Lindsey du Toit and Mike Derie conduct tests to create control soils at various levels of risk for harboring Fusarium wilt.
Lindsey du Toit and Mike Derie conduct tests to create control soils at various levels of risk for harboring Fusarium wilt.

Soil from area growers is being tested through mid-February at Washington State University Mount Vernon to determine field suitability for planting spinach seed in spring. Meanwhile, related spinach disease research recently was accepted for publication.

“This is the time of year we offer a greenhouse soil bioassay to test fields for the risk of spinach Fusarium wilt,” said Lindsey du Toit, WSU plant pathologist. This is the sixth winter of testing. The cost is $200 per field.

“Our purpose is to help growers and seed company personnel identify fields that can be planted to a spinach seed crop in shorter rotations than the current recommended rotation of 10-15 years out of spinach,” duToit said.

“More important,” she said, “the bioassay is intended to help identify fields that should not be planted to spinach seed crops because of a high risk of Fusarium wilt,” a fungal disease that can survive in soil many years.

Limestone yields temporary benefits

One way to help suppress the fungus is to treat susceptible fields with limestone, a treatment studied by former WSU Mount Vernon graduate student Emily Gatch.

“The maritime Pacific Northwest is the only region of the United States suitable for production of spinach seed – a cool-season, day-length-sensitive crop,” du Toit said. “However, the acidic soils of this region are highly conducive to spinach Fusarium wilt.

“Raising soil pH with limestone partially suppresses spinach Fusarium wilt,” she said, “but the suppressive effect is transitory.”

Gatch found that annual application of limestone for three years prior to a spinach seed crop was superior to a single limestone application for suppressing the fungus and increasing seed yield, du Toit said.

“She developed our soil-based greenhouse bioassay to characterize the spinach Fusarium wilt risk of soil samples submitted from stakeholders’ fields and explore the mechanisms of limestone-mediated Fusarium wilt suppression,” duToit said.

Gatch’s dissertation recently was accepted for publication in the scientific journal Plant Disease.

“Her research demonstrated relationships among soil properties and spinach Fusarium wilt development, increased the capacity for and profitability of U.S. spinach seed production, and will guide future research on soil-based management of this disease,” said du Toit.

Soil testing identifies risks

For the bioassay, spinach seed producers and growers bring in a 5-gallon soil sample from each field to be analyzed by du Toit’s research team, which includes Mike Derie, Barbara Holmes and Sarah Meagher.

Seed is planted of three parent spinach lines that range from highly susceptible to partially resistant to Fusarium wilt. The seed germinates within a week; within three weeks, symptoms of the fungus begin to show on the plants.

“If all the plants come out healthy, the field is considered low risk,” Derie said. “Nobody’s going to plant spinach in a high-risk field.

“No one else is doing this kind of service for growers,” he said. “It’s really a great deal for them, considering what it would cost to have invested in seed-bed preparation – ground work, herbicide treatment, fertilizer application, seed purchase and planting, and related labor and equipment – and then lose the whole crop to Fusarium wilt.

“The disease often won’t show up until the crop (flowers),” he said. “And by then it’s too late and the growers and seed companies have lost their investment.

“What we’re doing through this soil testing service is giving them informed options for making decisions,” he said, “like what seed lines to use and what fields to plant.”