Posted: Tuesday, December 11, 2012 1:58 PM
By JOHN O'CONNELL
RUPERT, Idaho -- New lab results suggest zebra chip infection in Idaho's 2012 potato crop resulted from a type of the disease-causing Liberibacter bacteria that's believed to be less aggressive than the other known type.
Type I -- the only type found by North Dakota State University in testing 50 tubers dug from throughout Idaho -- also appears to be predominantly responsible for infections in Oregon and Washington, said University of Idaho storage specialist Nora Olsen.
"If it truly is less aggressive, that is better for our industry," Olsen said.
She offered the caveat that much of Idaho's knowledge on zebra chip comes from studies in Texas, where type II is found, and additional research will be needed to determine how the types may behave differently. Furthermore, she said Oregon State University is researching if volunteers are more apt to sprout from tubers infected with type I, potentially creating a new source of the bacteria.
Zebra chip, which first arrived in the Pacific Northwest last season and has added to growers' insecticide costs, is spread by tiny insects called potato psyllids. It raises sugar levels in tubers, creating bands that darken when fried.
Of the three genetic biotypes of psyllids, most tested in Idaho have been consistent with the California biotype.
Olsen said the first psyllids of the season were found on June 18, early enough that she believes overwintering may pose a threat, and a point to study into the future.
U of I also participated in zebra chip chemical trials with Rupert-based Miller Research. The results showed no notable difference between any of the programs, including the untreated check. Jeff Miller, with Miller Research, advised growers not to give up on spraying regimes as the results may have been skewed by low disease pressure.
He said growers who have conducted their own strip trials will soon provide him with additional data. He suggests growers use two consecutive treatments of a given insecticide with applications on 10-day intervals, and then avoid using that product again.
U of I Extension entomologist Erik Wenninger said foliar zebra chip symptoms have been much heavier at the edges of fields. He was surprised when testing of spuds taken from six growers' fields this season found roughly equal infection rates in tubers dug from field perimeters and interiors.
Wenninger said Idaho's Magic Valley, where two adjacent Russet Norkotah fields were rejected for having more than 15 percent zebra chip, had the most disease pressure for the second consecutive year. He said sticky card data suggests proximity to the Snake River, where psyllids may be living on bitter nightshade along the banks, could increase a grower's risk.
Processors have agreed to share data on zebra chip distribution with Wenninger to develop a map of hot spots. Next season, Wenninger said researchers will rely less on vacuuming leaves for psyllid samples, but will have to increase significantly monitoring with sticky cards.