Spinach is suspect: A pathological mystery

The case started a few years ago when a farmer approached seed pathologist Lindsey du Toit at WSU Mount Vernon wondering what was damaging his spinach seed crop out in the field. He had planted on clean ground that hadn’t had spinach before. He wondered if maybe the stock seed had a problem.

“It didn’t make sense,” says du Toit, explaining that what happened to the plants didn’t fit with the known diseases. At the time, du Toit and one of her graduate students were looking at fungal pathogens in the seeds of spinach plants. About 75 percent of the spinach seed grown in the United States comes from Skagit, Snohomish, and Whatcom counties, with an annual market value of about $24 million.

They tested for a common disease called Fusarium wilt, but didn’t find the pathogen. However, while testing a sample of the stock seed, du Toit observed that a lot of it was infested with the fungus Verticillium, a soilborne fungus which is common in other crops but in only a few reports has been linked to spinach. Du Toit then inoculated healthy spinach plants in a greenhouse with isolates of the Verticillium that she got off the seed to see if the fungus might be pathogenic on spinach. No signs of the disease. “I was going to throw the plants out,” says du Toit. “But then I stopped and thought about how Verticillium wilt develops in potato crops.” In potato Verticillium wilt causes the plant to shut down late in the season, which is why the disease is also called “early dying,” says du Toit. Maybe in the spinach the disease also occurs late.

Du Toit lengthened the time of light in the greenhouse to 18 hours a day, effectively telling the plants it was high summer and time to go into their reproductive phase of flowering and setting seed. “In three days we started to see symptoms of wilting,” she says. She wanted to be sure she wasn’t jumping to conclusions, so she tested the plants and then their seeds. She found the corky material surrounding the embryo did contain the wilt fungus, but noticed it didn’t affect the plant until it was mature.

“Which explains why they never see the disease (in spinach) in California, Arizona, Arkansas, or Texas,” she says. In those states spinach is harvested before the reproductive phase, either as baby spinach when the leaves are just a few inches out of the ground or the more mature bunching and leaf phases. The discovery prompted du Toit and her student to look at seed lots produced by a number of different spinach seed companies around the world. That produced a shock. “We discovered that 90 percent of the commercial seed lots had Verticillium on them.” Once the discovery was made known, a buzz ran down the coast.

Farmers in the Salinas Valley in California had been finding Verticillium wilt in their lettuce crops and were struggling to find the source. “You knew it was there in other crops,” says du Toit—pointing to peppers and strawberries as two key examples. “But until recently you never saw it in lettuce.” Hearing that du Toit had found it on the spinach seed, baby leaf spinach being a major crop for the California valley, Salinas Valley farmers were very interested in her results. “The connection was made,” she says. “Was the spinach infecting the soil?”

The California scientists believed they had found their culprit and even published several papers on it. “It makes sense,” says DuToit. “Except that a lot of other crops in Salinas Valley are also susceptible to Verticilium wilt.” Spinach may be involved, but then again it may not be the one to blame, she says. Or if it is, it may not carry the blame alone.

Du Toit shared her evidence, an entire collection of 700 spinach seed isolates of Verticillium, with her California counterparts. “Because of this concern, I wanted to help the research.” Spinach is now their key suspect.

But du Toit is still not so sure. She’s hedging her bets. If the source of the wilt is spinach seed, then she’s hunting for ways to clean the fungus from the seed before it goes to the Salinas Valley. “There’s a wide range of conventional and organic ways to approach this,” she says. She has found one conventional fungicide treatment that is on its way to FDA approval by 2014, and one organic steam treatment that could be adapted to spinach seed.

But that’s just half the effort. Meanwhile, du Toit and several Salinas Valley farmers have set out to see if the seed really is transmitting the disease to the soil and from the soil to the lettuce. They’ve completed two years of field trials: with planting infested spinach seed that was not treated or treated with fungicide against the wilt followed with a lettuce crop, and one treatment of just bare ground (no spinach) before the lettuce crop. In all cases there was no symptom of Verticillium wilt in the lettuce plots, she notes. “In two years there doesn’t seem to have been any difference.” So maybe the scientists should keep looking.

The issue has produced a few dividends for the scientists, among them several new avenues of investigation. Du Toit has also discovered that there isn’t an international standard method for testing spinach seed for Verticillium. While some labs found the fungus on the seed, others missed it entirely. So she organized workshops on how to look for the fungus on spinach seed.

It may be a while before the mystery of how the wilt got into lettuce is solved, but if through seed treatment and testing, seed producers can remove spinach seed as a potential source of the inoculum, “then we can, at least, take the blame away from spinach seed.”