Resistance Problems Not Limited to Weeds and Insects
By John Wilson – Extension Educator and Loren Gielser – Extension Plant Pathologist
It is easy to see when weeds or insects develop resistance to a herbicide or insecticide. The weeds or insects are easy to spot in the field. Even some plant diseases are becoming resistant to some fungicides, as evidenced by the infected plants in the field.
A harder place to detect resistance may be when soybean cyst nematodes, SCN, start to overcome the benefits of using a soybean variety with a particular source of resistance to SCN.
Of the hundreds of SCN-resistant soybeans available to producers, the vast majority use the same source of resistance, PI88788. The explanation for this is simple: This is the easiest source of resistance to breed in and still maintain high yielding varieties. (This is done with traditional plant breeding techniques, not GMOs.) And for many years in many fields, those varieties did just what they were supposed to do. They limited SCN reproduction and allowed soybean growers to lower high SCN egg counts or to keep low counts low while maintaining yields.
Managing SCN is all about numbers. If you have SCN in your field, you will never be able to eliminate it., at least not with today’s management options. Rather, the goal is to reduce that number as much as possible. SCN populations are measured by egg density, the number of eggs in 100 cc’s of soil. For a reference point, that’s about enough soil to fill a pop can one-third full.
A certain portion of any SCN population in a field will reproduce on any source of resistance. There is no source of resistance that eliminates all nematodes from reproducing.
When the same source of resistance is used whenever soybeans are planted in a field, the same selection pressure is placed on those nematodes remaining in the soil. And just like when the same herbicide or same insecticide is used over and over, the nematodes that can survive on the PI88788 source of resistance multiply in the soil and the population numbers can increase….