ONTARIO, Ore. — Onion growers in Eastern Oregon and southwestern Idaho are dealing with a new plant disease that can damage the inside of onions but they so far don’t know what’s causing it or how to prevent it.
Oregon State University researchers are conducting field trials to try to answer those questions.
“We know what the problem is real well but we don’t know what’s causing it or how to manage it,” said Clint Shock, director of OSU’s Malheur County experiment station.
The disease is caused by a plant pathogen known as fusarium proliferatum and can damage the inside of the onion. An affected onion looks fine on the outside but is not desirable to consumers when it’s cut open.
That particular type of fusarium fungi has caused a few cases of so-called onion bulb rot over the years but it became a major issue in 2014 and 2015, said OSU Cropping Systems Extension Agent Stuart Reitz.
“Over the past couple of years we’ve seen it become a real serious problem,” he said. “We’re trying to figure out, is there some trigger that makes onions susceptible to getting the disease.”
One theory that is being explored in the Malheur County field trial is that high temperatures cause a condition known as dry scale, which is when the top of the onion doesn’t completely close, leaving a small opening where the fungal pathogen can enter.
The 2014 and 2015 summers in this region had unusually high temperatures and the OSU field trial includes heat strips that make the soil around the onion bulb hotter.
“By sampling onions every week, we’re trying to understand when the defects start to show up ... and see if temperature is a factor,” Shock said. “If temperature is a factor, there are various approaches we can take to try to reduce the bulb temperature.”
A separate trial is exploring the effectiveness of different fungicides that have proven beneficial in treating related fusarium pathogens that impact other crops.
Reitz is also collecting samples from various farms around the region “and looking at different varieties and growing conditions, trying to track when we see the problem coming on so growers can use fungicides at the right time instead of having to spray all year long.”
Bob Simerly, an agronomist with McCain Foods, a large processor of onions grown in this region, is skeptical that heat is causing the problem. High temperatures and dry scale have been around for a long time but the disease hasn’t, he said.
He thinks it could be linked with a significant increase in corn acreage in the region. Corn is a host for many different fusarium fungi, including the one that causes onion rot, he said.
“My theory is that the increase in corn acres ... has increased the amount of spores of this (pathogen) in the environment,” he said.
Paul Skeen, president of the Malheur County Onion Growers Association, said there are many different opinions on what’s causing the disease but as of now, “I don’t think anybody really knows.”