On a routine sweep of her research territory, plant pathologist Cindy Ocamb stepped out of her car to survey a field of fertile Willamette Valley farmland. Stunted and dead turnip plants spread out before her. She pulled out a hand lens and stooped to inspect the leaf spots pocking the foliage. She had seen these symptoms only once before. They pointed to black leg, a potentially fatal, difficult-to-control fungal disease of turnips and their crucifer relatives—radishes, kale, cabbage, broccoli, and cauliflower—all grown in western Oregon for the international seed market and the expanding fresh vegetable market.
Ocamb, an associate professor in Oregon State University’s Department of Botany and Plant Pathology, collected samples from the turnip plants, then hurried to survey a field of canola, another crucifer relative. Again, the symptoms were significant. A trip further afield unearthed more evidence of the disease in commercial turnip fields. Ocamb cut the survey day short and headed to her laboratory on the Corvallis campus.
A look under a microscope confirmed her suspicions and raised more concerns. Downstairs in the OSU Plant Disease Clinic, she asked her colleague Maryna Serdai to examine a sample. Yes, she told Ocamb, it is black leg.
“Well,” thought Ocamb, “I won’t be taking sabbatical any time soon.”
Instead, throughout the spring of 2014, Ocamb trudged through muddy fields of crucifer vegetables and canola. In 43 of 60 sites she discovered black leg and two other diseases she’d not found in the past, including one never before reported in the U.S.
“Trouble comes in threes,” she says. “It certainly did this time.”
The two most important diseases— black leg and the newly discovered light leaf spot—distress the foliage of crucifer plants. Eventually, the disease reaches the stem and roots and the plant collapses. Through her investigation, Ocamb theorizes that at some time prior to 2013 seeds contaminated with black leg (Phoma lingam) and possibly light leaf spot (Cylindrosporium concentricum) were planted in an unknown field. How they got into the seed stream is a mystery, she says, because after a 1970s outbreak of black leg in the Midwest, testing and treatment became customary and the disease became rare. Ocamb guessed that the infected seed had come from a company that neglected to do routine testing and treatment.
When Ocamb arrived on the scene of that disease-infected turnip field, back in March 2014, she expected to find some plant mortality, the result of a particularly cold winter. “Some of the specialty seed growers had been talking about cabbage fields that had died out,” Ocamb remembered. “But when I saw all those dead plants, I was shocked.”
Dead plants don’t often shock plant pathologists. But the discovery of the long-controlled black leg and a potentially more serious disease got her heart pounding. The consequences could be serious. “We’re one of only five places in the world to grow crucifer seed crops,” Ocamb noted. “It would have a huge impact if our production took a hit.”
Just one field planted with tainted seed can set off an outbreak. As some of those seeds germinate into diseased plants, they produce spores that splash from plant to plant during rain. Once spores pierce a leaf, the fungus takes hold and eventually kills the plant. The debris from dead plants harbors spores that can live for years and restart the cycle during the wet, cool weather of fall and spring. In those conditions, Ocamb estimates black leg and light leaf spot can move through a field within 2 months.
Through an ongoing experiment with graduate student Briana Claassen, Ocamb determined that the pathogen produced another form of spores, called ascospores, that blow on the wind. The two scientists placed uninfected plants in small pails hung alongside infected fields. After a week, they took the plants to a greenhouse and monitored them for 5 weeks. The black leg fungus showed up each time. Ocamb worries most about these windborne spores, which she thinks could be propelled up to 25 miles in all directions during storms.
Back in the lab at the Plant Disease Clinic, William Thomas, an OSU molecular and cellular biology scientist, examines the pathogens’ DNA. While black leg is easy to detect, light leaf spot can take an excruciating 2 months minimum to get accurate results. Thomas has developed a new molecular test that yields more comprehensive answers in less than 3 weeks, sometimes within days if the conditions are right.
“It’s like blood at a crime scene,” Ocamb says. “We collect it, run it, and match it to the victim.”
In 2015, cases continue to crop up, but the growing knowledge of control methods will lessen the death toll, says Ocamb with confidence. The key, she’s told growers, is to plow under or dispose of diseased residue, to deter windborne spores. Her research shows effective control also means spraying fall through spring while spores remain active.
“Management is not free and not easy,” she says, “but we can do it by getting everyone on the same page.” Ocamb thinks back to the drizzling day she came upon the scene of fields dying from disease. Now, she looks forward to closing the case.