Commercial varieties bred from line PA99N82-4 would be the first with resistance not only to CRN, but also to northern and southern root-knot nematodes, ARS geneticist Chuck Brown says.
"PA99N82-4 also resists the viral disease corky ringspot, which is transmitted by nematodes and causes unsightly blemishes in tubers," he says. "Corky ringspot is also controlled by soil fumigation."
CRN is problematic in the Pacific Northwest, where two-thirds of America's potatoes are grown, and in Florida. Though fumigating the soil before planting suppresses CRN numbers, the practice isn't cheap, with some chemicals costing $300 an acre. It can also harm non-target organisms, including beneficial soil-dwelling insects.
Genetic resistance, however, confines the fight to the potato's roots and tubers. But putting that resistance to work hasn't been easy.
Because resistance is absent from U.S. cultivated potatoes, Brown and colleagues used the wild species collection at ARS's U.S. Potato Genebank in Sturgeon Bay, Wisc. Painstaking screening of the material at Prosser showed Solanum bulbocastanum to be the most resistant.
The problem is, wild and cultivated potatoes are chromosomally incompatible. So the researchers resorted to "bridging," a technique that fused S. bulbocastanum and cultivated potato cells together, forcing the DNA of both to combine. Stimulants were then added to induce cells to become plantlets. Over several years, the researchers used backcrossing to eliminate unwanted traits - such as tiny tubers and poor taste - from resistant plants they had created.
The entire process to date has taken 20 years and the close collaboration of many scientists, including ARS postdoctoral researcher Lin-Hai Zhang, Washington State University scientist Hassan Mojtahedi; and John Helgersen, now retired from ARS.
PA99N82-4, the top pick of this intensive effort, is in its third year of field trials. Besides tests in Washington, Oregon and Idaho under the Tri-State Potato Breeding Program, it's also being evaluated in California and Texas.
Two more years of testing will follow before the line is released for development into commercial varieties.