Resistance Gene Pyramiding and Rotation to Combat Widespread Soybean Cyst Nematode Virulence

Abstract

Soybean cyst nematode (SCN) is an important pathogen of soybean causing more than $1 billion in yield losses annually in the United States. Planting SCN resistant soybean cultivars is the primary management strategy. Resistance genes derived from the plant introductions (PI) 88788 (rhg1-b) and PI 548402 (Peking; rhg1-a and Rhg4) are the main types of resistance available in commercial cultivars. The PI 88788 rhg1-b resistance allele is found in the majority of SCN resistant cultivars in the north central US. The widespread use of PI 88788 rhg1-b has led to limited options for farmers to rotate resistance sources to manage SCN. Consequently, an over reliance on a single type of resistance has resulted in the selection of SCN populations that have adapted to reproduce on these resistant cultivars. Here we evaluated the effectiveness of rotating soybean lines with different combinations of resistance genes to determine the best strategy for combating the widespread increase in virulent SCN and limit future nematode adaptation to resistant cultivars. Eight SCN populations were developed by either continuous selection of a virulent SCN field population (HG type 1.2.5.7) on a single resistance source or in rotation with soybean pyramiding different resistance gene alleles derived from PI 88788 (rhg1-b), PI 437654 (rhg1-a and Rhg4), PI 468916 (cqSCN-006 and cqSCN-007) and PI 567516C (Chr10). SCN population densities were determined for eight generations. HG type tests were conducted after the eighth generation to evaluate population shifts. The continued use of rhg1-b or 006/007 had limited effectiveness for reducing SCN type 1.2.5.7 population density, whereas rotation to the use of rhg1-a/Rhg4 resistance significantly reduced SCN population density, but selected for broader SCN virulence (HG type 1.2.3.5.6.7). A rotation of rhg1-a/Rhg4 with a pyramid of rhg1-b/006/007/Chr10 was the most effective combination at both reducing population density and minimizing selection pressure. Our results provide guidance for implementation of a strategic SCN resistance rotation plan to manage the widespread virulence on PI 88788 and sustain the future durability of SCN resistance genes.

Epistatic interaction between Rhg1-a and Rhg2 in PI 90763 confers resistance to virulent soybean cyst nematode populations

“…Thus, the results highlight that a pyramid of the rhg1-a allele with rhg2 offers an advantage against the virulent SCN populations tested. Since rhg1-b is the most widely used resistance allele in more than 95% of resistant soybean cultivars planted in the USA, current efforts have focused on pyramiding of rhg1-b with other resistance loci to enhance resistance to virulent SCN (Brzostowski and Diers 2017 ; Yu and Diers 2017 ; Meinhardt et al 2021 ). Here, we demonstrated that a combination of rhg1-a and rhg2 genes provides resistance against virulent nematode populations and resistance allele pyramid focused on these genes would be an effective strategy for the management of virulent SCN populations.…”

“…PI 88788 has been excessively utilized as a resistance source in modern SCN-resistant soybean cultivars due to resistance being derived from a single rhg1-b allele which allows for convenient breeding by introgression of a single resistance locus (Concibido et al 2004 ; Niblack, 2005 ; Mitchum 2016 ; McCarville et al 2017 ). Consequently, the monoculture of PI 88788-resistant soybean cultivars has facilitated the selection of virulent nematode populations that are capable of overcoming this resistance (Niblack et al 2008 ; McCarville et al 2017 ; Howland et al 2018 ; Meinhardt et al 2021 ). A crucial step to a successful SCN management strategy is the use of resistance sources capable of limiting the selection of virulent nematode populations which has often been undermined while breeding for high-yielding SCN-resistant cultivars (Niblack 2005 ; Chen 2020 ; Meinhardt et al 2021 ).…”

“…Consequently, the monoculture of PI 88788-resistant soybean cultivars has facilitated the selection of virulent nematode populations that are capable of overcoming this resistance (Niblack et al 2008 ; McCarville et al 2017 ; Howland et al 2018 ; Meinhardt et al 2021 ). A crucial step to a successful SCN management strategy is the use of resistance sources capable of limiting the selection of virulent nematode populations which has often been undermined while breeding for high-yielding SCN-resistant cultivars (Niblack 2005 ; Chen 2020 ; Meinhardt et al 2021 ). Due to the limited understanding of SCN virulence genes referred to as ror genes (for the r eproduction o n a r esistant host), a strategic rotation of SCN resistance sources that can counter-select nematode populations is a potential solution for combating SCN (Gardner et al 2017 ; Meinhardt et al 2021 ).…”

“…A crucial step to a successful SCN management strategy is the use of resistance sources capable of limiting the selection of virulent nematode populations which has often been undermined while breeding for high-yielding SCN-resistant cultivars (Niblack 2005 ; Chen 2020 ; Meinhardt et al 2021 ). Due to the limited understanding of SCN virulence genes referred to as ror genes (for the r eproduction o n a r esistant host), a strategic rotation of SCN resistance sources that can counter-select nematode populations is a potential solution for combating SCN (Gardner et al 2017 ; Meinhardt et al 2021 ). Counter-selection studies using different virulent nematode populations on PI 88788, PI 90763, and Peking have contributed to insights on the potential use of these sources in rotation to limit virulent SCN population build-up (Luedders and Dropkin 1983 ; Anand and Shumway 1985 ; Gardner et al 2017 ; Meinhardt et al 2021 ).…”

“…Due to the limited understanding of SCN virulence genes referred to as ror genes (for the r eproduction o n a r esistant host), a strategic rotation of SCN resistance sources that can counter-select nematode populations is a potential solution for combating SCN (Gardner et al 2017 ; Meinhardt et al 2021 ). Counter-selection studies using different virulent nematode populations on PI 88788, PI 90763, and Peking have contributed to insights on the potential use of these sources in rotation to limit virulent SCN population build-up (Luedders and Dropkin 1983 ; Anand and Shumway 1985 ; Gardner et al 2017 ; Meinhardt et al 2021 ). Some of these studies have highlighted a strong counter-selection between virulence on PI 88788 and PI 90763 (Anand and Shumway 1985 ; Gardner et al 2017 ).…”