Ten years ago Iowa State University nematologist Greg Tylka predicted that PI 88788, the resistance source in most US soybean varieties, would keep losing its power against soybean cyst nematode (SCN). A study released in late August confirmed it. "If you are a biologist and study selection pressure, that's no surprise," he said on an SCN panel hosted by BASF at the Farm Progress Show near Boone, Iowa, with Dylan Mangel of the University of Nebraska, co-leader of the SCN Coalition, and Pocahontas, Iowa, farmer Austin Behrendsen.

SCN, first found in the United States in the 1950s, is the top yield robber in soybeans, costing farmers an estimated $1.5 billion a year according to the SCN Coalition; the worm feeds on roots and cuts plant growth, and can take 30% or more of yield with no visible symptoms above ground. Since the late 1980s PI 88788 has been the primary resistance source. ISU field data from 2001 to 2023 show SCN populations' ability to reproduce on it rising 2% a year — the same rate as a decade ago — and project only about 40% control by the end of the decade, leaving growers open to average losses of about 9 bushels an acre, or 12% of yield potential. "If we don't diversify how we manage the pest, farmers can expect that trend to continue," Tylka said.

Behrendsen's wake-up call came when two fields half a mile apart yielded 15 to 20 bushels an acre differently: the weaker was PI 88788, the stronger carried Peking (PI 548402) resistance, the only other resistance line in commercial varieties, widely available for five years but still in only 20% of seed offerings. "Rotating varieties is so important. You don't want to plant Peking year over year, or SCN will become resistant to that faster," he said.

The Coalition's advice: test soil for SCN after harvest and run the egg counts through its Profit Checker; consider corn-on-corn where counts exceed 10,000 eggs per half-cup of soil; rotate PI 88788 and Peking so that in a corn–soybean rotation each trait appears once every three years — which Tylka estimates would stretch Peking's useful life to 20 years instead of 10; add nematode-protectant seed treatments, chemical or biological; and use nonhost crops such as corn, alfalfa and oats. "I fear the farmers that go for maximum yield in the short term and only plant Peking will hurt themselves in the long term," Tylka said.

New help is coming. BASF is on track to launch Nemasphere, a transgenic trait expressing the protein Cry14Ab-1 that kills the nematode when ingested — the first new mode of action against SCN in more than 60 years and the first biotech trait — in 2028, stacked with native resistance in Enlist E3 soybeans, with 28 varieties from maturity group 0 to 4.5 in development. "We should have all our regulatory approvals by the end of this year," said BASF trait technology specialist Justin Moritz, who expects growers to "get back to the level on SCN control that they used to have with PI 88788"; the varieties will also tolerate mesotrione alongside 2,4-D choline, glyphosate and glufosinate. Rotating native traits will still be recommended: "We're all going to have to continue to be stewards of these technologies."

Source: DTN/Progressive Farmer