Published online: Mar 22, 2012 Potato Harvesting, Fungicide, Seed Potatoes
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Potato virus Y rates in Idaho seed rebounded in 2011 following a sharp decline during the previous year, according to test results from the state's annual winter grow-out.

Of 722 seed lots planted in California for the 2011 crop test, 50.42 percent contained some level of the virus, up from 39.6 percent in 2010. The number of lots with more than 2 percent of the virus, making them ineligible to re-certify as seed, was 18.84 percent, up from 8.27 percent in 2010.

Idaho's seed certification program has required enzyme-linked immunosorbent assay testing of leaves from a winter grow-out since 2007.

The results, released March 13, are similar to 2009, when 50.72 percent of lots had some level of the virus and 15.52 percent were ineligible to plant for more seed.

"I really don't have a handle on why we had more. I'm confident the course we're on is one we need to stay on. We may just need to wait another year to see if we're going to stabilize," said University of Idaho Extension seed potato pathologist Phil Nolte. "It may be the normal ebb and flow of the amount of virus we're going to see in the future, even with the methods we're using."

Nolte emphasized the test helps growers keep the virus out of Idaho's seed system, and he doubts the state will see increased planting of seed with it this year. The virus seems to be up nationwide, he added. In Minnesota, half of the seed supply isn't eligible for commercial planting due to the virus.

"State certifications that do not rely on ELISA testing on their winter test are probably going to have some problems this year," Nolte said.

Nolte has heard from several growers who speculate mild weather in 2011 increased crop pressure by aphids, which vector the virus.

"I was a little bit apprehensive on what our readings would be like because I didn't get a frost and our potatoes stayed greener in the fall than they normally do," said Driggs seed grower Paris Penfold, adding his crop came out clean and ELISA testing fosters confidence in Idaho seed.

According to UI research, PVY poses a mean reduction per acre of 1.6 cwt for every 1 percent of virus by stunting plant growth.

Coolers with leaves from the grow-out plots are shipped to a laboratory in Idaho Falls for ELISA testing, which entails exposing juice from leaves to antibodies. If disease is present, an enzyme reaction turns the liquid yellow, explained Greg Lowry executive vice president of Idaho Crop Improvement Association, which oversees Idaho's certification program.

Formerly Idaho relied on a visual examination for the virus, testing only varieties that didn't tend to express symptoms.

"It's given growers more information and very accurate information as to what's really in their seed lot so they can make decisions as to whether they increase those lots or flush them out of the system," Lowry said.

Results of additional grow-out testing by UI gauging the prevalence of necrotic strains that also impair tuber quality should be available in a month.

SOURCE: John O'Connell, Capital Press