Prime apple growing areas in US face increasing climate risks

(Press-News.org) VANCOUVER, Wash. – Some of the most productive apple regions in America are facing big challenges from a changing climate, according to a Washington State University study.

Researchers analyzed over 40 years of climate conditions that impact the growth cycle of apple trees from bud break and flowering through fruit development, maturation and color development.

While many growing areas are facing increased climate risks, the top three largest apple producing counties in the U.S. were among the most impacted: Yakima in Washington, Kent in Michigan and Wayne in New York. In particular, Yakima County, the largest of the three with more than 48,800 acres of apple orchards, has seen harmful trends in five of the six metrics the researchers analyzed.

“We shouldn’t take the delicious apples we love to consume for granted,” said Deepti Singh, a WSU climate scientist and the study’s corresponding author. “Changing climate conditions over multiple parts of the growth cycle pose potentially compounding threats to the production and quality of apples. Moving forward, it would be helpful to think about adaptations at different stages of apple growth that can minimize overall harmful impacts.”

The team’s immediate next step is to look at projections, Singh added, to inform planning and management in the tree fruit industry.

For this study, published in the journal Environmental Research Letters, the researchers analyzed climate data from 1979-2022. They specifically looked at six metrics that impact the apple growth, including two metrics of extremes—extreme heat days, defined as days with a maximum temperature greater than 34 degrees C (93 F) that can cause sunburn as well as other problems, and warm nights when the minimum temperatures were greater than 15 C (59 F) that can adversely affect coloration.

The researchers also looked at the number of cold days; “chill portions” or the number of colder hours an apple tree needs to be dormant; the last day of spring frost; and growing degree days, meaning the number of days above a certain temperature that are conducive for apples to grow.

Changes in these metrics can impact apple production, changing the time when apple flowers bloom, increasing the risk of sunburn on apples as well as affecting apple appearance and quality. The western U.S. has experienced the strongest trends in multiple metrics that are detrimental to apples.

The challenges are complex partly because apple trees are perennials, said co-author Lee Kalcsits, a WSU tree physiologist who leads programming at the Wenatchee Tree Fruit Research and Extension Center.

“What goes on in different seasons can affect long-term health as well as the performance and productivity of the apple tree during that specific season,” he said. “So what goes on in the winter affects what happens in the spring, which affects the summer, and it just keeps going around in a cycle.”

Growers are already adapting, he added, noting that in Washington state, producers are employing measures such as netting and evaporative cooling to fend off sunburn during the more frequent extreme heat.

With more climate risks likely, researchers are also taking steps to help the industry adapt. Kalcsits is leading a project funded by a $6.75 million grant from the U.S. Department of Agriculture to help mitigate the impact of extreme climate events on apple and pear crops. It is a multi-institution grant with work planned across the nation as well as in the apple growing regions of Washington state.

“Washington is a great place to start to talk about adaptation,” said Kirti Rajagopalan, a WSU biological systems engineer and study co-author, who is also working on the grant project. “A lot of the commercial apple production happens in the northern U.S. There are also parts of Washington where the summers can get pretty hot, so this is a good place for a case study – and if we can manage it here, then it's likely manageable elsewhere too.”

Additional researchers on the current study include first author Shawn Preston and Matthew Yourek of WSU. This research was supported by the National Science Foundation and WSU’s Emerging Research Initiative.

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