Onion World July/August 2021

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Postharvest Disinfectants

To Disinfect or Not?

Do Postharvest Applications of Disinfectants Reduce Losses to Bacterial Bulb Rots in Storage? By Lindsey du Toit and Tim Waters, Washington State University

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diversity of bacteria can infect onion plants and bulbs. Some infections cause symptoms on the leaves. Some infections result in leaves dying back into the neck, and some cause onion bulbs to rot. If infection of the neck and bulb occurs late in the season, symptoms usually do not develop by harvest. However, these latent infections can begin to rot bulbs after they have been harvested, cured and placed in storage or, even more significantly, after all production, storage and shipping costs have been incurred and the bulbs have been received at the final market. In fact, bacterial diseases of onion have been estimated to cause $60 million in losses annually in the United States. “Stop the Rot” is a four-year research and extension project funded by the Specialty Crops Research Initiative of the USDA National Institute of Food and Agriculture (NIFA). This team of scientists from across the U.S. and South Africa brings diverse expertise to research bacterial diseases of onion. The goal is to develop practical, economically sound strategies for bacterial pathogen detection and management to improve the profitability and sustainability of onion production.

A vital part of developing the project and guiding the research as progress is made is the input received from a Stakeholder Advisory Panel of 15 onion growers and industry representatives from across the U.S. Their grassroots expertise and routine guidance ensures the project addresses grower priorities and provides tools that will be of value to onion growers.

Postharvest Application of Disinfectants to Onion Bulbs in Storage In an effort to reduce losses to bulb rots in storage, some growers treat onion bulbs with disinfectants immediately after harvest. The bulbs are treated in storage with products such as ozone or various formulations of hydrogen peroxide + peroxyacetic acid. When developing the Stop the Rot proposal, the Stakeholder Advisory Panel placed a high priority on evaluating how effectively such postharvest treatments of onion bulbs with disinfectants prevent bulb rots from developing in storage. Researchers on the team hypothesized that treatment of bulbs with disinfectants has little to no effect on internal bulb rots as the products do not penetrate the outer, dry wrapper scales of onion bulbs. Bacterial and fungal bulb rots do not spread between bulbs in storage. Rather, bulb rots develop during storage as a result of latent internal infections becoming active as the bulbs age physiologically and/or conditions become favorable (warmer and more humid) in storage or during shipment.

2020-21 Postharvest Disinfectant Trial

Figure 1. Tim Waters works inside a trailer that is used to treat onion bulbs in commercial storage facilities. The equipment was used to dose each disinfectant product onto a plate heated to 750 degrees Fahrenheit inside the pipe with propane, while a blower forced air through the metal pipe and into the storage container.

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Onion World • July / August 2021

A trial was planted at the Washington State University Pasco Vegetable Extension Farm in 2020 to evaluate the effects of applying disinfectants to onion bulbs in storage, immediately after harvest, for management of bulb rots caused by two bacterial pathogens found commonly in the Columbia Basin of Washington and northcentral Oregon, Pantoea agglomerans and Burkholderia gladioli pv. alliicola. About 24,000 acres of storage onion crops are grown in the Columbia Basin annually. The trial was a split plot, randomized complete block design with five replications of a factorial treatment design, with two inoculation treatments applied to main plots (plants inoculated or not inoculated with the two bacterial pathogens during the month before harvest) and six postharvest disinfectant treatments applied to bulbs harvested from the split plots: 1. Jet-Ag (hydrogen peroxide + peroxyacetic acid) 2. StorOx 5.0 (hydrogen peroxide + peroxyacetic acid) 3. Sanidate (hydrogen peroxide + peroxyacetic acid) 4. Ozone