Researchers say weed aids zebra chip
Bittersweet nightshade common in Pacific Northwest
By MITCH LIES
HERMISTON, Ore. -- The insect that vectors zebra chip can survive the Northwest's harsh winters on a weed plentiful in the Columbia Basin and Idaho, according to a Northwest scientist.
Andy Jensen, who works for the Oregon, Washington and Idaho potato commissions, said at the Hermiston Farm Fair he found abundant populations of potato psyllids in winter months on bittersweet nightshade, a weed common in the Northwest.
And, he said, cold winters don't appear to eliminate populations of the pest.
"It's not like the cold, frosty weather is going to kill the insects," he said. "That is definitely not the case."
Jensen said he found potato psyllids on defoliated bittersweet nightshade throughout the winter. And, he said, the weed is prevalent throughout the Northwest.
"Once you start looking for (bittersweet nightshade), you'll see it everywhere you go," he said. "It is very abundant."
The finding that potato psyllid overwinters in the Northwest is contrary to a long-standing belief that the pest's sole means of arriving here each year is through a northern migration from warmer southern climates.
In the Hermiston area, potato psyllid was first detected last year on July 5. About two weeks later, growers began treating for the pest.
Zebra chip has been a significant pest for Southwestern U.S. potato growers since it was first discovered in Texas in 2000.
The disease causes purpling and curling of foliage and black striping on tubers, which makes potatoes unmarketable.
USDA scientist Jim Crosslin, who spoke following Jensen's presentation, said he has evidence potato psyllids were present in the Northwest since the late 1990s. But the pest was not considered a significant problem until 2011, when the disease it transmits, zebra chip, appeared for the first time in the Columbia Basin.
The disease caused significant crop loss in isolated fields in 2011 and led to a temporary ban on fresh potato shipments from the Northwest to South Korea in 2012.
At the farm fair, Northwest scientists unveiled several findings from two years of research. Among the findings is the fact that the liberibacter virus that causes zebra chip appears to survive better in mild temperatures.
"The bacterium is heat sensitive," said Silvia Rondon, an Oregon State University Extension entomologist. "In temperatures above 90 (degrees Fahrenheit), the bacteria shut down."
Rondon said only about 1 percent of the psyllids she trapped tested positive for the liberibacter virus this year, when temperature in the area rarely exceeded 90 degrees.
Researchers also found that monitoring the edges of fields may be sufficient in cases where insect pressure is low.
Alexzandra Murphy, a scientist working at the Hermiston Research and Extension Center, said there was no significant difference between monitoring psyllid populations in the middle of fields and the edges.
Identifiable marks for potato psyllids include a white stripe around the abdomen and clear wings.
Rondon ended her presentation by encouraging growers to monitor for the pest in 2013.
"Keep monitoring," Rondon said. "I know it takes a lot of time and it is a lot of work, but we have to do it."