Jun 24, 2016
Pathologist hits back against fire blight

Fire blight and apple scab are the two biggest disease problems for Michigan’s commercial apple orchards. Scab hits annually, with damage to fruit the main concern. Fire blight is more sporadic – but when it hits, it hits hard, said George Sundin, a professor and tree fruit pathologist with Michigan State University (MSU).

Fire blight’s tendency to kill trees can devastate apple orchards – especially modern, high-density orchards. A fire blight epidemic hit southwest Michigan in 2000, killing about 20 percent of the acreage in the region. About 400,000 trees died – and that’s when plantings were lower- density. If they’d been high-density like they are today, there’s no telling how many trees would have been killed, Sundin said.

Another fire blight epidemic hit Michigan in 2005 or 2006, but that was the last major incident. There’ve been minor outbreaks since, but nothing disastrous. In humid Michigan, growers expect a fire blight epidemic about every five years, so they’re on a lucky streak. And as of early May, the 2016 apple crop was looking like it might escape an epidemic again, thanks to temperatures that weren’t quite warm enough during bloom to facilitate the disease, he said.

Sundin was walking through an old block of Jonathan and Golden Delicious as he talked, one of about a dozen test blocks at the MSU Plant Pathology Farm. He’s been with MSU since 2002. A New Jersey native, he worked for Texas A&M University for five years before moving to Michigan.

On top of his teaching and Extension duties, Sundin’s job is to study Michigan’s tree fruit diseases (including cherry leaf spot and various fruit rots and fungal cankers) and find viable ways to control them. His team includes Cory Outwater, a field research technician, Tyre Proffer, a visiting professor, and several graduate and undergraduate students.

The pathology farm, part of MSU’s Department of Plant, Soil and Microbial Sciences, is just south of the main campus in East Lansing, part of a large plot of contiguous farmland owned by the university. The tree fruit blocks (including cherries and peaches) are scattered around the property – far away from Michigan’s major fruit regions, so there’s little chance the inoculum treatments will spread to commercial production, Sundin said.

Fire blight

It was early May, during bloom. Sundin and his team were spraying in the test blocks and analyzing data in the lab. They tackle fire blight from multiple angles. The goal is to find the best controls – and to preserve those controls for the long haul, he said.

The Jonathan/Golden Delicious block Sundin moved through was planted in the 1970s. The trees must be tough, because MSU researchers have been bombarding them with fire blight inoculum for more than three decades.

“It’s amazing how these trees have survived that onslaught,” Sundin said.

It helps to be big and old. If they were to inoculate a block of young, high-density Galas, for example, the pathogen would most likely move through the trees like wildfire, right down to the trunk, and kill them. Older trees, however, have a lot of big branches, which block the spread of the pathogen. If they prune the infected branches out in winter, the tree will survive, he said.

After spraying the fire blight pathogen on the trees, Sundin and his team treat them with various experimental treatments, trying to find the best control methods. They want disease pressure to be as high as possible.

George Sundin points out ooze coming out of a Ginger Gold tree, ooze that contains the fire blight pathogen. Flies landing on the ooze spread the pathogen to other trees.

“With high infection pressure, you separate the best treatments from the marginal treatments, and from the ones that don’t work,” he said.

They prefer to work with varieties, like Jonathan, that are highly susceptible to fire blight.

“We always choose the most susceptible varieties,” he said. “If we can control diseases on the most susceptible, we can control them on any variety.”

Unfortunately, some of the most popular varieties today are highly susceptible to fire blight. Sundin listed a few: Gala, Fuji, Pink Lady, Ginger Gold, SweeTango, RubyFrost.

“Growers have to grow these varieties to make money, but blight can just eat them for lunch,” he said. “Some of the new varieties have been pushed to market so quickly; I don’t think there’s enough testing done on disease susceptibility.”

The main breeding emphasis is taste and texture, not disease resistance.

“We have resistant varieties, but they are not widely grown because they don’t sell very well,” he said. “Without the consumer driving desire for disease tolerance and fewer sprays, I don’t see apples with tolerant traits happening in the near future.”

The best controls currently available for fire blight are the antibiotics streptomycin and kasugamycin (Kasumin). Sundin’s team is studying other methods, such as virulence inhibitors and biological controls, but nothing yet works as well as the antibiotics, he said.

His team’s biggest accomplishment so far has been their work on Kasumin, which helped convince EPA to register the bactericide on pome fruit a couple of years ago. Kasumin was developed by Arysta LifeScience, and Sundin’s lab did the “leg work” for the registration, gathering data, showing controls in the field and monitoring for antibiotic resistance. Getting Kasumin registered was especially critical in Michigan, where fire blight has developed resistance to streptomycin, he said.

“The only materials we would have had without Kasumin are marginal to good,” he said. “Under epidemic conditions they wouldn’t control disease very well, but Kasumin will. That’s a huge thing for growers.”

MSU research has also revealed the efficacy of Apogee, which is now a major control mechanism for shoot blight. Kasumin (or strep) followed by Apogee will give growers “pretty close to season-long control” of fire blight, Sundin said.

A dead Ginger Gold tree. That variety is highly susceptible to fire blight, Sundin said.
A dead Ginger Gold tree. That variety is highly susceptible to fire blight, Sundin said.

Dying Breed

There aren’t many people left who do what Sundin does. He said there might be half a dozen tree fruit pathologists running active programs in the Midwest and East today. There used to be more, but retirements and funding cutbacks have whittled them down. Fortunately, Extension programs are relatively robust in Michigan, and the recent creation of the Michigan Tree Fruit Commission has been a real boost – for funding as well as morale.

“It makes you really want to work hard, because you know the growers support what you’re doing,” he said.

Sundin’s expertise has been in demand across the country, as well as in Canada. He’s visited Nova Scotia three times in the last 16 months. The province was hammered by a massive fire blight epidemic in 2014, courtesy of Hurricane Arthur. Sundin has been helping Nova Scotia’s apple growers deal with the aftermath. The hurricane spread the pathogen far and wide around the province, and it will be more of a problem for commercial orchards going forward, he said.

— Matt Milkovich, managing editor

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