Fusarium wilt, a soilborne pathogen, is like that relative you just can’t stand — the one you aren’t happy to see when he shows up because you know once he arrives, he’s going to be around for a long time.

That unwelcome arrival was first reported in Japan in the 1950s before it started to spread, arriving in California (primarily Fresno, Monterey, Santa Cruz, and southeastern Imperial County) in the early 1990s, then it hit the western Arizona lettuce fields, where it was first spotted in 2001 before it really began to branch out. “It’s marching around the world,” says Barry Pryor, a researcher at the University of Arizona (U of A).

Fusarium wilt is dreaded by those who farm open fields as well as those who grow under glass. It will attack tomatoes and can render iceberg and romaine lettuce unmarketable.

“It’s boxing growers into a corner, unable to meet market demand at certain times of the year,” says Paul Brierley of the Yuma Center of Excellence for Desert Agriculture (YCEDA), where fusarium wilt on lettuce management techniques is their top priority.

“Other soilborne diseases are treatable. Fusarium wilt of lettuce is not,” Brierley warns. “Once you’ve got it, it’s there to stay because we don’t yet know a good way of getting rid of it.”

The aggressive pathogen causes disease in lettuce but may be sustained on the roots of many plants, according to researchers from the University of California. It exists in three categories — Race 1, Race 2, and Race 3. Race 1 is found in lettuce and favors high soil temperatures. Once introduced into a field, the fungus will probably remain indefinitely.

While lettuce and tomatoes are susceptible to Race 1, Race 2 and Race 3 will attack a variety of crops like peppers, soy beans, eggplant, and potatoes, which are especially vulnerable to Race 3.

Field Trials Ongoing

The U of A and YCEDA began a public-private partnership more than three years ago, collaborating with lettuce growers in Yuma — where 90 percent of America’s winter leafy green vegetables come from — to figure out what to do with the disease that causes so much loss. Researchers say about $9 million in lettuce is lost annually in Yuma fields alone.

Brierley’s researchers have been examining several different angles involving the pathogen by working with it in trials in heavily infected fields. “You need trials to be ongoing with current seed varieties and soil combinations as well as current biological and chemical products, which, to date, have been discouraging to be honest,” says Brierley. “We haven’t found anything to be effective yet.”

Steaming infected garden beds and breeding resistance into seeds are two methods being discussed, as is genomic analysis. “We’re working with a start-up company on genomic sequencing and DNA analysis of the soil, so they can tell in a single analysis what different types of soilborne diseases might be present, as well as different pressures expected for the disease,” says Brierley. “That would offer advanced indication of what kind of pressure you’d have for Fusarium wilt as you decide where to plant your lettuce.”

Dependent on Heat

Fusarium wilt is dependent on heat, a big problem in the Yuma area. “The problem is mainly in our fall crop planted in September when it’s still 100°F-plus in the field,” Brierley says. “When we plant winter lettuce in the November timeframe with cooler temperatures, we have virtually no problem at all.”

But avoiding the hot months when planting isn’t an option. “We plant every day because lettuce is a perishable commodity that requires daily delivery. Waiting until temperatures drop a bit would be nice, but the product is driven by market demand and existing contracts,” added Brierley.

Seed companies are also involved in the trial research, seeing if they can breed a variety that will fight Fusarium. In one field trial, Fusarium wilt had a 98 percent kill rate, but there were two seed varieties from one company that mostly survived it. If those seeds can withstand further testing, they may become a commercially viable option.

Type of soil is another area to study as some soils holding plants with Fusarium didn’t lead to actual plant damage. Researchers are experimenting with fumigation — killing everything in the soil to eliminate disease — and biome build-up, creating a stronger, healthier environment in the soil to naturally fight off disease.

Mike Matheron and Barry Pryor split Fusarium research duties — Matheron in the open field, Pryor doing greenhouse research.

“I focus on variety of product rather than the soil it’s grown in. I’m looking for head lettuce cultivars that are resistant to the disease,” says Matheron. “There aren’t many that are… and that’s the problem.”

Currently studying the disease and its relationship to different types of soils, Pryor is using polymerase chain reaction-based technology and loop-mediated isothermal amplification methods to determine how long it takes the lettuce to become infected after pathogen exposure. “The great thing about these methods is we can now have detection in the field in as little as 20 minutes,” he says.

While wilt research continues with a hint of hope, Yuma lettuce farmers contend with today’s realities. Matt McGuire of JV Farms, which has more than 5,000 acres of iceberg and 3,000 acres of romaine, says farmers have become more production- and schedule-oriented in volume and timing of harvest. “We need the right volume on a consistent basis because our product isn’t storable like grains or dry goods,” he says.

McGuire says cultural practices, seed treatments, chemical treatments, and field rotation have yet to show any positive effects. Only the seemingly resistant seeds and DNA soil testing that identifies infected fields are maintaining hope.

“We should see more new items like this in the coming years, but those, presently, are the only things that give me optimism,” says McGuire.

(For additional info on growing lettuce, read Grow Lettuce Indoors All Winter.)