Erik Wenninger
Researchers say the number of potato psyllids found harboring the Liberibacter bacteria that causes the crop disease zebra chip dropped significantly in the Pacific Northwest during 2014.
A new study out of Washington, however, has heightened concerns about resident psyllid populations overwintering in the Pacific Northwest.

Zebra chip, which causes bands in potato flesh that darken when fried, first arrived in the Pacific Northwest in 2011. By 2012, infections remained minimal as a percentage of the overall Idaho crop but ran as high as 15 percent in certain spud fields, said University of Idaho Extension entomologist Erik Wenninger.

In 2013, 33 of 1,093 psyllids captured through a UI monitoring program tested positive for Liberibacter.

Just 170 psyllids were captured in 2014, with four testing positive for Liberibacter. No infected spud plants were found.

Almost all of Idaho’s 2014 psyllids were of the Northwest haplotype, compared with 2013 when many Western psyllids were also found, suggesting to Wenninger that there may have been more “homegrown” psyllids this year, rather than insects blowing in from other regions.

Oregon State University tested 15,000 psyllids in 2014 — half of the 2013 total — finding 0.5 percent were positive for Liberibacter and “few plants in the field were symptomatic.”

Washington’s monitoring program confirmed only two Liberibacter-positive psyllids of 1,000 tested, said Joe Munyaneza, a research entomologist with USDA’s Agricultural Research Service in Yakima, Wash. Munyaneza led an experiment showing psyllids that feed on bittersweet nightshade — a common plant in the Northwest known to support the bugs between potato crops — seem to have greater cold hardiness than insects that feed on potatoes alone.

“We believe the bittersweet nightshade is conferring some cold-hardy property to the psyllids,” Munyaneza said.

Munyaneza’s team has also found a second psyllid host plant, called goji berry, or Lycium. His team continues to find psyllids surviving at five Washington goji berry sites.