struggled to leaf out this year in South Carolina due to lack of chilling this winter, including these trees seen at Clemson University’s Musser Fruit Research Center in April. Lower chill varieties bloomed well and early, but were then hit with a hard frost. (Courtesy of the Clemson Peach Breeding Program)

A one-two punch of a warm winter followed by a hard spring frost hit peach growers in the southeastern U.S. hard this year.

Growers in Georgia and South Carolina expect harvest to be about a quarter of normal yield. That’s a big hit to the country’s second- and third-largest peach producing states, which combined represent a $100 million industry and about 12 percent of the U.S. peach harvest.

“Only the low chill varieties got enough chilling. They came out three weeks early and then we got a terrible freeze, and it killed all that fruit off,” said Clemson University horticulturalist Greg Reighard.

Bad weather is certainly not a new problem for growers, but this one is expected to be exacerbated by climate change: as winters trend warmer and growers respond by planting lower chill cultivars, they end up with earlier blooms at greater risk of frost damage. This season shows what’s at stake if the industry doesn’t adapt.

“We’ve had four of the last six years that have been short; this year was exceedingly short,” Reighard said. “If it continues like this, we’ll have to switch to medium or low chill varieties. If we switch, we’ll have more problems with frost.”

Fortunately, research is well under way to help growers find ways out of that catch-22. In the long term, Reighard is optimistic that breeders will be able to combine lower chill varieties with later bloom timing to maximize resilience.

Meanwhile, growers can consider cultural practices, such as pruning or dormancy breaking chemicals, to maximize bloom on insufficiently chilled trees or long-term investments in frost protection such as wind machines.

And the silver lining to a season like this is that it’s a good opportunity for researchers to test strategies that will help growers in the future, said Dario Chavez, a University of Georgia horticulture professor.

The chill curve

In central Georgia, where most of the state’s peach acreage is, winters average about 800 to 1,000 chill hours, and growers have planted a number of suitable varieties, with requirements ranging from 500 to 850 chill hours, Chavez said. But this winter, they got just 420 chill hours.

That doesn’t mean that every tree was stuck in dormancy. The chill hours reported for each cultivar are an average, and as long as a tree gets about 50 percent of its chill hours, it will likely get some blooms, stretched out over several weeks.

And since buds located on new growth tend to need less chilling, strategic pruning can help growers make the most of the buds they will get.

“If you think of the tree as a population of buds, there is a normal distribution of the chill requirement within the tree. If you know how they are located, you can change your pruning and leave the tops of some limbs or some shorter caliper wood that you would prune in a normal year,” Chavez said.

Calculating chill hours is also key to using dormancy breaking compounds such as Dormex (hydrogen cyanamide) to spur more buds into action. It’s commonly used in Florida peach orchards where lack of chill is a common problem. But, the chemical only works to make up a few hundred chill hours, Chavez said.

“What we found is Dormex works better when lack of chill is smaller. At 400, that effect just goes away. There’s only so much a chemical can do, physiologically speaking,” he said.

And no chemical can help the other problem growers faced — a freeze on March 15 and 16 when most of the early varieties that had sufficient chill were in a tender stage of bloom.

Breeding for later bloom?

Peach growers have relied on chill hour requirements for decades to make planting decisions, but breeders are just now starting to pay attention to the less understood warm hour requirement that dictates how much warm spring weather is needed to convince a tree that it’s safe to bloom.

A short warm requirement means earlier blooming, and it’s typically found in cultivars bred for warm climates, where chill hours are harder to come by and spring frosts are not much of a risk.

Ideally, breeders would like to combine a cultivar with a moderate chill requirement — something suited to a winter like this past one — with a longer warm requirement that delays bloom until more frost risk has passed. The Clemson breeding program is taking on this task, breeder Ksenija Gasic said.

“Up until now, the thinking was that chill requirements was the only thing we should worry about, and the higher the chill requirement, the later the bloom. Now, with these last few years with how the climate is changing, we know that’s not enough; we also need to play with the warm requirement,” she said. “Later bloom, to avoid the early spring frost, is on the radar of every breeding program now.”

But it’s easier said than done, even with advancements in genetic analysis that provide new tools like DNA marker tests for key traits.

That’s because there are many different genes involved in how fruit trees have adapted to survive winters and bloom safely in spring, Gasic said. Sorting out how they all work together to create what we think of as chill or warm-hour requirements is complicated.

“Trees are smart. They evolved mechanisms to protect themselves, so if you have one or two days that are really warm, it’s not going to fool them into starting to bloom,” Gasic said.

Measuring how the trees respond to temperature requires painstaking work: cutting branches from each tree every day to bring into a greenhouse, so that researchers can watch for a full bloom to indicate that they’ve acquired enough chill.

Then, warm requirements must be assessed separately, after controlling chilling in a freezer. To complicate matters further, different trees seem to have a different starting temperature for what counts as a warm hour, Gasic said. That means running multiple experiments to find that “base temperature.”

But she needs this data on how each tree behaves — known as phenotypic data — to match up with its genetic data to sort out what genes control the chill and warm requirements.

Once she’s matched up enough trees to see patterns emerge, they can create predictive DNA tests. That’s the same process Gasic and other breeders collaborating with the U.S. Department of Agriculture-funded RosBREED 2 project are using to improve disease resistance in rosacea crops.

Work is well underway for the genes controlling chill requirements, and Clemson scientists are seeking a USDA grant to fund research on the warm requirements so they can ultimately breed for delayed bud break and increased frost protection.

“I want a marker to tell me the heat requirement, so we can breed a tree that requires 600 to 700 chill hours and has a high heat requirement or high base temperatures to start counting, which would push it to bloom late,” Gasic said.

Management

In the meantime, southeastern growers may want to adopt frost protection techniques common in other regions, such as wind machines or overhead irrigation, along with switching to lower chill cultivars, Chavez said. Right now, he knows one large grower using wind machines, while others rely on passive methods of frost protection, such as planting more at-risk varieties in locations that rarely freeze.

Protecting thousands of acres of peaches would be prohibitively expensive, but for early varieties that bring a premium price, it might be worthwhile.

Reighard credits the wind machines, and a location along a lake, for why he has more fruit in the Clemson research orchard this year than many other South Carolina growers.

There’s growing interest in wind machines, according to suppliers who said they have received a lot of inquires this year.

That’s why Chavez and his colleagues are working on developing models of freeze damage thresholds so that they can tell growers when and where wind machines could be deployed to protect the crop.

“The growers and researchers here, we’re not used to those tools. We need to learn how to they could work for us,” he said.

He also collected data this year to build a decision aid tool to guide the use of dormancy breaking compounds, which can damage flower buds that are developing normally if applied too late or be used to save trees with such significant lack of chill that they won’t leaf out on their own at all.

“Hopefully, the chill requirement problem won’t come up again in the next few years, but I do think it will be an issue coming up more often in the future,” he said. “We have to start moving to a different mentality as we start having issues of warmer winters.” •

– by Kate Prengaman