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Student researcher seeks roots of apple resistance to fire blight

WSU researchers inoculate young apple trees near Wenatchee. Doctoral student Sarah Kostick is searching for areas of the genome that help apple trees resist the disease.

Named for the crooked, blackened branches it leaves behind, fire blight can wreck entire Northwest apple orchards in a single season.

Growers depend on antibiotics to keep the fire blight bacteria, Erwinia amylovora, in check, but the pathogen has already developed resistance in many growing areas. It’s only a matter of time before antibiotics won’t be able to stop it.

Sarah Kostick, student researcher at Washington State University’s Tree Fruit Research and Extension Center, is racing against that clock.

As she pursues her doctorate in horticulture, Kostick is working in the heart of Washington’s apple country, experimenting with hundreds of apple cultivars in the search for genetic resistance to the devastating disease.

An infected apple shoot
An apple shoot curls due to fire blight infection.

$100 million disease

“Fire blight is one of the few diseases that can cause serious problems for Washington apple growers,” says Kostick. Minor outbreaks strike the state annually, but significant damage was reported in 2015, 2016, and 2017. Nationwide, the disease costs apple growers about $100 million every year.

Fire blight prefers to strike at younger, vigorous trees. Older, established varieties, like ‘Red Delicious,’ tend to be more tolerant to fire blight than newer, popular varieties.

“We need new apple cultivars that resist fire blight, but keep that high fruit quality,” says Kostick.

In WSU’s Columbia View Research Orchard, near Wenatchee, she spent the last two years inoculating more than 1,600 trees, from a genetically diverse list of species, with the bacteria. Dipping scissors in a solution of fire blight pathogen, she cut the tips of shoots on young trees, then measured the resulting infection. Kostick is now analyzing the results to find the genes that determine if trees will stand up to the disease.

“Once we find them, we can develop DNA tests so that breeders can test for resistance with a simple tissue sample,” Kostick said. “If we’re successful, breeders will know years earlier if a promising new apple variety will be tolerant of fire blight, or susceptible to it.”

Sarah Kostick, student and researcher with WSU Tree Fruit, studies the genetic roots of fire blight resistance.

Food and genetics

Kostick is a relative newcomer to the apple industry. As a 4-H youth, she was drawn to agriculture and science. Originally from Minnesota, she learned about apple breeding during an internship at the Minnesota Landscape Arboretum Horticultural Research Center.

Master’s degree in hand from the University of Minnesota, she immediately joined WSU researcher Kate Evans’s apple program two years ago.

“I love being in the heart of the apple industry,” says Kostick. “I have so many opportunities to talk with growers, share my research and connect with the industry.

“Growers treat me as a researcher and are interested in what I’m doing, since the solutions I work on will affect them,” she added. “They aren’t afraid to ask hard-hitting questions, which is really good for a student—it challenges you to really focus your ideas.”

As an aspiring fruit breeder herself, Kostick is thankful for the opportunity to work directly with growers.

“I’m a student, here to finish my degree, and at the same time, I’m a researcher, working on a real problem and finding answers,” says Kostick. “There’s just something so fascinating about food, genetics, and putting it all together.”

• Contact: Sarah Kostick, Doctoral Student, WSU Tree Fruit Research and Extension Center,