Stop the rot. It seems like a simple objective for those growing produce after they harvest it.

It’s not always an easy problem to solve though. For example, onion bulb crops are grown on approximately 140,000 acres annually in the U.S., with a farm-gate value of $925 million. But bacterial diseases of onion cause more than $60 million in losses each year. Those losses can be particularly severe for stored bulbs as bacterial rots typically develop in storage – after all production costs have been incurred.

That’s where the Stop the Rot project comes into play. The project involves 24 scientists across the country researching the complete system (host, pathogen, environment) of bacterial diseases of onion. Their aim is “to develop practical, economically sound strategies for pathogen detection and management that will improve profitability and sustainability of onion production,” according to AlliumNet.

Stop the Rot has two primary research objectives:

  • A national survey across all onion growing regions to compare the genomics of onion bacterial pathogens collected in different regions to identify virulence factors, develop practical molecular diagnostic tools for identifying specific bacterial pathogens and develop phenotypic resistance screening methods
  • Through research trials, identify onion production practices, environmental factors and inoculum sources that impact bacterial diseases, then use that knowledge to develop effective, practical solutions for managing bacterial diseases

Associate Professor and Extension Vegetable Disease Specialist Bhabesh Dutta of the University of Georgia provided updates from Stop the Rot at the most recent Great Lakes Expo.

He noted one of the biggest pest issues for onions continues to be thrips. A thrips infestation can reduce the efficacy of protective chemical treatments – the tiny insects can scratch right through them and feed on bulbs, which in turn invites diseases in.

Dutta noted that the fungicide LifeGard®, used in rotation with copper products, can help reduce thrips populations.

“A thrips management program is critical to reducing bulb rot,” he added.

Onion bulbs cut open to reveal damage caused by Pantoea bacterial blight. Photo by Howard F. Schwartz, Colorado State University,

Solid cultural practices on the farm are also critical for preventing internal bulb rot. “Wait until the proper time to harvest,” Dutta stated. (That’s when the neck is falling over on its own; otherwise, any early tear in the neck invites disease in.)

Dutta noted a study which took place in New York which found that rolling the tops that died “standing up” helped to reduce bulb rot. He also suggested letting a field of onions cure for at least 48 hours after the necks have been cut before harvesting.

The growers of many other crops have benefitted from advances in automation, but the majority of onions (about 95%) are still manually harvested – and because of that, we often see a lot of variation in neck length. The optimized neck length of two inches or more helps reduce internal rot significantly, Dutta said – and we see that length more often with mechanical harvesting.

But until more mechanical harvesting options are available, Dutta said combatting onion bacterial diseases with pathogenic tools and enhanced management strategies is an ongoing process.

Growers need to be able to properly identify pathogenic and virulent factors in their crops. They also need improved diagnostics to identify the onion-pathogenic Pantoea species that lead to bulb rot. There are 20 different species in the genus and their bacteria are usually yellow pigmented, ferment lactose, are motile and form mucus-like colonies – exactly what consumers do not want to find inside their onions.

To that end, Stop the Rot aims to help with improved management of Pantoea species, ideally with a bactericide program that overlaps with a thrips control program. This would be a growth-stage targeted bactericide program, since post-harvest treatment of bulb rot does not exist.

The project is also working on finding improved harvesting practices to help reduce internal rot.

More information on Stop the Rot (including a wealth of resources) is at

by Courtney Llewellyn