The United States is one of the top world producers of peppermint and spearmint. American mint is known for its consistency and purity. In 2017, 22,300 acres held mint crops and produced 5,778 pounds of mint oil. Most U.S. mint is grown in Washington, Oregon and Idaho. Mint production shifted to the Pacific Northwest from the Midwest in the 1920s because of Verticillium dahliae, a soilborne fungus. 

Unfortunately, Verticillium travelled with the transferred mint plants and now forces Pacific Northwest growers to abandon producing mint in certain fields. Decades later, Verticillium still lives on in Midwest fields as well. There is no control for the soil fungus — yet. Dennis Johnston, a professor of plant pathology at Washington State University, Pullman, wants to change that. 

“We’re running out of new fields,” Johnston says of the unsolved problem.

Peppermint and Scotch spearmint are very susceptible to Verticillium, but native spearmint is resistant. Verticillium survives in soil for at least 10 years. It infects the roots of the host plant, such as mint, when they grow close to the soil’s microsclerotia.

“Microsclerotia is a mat of resistance fungus mycelium,” Johnston explains. “It germinates when close to a mint plant in response to exudation of roots. The fungus will colonize the root. If it gets into the water-conducting vessels, it plugs them up and causes the mint plant to wilt.”

Johnston and fellow researchers discovered that weeds in the mint family also host Verticillium. Another finding is that the microsclerotia will travel with surface irrigation water.  

“A field is surface-irrigated, and then the farmer reclaims that water,” Johnston says of the fungus spread. “The reclaimed water is used downhill to irrigate another field, which sometimes happens several times, and that water can pick up contaminants of the Verticillium microsclerotia and be a real problem. The treatments to get rid of it ... we’re still looking for those.”

Johnston is developing a real-time PCR (polymerase chain reaction) assay to amplify a DNA segment of Verticillium for research. In 2018, his team, which includes Oregon State University, will sample soil in fields before mint is planted.

“We’re building a forecasting model,” he says of their work. “We can then predict, based on Verticillium presence, root lesion nematodes and other factors, whether or not there’ll be a good mint crop in a particular field.”

Integrated management needed
Travel west across the state to Prosser, Wash., and Doug Walsh, professor of entomology and an environmental and agrichemical education specialist, studies mint pests at Washington State University’s Irrigated Agriculture Research and Extension Center. Walsh proved last year that spider mites affecting mint are susceptible to registered miticides. WSU will also soon register a synthetic pyrethroid insecticide to help mint growers with late-season caterpillar pests. His lab is now preparing for its 2018 project with the mint root borer.

“The mint root borer only has one generation per year,” Walsh explains, “but it overwinters as small, immature larvae that consume stolons during winter and spring months. Growers think they have a perfectly healthy peppermint field in the fall after cutting. Then in the spring, the field appears to never break dormancy. The root damage caused by borers is hidden for months.”

Currently mint growers apply, postharvest, the insecticide chlorpyrifos to control mint root borers every autumn. There are no data to show at what population level the insecticide does not need to be applied. Walsh and his team will develop a threshold of how many mint root borers it takes to cause damage to a mint stand.

“We’ll mechanically collect soil, and separate out mint root borers from the roots and soil to determine how many borers are present,” Walsh says of their research process.

Sex pheromones for the mint root borer, a synthetic analog of the female pheromone, are commercially available and attract males to pheromone traps. “We’ll use these traps during the mating flight period in early summer,” Walsh says. “We want to see if we can develop a relationship between how many male moths we catch in those traps, in comparison to the actual numbers of larvae feeding on roots later on in the season in those same fields.”

This work will inform an Integrated Pest Management plan for the mint root borer. By determining the severity of the borer, applications of pest control products can be based on specific thresholds. “We want to come up with a method to assure growers when it’s OK not to treat with insecticide,” Walsh says of reducing input costs and chemical applications.

Most mint growers distill their crop for mint oil, which enjoys increased demand because the citrus greening disease has drastically reduced the amount of citrus peels available for citrus oil. While mint oil is used to scent cleaning supplies, it also flavors things from chewing gum to chocolate to toothpaste.  

“We'd like to think most people brush their teeth daily,” Walsh says, “so most people are touched by mint oil every day. About 15 years ago, there was a push to sell U.S. mint oil in the China market. It didn’t work. The favorite toothpaste flavor of the Chinese is green tea.”

Hemken writes from Lander, Wyo.