Soybean resistance to southern root-knot nematode reduces nematode population density under field conditions

Abstract

The southern root-knot nematode [SRKN; Meloidogyne incognita (Kofoid & White) Chitwood] is one of the most damaging soybean (Glycine max [L.] Merr.) pathogens in the southern United States. Genetic resistance is the primary management strategy. A major quantitative trait locus (QTL) on chromosome 10 (QTL 10) is widely utilized in SRKN-resistant soybean, yet the extent of population suppression in infested fields remains unclear. This study aimed to evaluate the effectiveness of QTL 10 in reducing SRKN population density under naturally infested environments, with a secondary post hoc objective of assessing the impact of off-target dicamba exposure on host resistance. Seventy-six elite breeding lines, 38 carrying the resistant allele, were evaluated in replicated field trials with natural SRKN infestation over 2 years. Soil samples were collected at the R5 stage, and SRKN second-stage juveniles (J2s) were extracted and quantified. Resistant lines had 1620 SRKN-J2 per 100 cm3, while susceptible lines had 2126 per 100 cm3. In 2019, prolonged off-target dicamba exposure significantly increased nematode density across all lines. Dicamba-tolerant breeding lines had lower SRKN densities, while susceptible lines had higher densities, regardless of SRKN resistance. These findings suggest that prolonged off-target dicamba exposure imposes additional stress that compromises plant defense response and increases nematode reproduction. Variation in SRKN-J2 densities among resistant lines further indicates loci beyond QTL 10 may influence nematode infection and/or reproduction. Future genetic studies could identify additional resistance sources, leading to stronger and more durable SRKN resistance in soybean.