Think Out Loud

Oregon’s peppermint farms threatened by fungus

By Sage Van Wing (OPB)
Sept. 11, 2023 3:44 p.m. Updated: Sept. 18, 2023 8:34 p.m.

Broadcast: Monday, Sept. 11

Oregon is one of the leading exporters of peppermint oil in the U.S. But a fungus called verticillium wilt, which affects the leading variety of peppermint, is spreading throughout the Willamette Valley. Reporter Berit Thorson recently wrote about the issue for Capital Press, and joins us to talk about what she learned.


Note: The following transcript was created by a computer and edited by a volunteer.

Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. We start today with the challenges that mint farmers are facing and a possible solution. Oregon is one of the leading exporters of peppermint oil in the US. But a fungus that affects the most common variety of peppermint is spreading throughout the Willamette Valley. Reporter Berit Thorson recently wrote about this issue for the Capital Press and joins us to talk about what she has learned. Welcome to the show.

Berit Thorson: Happy to be here.

Miller: You focused on something known as Black Mitcham, which I’d never heard of. What is it?

Thorson: Black Mitcham is an heirloom type of peppermint. So it’s basically like an older variety or the traditional type of peppermint. When you think of peppermint, you’re probably thinking of Black Mitcham. It’s used in peppermint ice cream and toothpaste, you name it, and it’s sterile. So it’s cloned rather than reproducing. When it came over from England in, I think the 1800s, it really hasn’t changed since then, which I think is particularly interesting because it just keeps propagating and cloning itself.

Miller: I hope I’m pronouncing this correctly. But what is verticillium wilt, which is the villain in your story?

Thorson: [Laughing] Yeah. Verticillium wilt. That’s correct. It’s a disease caused by a fungus that’s found in the soil. So it’s a disease that shows up as discoloration of the plant leaves or wilted leaves. It can also affect growth or even just kill the plant. It affects more than just peppermint but Black Mitcham in particular, partly because it just clones and doesn’t reproduce, doesn’t really have any protection against it. There’s no way that it is selected for certain traits.

Miller: How widespread is the fungus that causes this disease?

Thorson: It’s pretty widespread. When it came over from England, Black Mitcham peppermint was originally grown in the eastern US, but as the fungus infected the soils there, it spread west. And once it’s in the soil, it really doesn’t leave it. So there aren’t really great ways to get rid of it. There aren’t really a ton of ways to prevent it. So it’s becoming increasingly widespread but it’s already very widespread.

Miller: Well, how big an impact has it had?

Thorson: A pretty significant impact. I mean, as I said, it’s been spreading across the US. So originally peppermint was grown in the east now it’s grown on the west coast. It’s had a pretty significant impact specifically, you know, in Oregon, in the Willamette Valley, we’ve seen peppermint growers and farmers switching away from Black Mitcham in particular, to other types of peppermint or just stopping growing peppermint altogether because of verticillium wilt, plus a little bit because of the impact of global competition. But I mean, production of peppermint, specifically, has decreased 47% since 2013. And it’s now basically half of what we used to produce in peppermint and a lot of that has to do with verticillium wilt, as well as global competition.

Miller: Are other US states picking up mint oil market share as this fungus has spread through Oregon?

Thorson: Yeah, definitely. Oregon used to be number one in market share in the US, but now that title goes to Idaho – they aren’t yet facing the same problems of wilt that Oregon is. They have what’s called virgin soil. It’s soil that doesn’t have the fungus in it. And so they are able to get higher yields. It’s easier to grow mint in Idaho right now than it is in Oregon. But Oregon is still number two and then Washington is number three and interestingly, Indiana is four.

Miller: But is it fair to say that Idaho mint farmers are just holding their breath? I mean, this came from the east, went west, is in Oregon, hasn’t quite made it to the soil of Idaho. I mean, you have to go through Idaho. If you were marching as fungus, you’d have to go through it. But are they worried too?

Thorson: Yeah. I think so. I think everyone is really worried. The Mint Industry Research Council, who is helping to fund a lot of this research that’s being done, is very concerned that it’s gonna be everywhere, and that it kind of already is everywhere. It really hasn’t gotten into Idaho, as you said. But I mean, as you also pointed out, it started east and now it’s west and once it’s there, you really can’t go back, and it’s really harmful to the plant. So everyone, I think, in the mint industry in the US is really worried about it because Black Mitcham is that kind of traditional classic peppermint. It’s definitely a concern.

Miller: We’re also going to talk now about efforts to find a new variety of mint that could be more resistant to it, which gets us to what researchers at the University of California at Davis are doing. Can you give us a sense for what’s happening there?

Thorson: Absolutely. I will do my best. It is kind of complicated but they have this three pronged approach that they’re working on, and they got funding from the Mint Industry Research Council about a year ago, maybe a year and a half now. And basically the first part of this was… the goal was to map the genome or the genetic material of the three parent plants of peppermint. Peppermint has three parent plants: water mint, horse mint and apple mint. Horsemint and apple mint actually create spearmint. Spearmint and then water mint are combined and that gives you peppermint. And so they really focused on water mint and mapping the genome of water mint because that makes up the majority of the genetic material in peppermint.

Then the second part of this approach is looking at the mutated peppermint plants. From the 1940s through the 1960s, they used gamma irradiation to mutate the genes of peppermint to kind of see if they could randomly make more resistant peppermint...

Miller: That’s a brute force way to do genetic engineering. Wow.


Thorson: Yeah. So they did it to a ton of the plants and then they are now able to use those plants and the genomes of those to figure out where the genes overlap with the parent plants and with Black Mitcham, to try and identify which parts of the genes relate to resistance. And then, which are oil composition and flavor and the things that we really care about in the quality of the mint.

Miller: How much progress have they made so far?

Thorson: They’ve made pretty good progress so far. I mean, they’re only a year into this specific part of the research and this specific plan, but this research has been going on for a long time, obviously. And it’s building on work that’s been done particularly at Oregon State and Washington State, as well as at University of California.

So they’re done with that first step of mapping the three parent plants, but they’re now going through the second, looking at the mutated peppermint plants and comparing the genome, as well as the third, which is they’re trying to double the genome of Black Mitchum since it’s sterile. But they’re trying to double the genome so that it can actually reproduce, because the reason it can’t right now is because some of the genes are only present once. So if they double it, they’ll be present twice and then it can maybe cross.

So they’re going through the second and third steps. And they’re probably at least a few years out from anything that’s commercially viable on kind of the faster track and they’re at least a couple of years out.

Miller: Mint is in some ways an unusual crop. I mean, it’s different than one that people can grow and eat. How is it processed?

Thorson: It is very different from apples or cherries or anything like that. It’s first cut. So in the fields it’s grown and then it’s cut by a process called ‘wind growing,’ which is what they use for hay, and then they leave it in the fields to dry for a couple of days. It’s then picked up and harvested into these huge trailers. I think they look kind of like box cars on a train which are then hooked up to the mint distillery factory thing. And then they have super hot water that runs underneath the trailers, creating steam that rises through the huge trailers of mint, heating it and then turning the mint oil from like a physical liquid state into steam as well, which then rises through these pipes. It almost looks like a ‘Wonka Factory.’ And it moves through the still and then it cools back down into this oil and water mixture which is then separated based on density. And the mint oil stays at the top while the water sinks and then there are pipes at the top and at the bottom that separate the water and the oil and move them to their respective areas. The oil going into barrels and the water being taken back into the still to be used again and reheated.

Miller: What does it smell like?

Thorson: Oh, at first it’s this really clean and fresh, almost sweet smell and then after a little while it kind of becomes stronger and almost overwhelming and sharper, and it really stays with you. So when I was going to the mint farm, everything that I wore and everything that I brought with me, smelled like mint for a few days after. I really didn’t realize how strong of a smell it was until I was there for a little while and then left and was like, oh, I still smell it.

Miller: Where does most northwest mint end up?

Thorson: That’s a great question. And I didn’t track the whole process of what happens after the mint oil is collected in the barrels, but generally it goes to big companies or corporations - there’s Colgate, Palmolive, there’s Wrigley, and they’ll use it for their products like toothpaste or soap or that kind of thing. But there’s the farmers and then there’s this middleman and then there’s the end users which are like the big companies like Colgate and Palmolive and Wrigley.

Miller: What would it take for those big industrial buyers, which are making gum or toothpaste or whatever, to accept mint oil from a different mint plant. I mean, the way you described it, they’ve been using the same one for decades. It’s literally the same plant. Just a bunch of clones. Do they want a different one?

Thorson: I think, yes and no. They do use other mint plants, they already do. It’s just those peppermints don’t have the same quality of oil and the flavor balance that they like. So when the mint leaves the mint still, it goes to this middle man that evaluates the chemical composition and these different companies have really particular goals of what they want their composition to be. And so they will mix different oils. If they evaluate that one oil has too much of a certain chemical compound and not enough of another, they’ll mix the oil until it gets closer to what those companies want.

Because Black Mitcham does have such good qualities, it has what they really like, [and it’s] really close already to what they want. It has that kind of higher market price, but then with the wilt, it yields less. And so there’s this kind of push and pull of yes, they would accept oil from a different mint plant but then it becomes much more complicated, trying to mix different oils from different places around the world or with different compositions. Whereas Black Mitcham is really close to what they already are looking for a lot of the time.

Miller: Which goes back to what you’re saying, the dream of finding a plant that will yield the Black Mitcham-like oil but not be so susceptible to that fungus. What’s the timeline like right now?

Thorson: For finding the plants or getting to a plant that is more resistant?

Miller: Yeah, exactly.

Thorson: It sounds like it’s at least a couple of years. From talking to Isabelle Henry, the researcher at UC Davis, it sounds like with research that they’re doing like this, it could be anywhere from a year or two years, to five or more. It kind of depends on how quickly they find a more resistant strain because it’s a lot of trial and error right now.

Miller: Berit Thorson, thanks very much.

Thorson: Thank you, Dave.

Miller: Berit Thorson is a reporter for the Capitol Press. She joined us to talk about the challenges that mint farmers in Oregon, and really in a lot of the country, are facing right now, but also some research hope on the horizon.

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