The popularity of packaged salads has fueled increasing demand for organic spinach, but the success of the product has come with growing pains—namely, a fungal disease so widespread and destructive that it threatens the viability of the crop.
The race has intensified to find solutions to downy mildew, a problem especially difficult for organic farmers because there is currently no effective fungicide approved for organic production. Organic farmers rely largely on resistant spinach varieties to ward off the disease.
But with demand for organic spinach continuing to grow and farmers planting more acreage, the pathogen is evolving much more rapidly and overcoming resistance. In recent years, the pathogen’s rate of proliferation has begun to outpace researchers’ ability to develop new resistant varieties.
“Hands down, our biggest challenge is downy mildew,” said Nick Trebino, vice president of farming for Braga Fresh Family Farms, which grows organic and conventional spinach in the Salinas Valley, Imperial Valley and Yuma, Ariz.
At least 40 percent of the state’s spinach acreage is organic, said Mary Zischke, CEO of the California Leafy Greens Research Board. But organic farmers are having to destroy 30 percent of their crop every season due to downy mildew, she added.
“It’s not sustainable,” she said. “People who aren’t disking fields on a regular basis just don’t understand what we’re going through and what this could mean to a lot of other crops in the future.”
The disease is such an important topic, she noted, that half of the research board’s annual conference agenda was devoted to downy mildew, even though spinach represents just 15 percent of the leafy-greens sector the board serves.
The problem is “a cautionary tale” for other crops that are also ramping up organic production, she said, because the disease has been “lock-step with that increase.”
“The more we grow organically, the more disease we have,” she said.
Spores of the pathogen are airborne and can easily spread by wind, rain and equipment. The spores also can lie dormant in the field, making crop rotation ineffective at controlling the disease. New strains of the pathogen have popped up much faster in recent years. Before 1990, there were just three races of downy mildew identified; now, there are 16.
The increase in downy mildew races is thought to be linked to the use of resistant spinach varieties, said Sierra Hartney, a plant pathologist for Sakata Seed America. Because there is now more spinach being grown and farmers are planting resistant varieties, the pathogen is working harder to overcome resistance by proliferating new races at a faster rate.
“The organic spinach industry, to some extent, is a victim of its own success,” said Bill Johnson, a spinach breeder for Sakata.
The fungus can reproduce both asexually through spores and sexually by genetic recombination, he noted. With 16 races of the pathogen and spinach being grown in increasingly wider geographic areas, there is more opportunity for the pathogen to reproduce sexually and create new races.
This presents a problem for seed companies that are working to develop new, race-specific resistant varieties, as the resistance is never long term, Johnson said. Managing inventory of spinach seed becomes a risky endeavor because of how quickly new varieties lose resistance and value.
“You invest a lot of money in production of the seed and maybe six months later there’s a new race, and then you can’t sell those seeds,” he said.
This year, varieties that offer the best levels of resistance are also hard to come by, Johnson noted, as there is a shortage due to weather problems in seed-production areas, which are largely limited to Denmark, the Netherlands and northern Washington because of the long day lengths required to produce spinach seed. Companies must produce a lot of seed, because growers typically need about 4 million seeds per acre to produce baby spinach.
For farmer Trebino, the tight supply of desirable seeds means having to plant multiple resistant varieties and planting them in different locations, in hopes that some of those plantings will make a crop.
“It’s not very efficient, but in order to diversify, you’ve got to do that,” he said.
Another technique organic spinach growers use is harvesting the crop early, before disease symptoms manifest and take over the entire field. Though that means less yield, Trebino said, it’s better than losing the whole crop to mildew.
“That’s why most of the bagged organic spinach you see is baby, because they’re cutting them earlier to avoid the disease problems,” said Richard Smith, a University of California Cooperative Extension farm advisor.
Zischke said efforts continue to try to find a biofungicide that works for downy mildew, but so far none has proven effective.
Johnson noted UC is also taking a different approach in its breeding program, by trying to improve the overall level of resistance to all mildew races simultaneously, using nonrace-specific genes rather than breeding for race-specific resistance. This strategy likely would not yield varieties that are immune to any race of the disease, he said, but it could be successful at delaying disease symptoms by days or weeks, which would be “a significant advantage to growers.”
But with lower yields and overall crop losses, Trebino said the organic spinach business is now “a break-even deal.”
“We’re kind of forced to grow it just to keep our suppliers happy,” he said. “There are only so many people doing organic spinach, and if we tell them no, they’ll just go somewhere else for their greens and all the other (vegetables).”
Some farmers may be getting out of the organic spinach business because of mildew problems, Johnson said, but the fact there is still plenty of product on the market indicates “there’s some profitability left in it for some growers.”
“I think there’s a good industry there and I think the demand for the product exists,” he said. “Farmers are very resourceful and innovative people. There’s always going to be a way to meet some of the market demand.”
Ching Lee is an assistant editor of Ag Alert. She may be contacted at firstname.lastname@example.org. Permission for use is granted by the California Farm Bureau Federation.