Joining Waite-Cusic as co-principal investigators are Stuart Reitz, Ph.D., with Oregon State University; Faith Critzer, Ph.D., with the University of Georgia; Tim Waters, Ph.D., with Washington State University; and Linda Harris, Ph.D., with the University of California, Davis.
“We’re just trying to figure out what possibly occurred so we’re able to understand the risks,” Waite-Cusic said, referring to Salmonella outbreaks in 2020 and 2021 linked to bulb onions.
This year, the researchers conducted onion field trials in Pasco, Washington, and Ontario, Oregon, using a three-strain E. coli cocktail as a Salmonella surrogate.
In the Washington trials, which involved a yellow onion variety, they made the season’s last overhead irrigation application using E. coli-contaminated water.
In the Oregon trials, which included red and white onion varieties, the researchers used the E. coli-contaminated water to mix the season’s last pesticide or clay sunscreen application. They focused on the late timing because the closer to harvest a contamination event occurs, the higher the risk. Waite-Cusic said they also chose the two different sites to represent diverse climatic conditions.
In both trials, the researchers monitored pathogen levels after application as the onions matured and cured in the field for 30 days.
“Spoiler alert — we just collected the last samples, and everything died. Over both field trials, only one onion out of 440 tested at the end of field curing was still positive for E. coli." Field curing did a great job of mitigating any contamination that happened in any of those water applications.
“This doesn’t tell us the answer to the outbreaks, but it’s great news for the industry and food safety.”
This winter, Waite-Cusic said she plans to conduct laboratory dye studies to see if water applied to leaves of younger plants can move into the tissue and possibly pose a contamination entry route.
Joshi’s project seeks to better understand how Salmonella colonizes and internalizes in onion bulbs. He also plans to identify production practices that may reduce plant susceptibility.
Joining him as co-PIs are Alejandro Castillo, Ph.D., with Texas A&M University, and Daniel Leskovar, Ph.D., and Subas Malla, Ph.D., both with Texas A&M AgriLife.
Joshi pointed to studies conducted with other produce that found when plants detect enteric pathogens, such as Salmonella, they trigger immune-like responses. The researchers plan to conduct genetic sequencing to determine whether genes responsible for similar responses occur in onions. If that proves true, the information on synthesis and regulation of antimicrobial compounds from plants could be used to devise strategies to control Salmonella in onions.
“Unlocking the genetic potential of plants to combat Salmonella is a novel avenue in improving food safety,” he said.
Characterizing the genetic background and chemical and physical factors influencing Salmonella internalization also will help describe the dynamics involved.
With this information in hand, the researchers plan to conduct field trials to determine how bulb quality traits — including nitrogen content, moisture level, and macro- and micro-elements — influence Salmonella internalization.
After harvest, onion growers let the crop cure in the field for several weeks to heal harvest wounds, dry the neck and seal the tops of scales, i.e., the modified layers of leaves within the bulb.
Drying leaves or damaged bulbs provide entryways for plant pathogenic bacteria that cause sour rot. Because little is known about whether enteric pathogens like Salmonella use the same route, Joshi said they also will look into that.
In addition, the researchers plan to sample onion production sites, packing sheds and storage facilities to monitor whether the environments are conducive for Salmonella establishment.
Their goal is to develop recommendations on onion crop management, harvesting, curing, storage and distribution that will help the industry minimize the risk of Salmonella contamination.