Green Manures Showing Promise as Biofumigants

Two years of research into how green manures act on Verticillium wilt is showing that the green manures incorporated into wilt-infested soils reduce soil populations of the wilt fungus and that some work better than others.

In a presentation January 26 in San Diego at the Mint Industry Research Council’s Annual Meeting, Oregon State University Extension Plant Pathologist Jeremiah Dung provided participants a snapshot of what he’s looking for and what he’s found to date.

In his project, Dung is evaluating green manures’ effectiveness as a soil biofumigant in hopes of providing growers a method to reduce the wilt impact in fields that are already infested.

He also is working with Washington State University plant pathologist Dennis Johnson on developing novel sensitive molecular methods and sampling techniques to help detect and quantify the mint strain of Verticillium dahlia in soils in hopes of providing growers a risk-based management decision tool to decipher results of sampling for Verticillium wilt levels.

A third leg of his project involves collaborating with OSU Assistant Professor Kelly Vining to characterize Verticillium populations in mint in hopes of devising more effective management techniques. “We want to know our enemy so that we can face our enemy better,” he said. Also, as part of this portion of his research, Dung is hoping findings will help in the resistant-breeding efforts being conducted by WSU Associate Professor Mark Lange.

The research has its basis in the fact that some brassicas naturally produce a compound that when broken down can serve as soil biofumigants.

Because not all green manures are equal, as Dung put it, the researchers are hoping to determine which green manure crops have the highest efficacy against Verticillium.

In research to date, researchers have used a growth chamber to look at how different brassicas and different rates of green manures affect Verticillium. They grew the different brassicas in pots with soil that was artificially infested with Verticillium, then chopped up the brassicas and manually incorporated them into the soil.

Researchers found that arugula, mustard and broccoli green manures outperformed the untreated check, or control, pot in reducing Verticillium populations and that arugulas and mustards “significantly reduced” Verticillium populations compared to the untreated control pot.

Dung said several factors need to be considered in any management decisions to incorporate green manures into operations, including that green manures require water and other inputs. “So, that should go into the cost analysis,” he said. “Also, some of these brassicas can be persistent weed problems if we let them go to seed.

“We want these green manures to go to flower,” he said, “which is when they have the highest concentration (of the desirable ITC compound). But we don’t want them to go to seed.”

Another factor that should be considered is the timing in getting the green manures incorporated into soil.

In Central Oregon, where the research has been conducted, growers harvest seed in late summer, giving them only a short window to grow and incorporate a green manure crop and get a mint field planted, Dung said.

“It is about a two-month window we can utilize after harvest and before planting,” he said, “so we want to know will it produce sufficient biomass in that time to suppress and reduce Verticillium populations.”

A third year of the research is planned for this summer.