When producer growers donated to form the Yuma Center of Excellence for Desert Agriculture (YCEDA) several years ago, their chief concern was Fusarium wilt in lettuce.
Fusarium wilt in lettuce was first reported in the Yuma area in 2001, according to Alex Putman, plant pathologist with UC Riverside. The soilborne pathogen was discovered about the same time in Bard and Winterhaven, a farming region technically in California, but separated from the Yuma area only by the Colorado River.
Discovery of the pathogen in California goes back to 1990, when it was first reported in the Huron area of Fresno County.
Putman says Fusarium is a pathogen that exists in many different plant systems but causes symptoms in a host-specific way. It's existence in fields used for lettuce may not be bothersome to other plants, though it can feed and multiply there.
Since its discovery in Yuma and Winterhaven in 2001 it was confirmed in Watsonville, Calif., in 2002, southern Monterey County, Calif. in 2005 and the Salinas area in 2015. It was also confirmed in Santa Barbara County, Calif. in 2013 and the Everglades area of Florida in 2017.
The soilborne pathogen leads to severe lettuce wilt – plants look like they've melted. Once a field has the pathogen there are no known ways to eliminate it from the soil.
YCEDA Executive Director Paul Brierley said Fusarium was high on the list of farmer concerns when growers donated large sums from the public-private partnership that is YCEDA. The organization partners with University of Arizona Cooperative Extension to study critical issues affecting growers in the region.
According to Brierley, lettuce farmers want to know more about Fusarium than simply which fields to avoid. They want answers and solutions to protect their lettuce crops. That is what Slinski and a host of researchers from across the country are trying to achieve.
In mid-September Extension and YCEDA researchers planted more than 70 different lettuce varieties in a Yuma, Ariz. as part of ongoing work to learn about the pathogen, how it affects lettuce, how it can move and replicate, and what might be done to prevent its devastating affects in lettuce. Part of this work includes testing lettuce varieties for resistance and tolerance to the pathogen.
The field where Stephanie Slinski, a plant pathologist, and the associate director for YCEDA, is studying Fusarium was donated by a grower. It continues to be used each growing season by Slinski to study the disease and new lettuce varieties. Her work is part of a collaborative effort into the disease that includes researchers from Land Grant universities in Arkansas, California, and Florida to understand the pathogen.
The form of Fusarium that infects desert lettuce was originally thought to affect the crop at the beginning and end of the desert growing seasons, when soil and air temperatures are warm, Brierley said.
In the Yuma region the pathogen seems to be spreading as growers now report symptoms in the middle of the growing season.
"Now farmers are starting to see it in December and January," she said.
The Fusarium Slinski is studying infects head lettuce most severely but can impact Romaine as well. "Romaine has resistance," she said.
Other types of lettuce, such as red leaf, tend to be less affected by Fusarium, she added.
No "quick fix"
Brierley says there is no known "silver bullet" solution for growers. Early efforts to treat the soil with agricultural chemicals proved worthless.
For growers who know they have the pathogen in certain fields, crop selection is the only known tool to avoid it. Lettuce crops are susceptible, but other produce crops are not, Slinksi said. Crops not affected by the Fusarium pathogen that infects lettuce can help it multiply, even if they show no damaging symptoms. There are no known rotational crops that can "clean up" a field once it is infected.
According to Putman, growers who know they have the pathogen in their fields can elect to plant or harvest those fields last then clean their equipment before moving onto the next project to avoid spreading it around.
Because the pathogen is soilborne, it is known to be spread by moving the soil to other locations. This includes equipment and foot traffic. Blowing dust is also thought to spread the pathogen, he said. Putman confirmed the pathogen can also be seedborne.
Slinski will study about 70 different commercially available lettuce cultivars in the ongoing trial. A field day at the Yuma site is planned for early December to share her results.
Additionally, Slinski wants to know if there is variability in the Fusarium populations, which is why she has surveyed soils across the Arizona growing region. Fusarium isolates from a wide cross-section of land are being studied at the University of Arkansas.
The idea is to look for genetic differences in the pathogen and what that could mean for growers.
Robert Masson, a Cooperative Extension advisor with the University of Arizona, will add to this study by investigating unreleased seed banks from various seed companies. Part of that effort could eventually help the various seed companies breed resistant or tolerant varieties. It was thought early on that the pathogen could be spread by infected seeds. That does not seem to be the case at this point.
Though not a direct part of Slinski's project, Masson will also look at soil amendments, such as biochar, to see if there are any soil health efforts that can build plant defense mechanisms to protect themselves from such pathogens.