Possible breakdown of SCN resistance
Iowa farmers produce soybeans profitably in fields infested with the soybean cyst nematode (SCN) by growing SCN-resistant soybean varieties that yield well and prevent large increases in SCN egg population densities. There are hundreds of SCN-resistant soybean varieties for Iowa. Almost all of the varieties contain SCN resistance genes from a single breeding line, called PI 88788.
click image to zoomFigure 1. SCN females on roots of an SCN-resistant soybean variety with PI 88788 SCN resistance. Because of widespread, repeated use of varieties with the same PI 88788 source of resistance, many SCN populations have developed increased reproduction on that type of resistance. It is no longer uncommon to see SCN females on roots of SCN-resistant soybean varieties in the Midwest (Figure 1).
How does increased SCN reproduction on soybean varieties with the PI 88788 source of resistance affect soybean yields and the buildup of SCN egg population densities (numbers) in the soil? Results of Iowa State University field experiments reveal the answers.
Experimental results show PI 88788 resistance is still working
Each year, SCN-resistant soybean varieties are studied intensively in nine experiments throughout Iowa as part of the Iowa State University SCN-resistant Soybean Variety Trial Program. The research is funded by the soybean checkoff through the Iowa Soybean Association. Since 2007, more than half of the fields in Iowa where variety trial experiments were conducted had SCN populations with elevated (greater than 10 percent) nematode reproduction on the PI 88788 source of resistance. The 10-percent level of SCN reproduction is a critical level above which plants are not considered resistant. Some of the SCN populations in the experimental fields had 35 to 50 percent reproduction on PI 88788.
But despite the increased SCN reproduction, soybean varieties with the PI 88788 source of SCN resistance in the experiments usually yielded the highest. Also, SCN population densities did not increase dramatically on the resistant varieties in the experiments. These results were fairly consistent throughout the past five years (results available online at http://www.isuscntrials.info/).
Overall, the results indicate that SCN-resistant soybean varieties with the PI 88788 source of resistance are not “failing”. The resistant varieties are still producing profitable soybean yields and preventing large increases in SCN population densities.
Because SCN is considerably more damaging in hot, dry years than in growing seasons with more moderate weather, soybean varieties with PI 88788 SCN resistance may suffer significant yield loss due to nematode feeding this season in fields with SCN populations that have elevated reproduction on PI 88788. The 2012 growing season is the type of environmental conditions under which increased SCN reproduction on soybean varieties with PI 88788 SCN resistance may result in significant yield reductions.