Onion growers around the country continue to shore up their defenses against iris yellow spot virus while researchers seek resistant varieties and other aids.
The yield-reducing disease, spread by onion thrips, is an ongoing concern, particularly in warmer onion-growing regions where thrips populations thrive.
“By the end of the season all the onions are 100 percent infected,” says Ron Mio, owner of Mio Farms Inc. in Fruitland, Idaho. “It’s real discouraging for growers. They spray as much as possible,” as many as eight applications during the growing season.
The area also produces seed and overwintered onions. A mix of allium crops adds to the pressure, offering more hosts.
Once iris yellow spot virus appears in an area, the severity of infections varies from year to year and field to field, says Howard Schwartz, a professor of plant pathology at Colorado State University in Fort Collins.
“But it doesn’t seem to go away.”
Schwartz, who heads up a multistate research team addressing the problem, says iris yellow spot virus seems to be spreading into new territory.
“In that sense it’s getting worse,” he says, though he raises the possibility that greater awareness and better reporting are inflating that impression.
Breeding to reduce thrips attractiveness
Breeding research is focused on identifying the most promising germplasm from which to begin developing crosses.
Michael Havey, a research geneticist with the U.S. Agriculture Department’s Agricultural Research Service in Madison, Wis., says the answer may lie in making onion plants less attractive to thrips.
“We’ve found no bona fide source of resistance” so far for developing truly resistant onion varieties, he says. But the researchers have seen that some foliage types tend to avoid severe infections, presumably because the thrips vectoring the disease avoid feeding there.
The semi-glossy foliage of onion varieties most common west of the Mississippi seems to suffer less thrips damage than normal waxy foliage, Havey says.
He theorizes that the more waxy leaves help the pest cling to the plant for feeding.
Onion plants need some wax to help ward off spray damage and water loss. Identifying the genetic markers for the specific waxes that attract thrips should speed up crossbreeding efforts.
If the theory bears out, the resulting plants still would be susceptible to iris yellow spot, but they’d have this extra protection from thrips, he says. That might lead to fewer insecticide sprays or a delay in the buildup of virus-spreading thrips populations.