A New Method for Single-Plant Selection of Wheat Genotypes for Tolerance and Resistance to the Root-Lesion Nematode Pratylenchus thornei by Low-Density Sowing

A new method of selecting wheat genotypes for tolerance and resistance to Pratylenchus thornei could enhance early-generation selection in wheat-breeding programs. Currently, the tolerance of fixed genotypes is determined in field experiments at a P. thornei-infested site, and resistance indices are determined by inoculated glasshouse experiments. For early-generation selection from segregating populations, resistance screening is limited to assessing single plants for resistance only using glasshouse experiments. The objective of this study was to develop a novel method that evaluates a single plant for both tolerance and resistance by using low density (LD) sowing in the field. Four replicated LD (1, 4, 16 and 32 plants/m2) field experiments evaluated 14 or 15 fixed wheat genotypes over two growing seasons in a field with damaging population densities of P. thornei (>2500 P. thornei/kg soil). To check the validity of these experiments, a linear regression analysis was performed for each experiment between the single plant grain yield and the population density of P. thornei with the published tolerance and resistance indices derived from multiple field and glasshouse experiments, respectively. Tolerance was best determined by the grain yield of each single plant grown at a density of 16 plants/m2 in 2021 (R2 = 0.63, p < 0.001) and 4 plants/m2 in 2022 (R2 = 0.79, p < 0.001), when compared to published results of tolerance indices assessed by grain yield from plots grown at 100 plants/m2. Resistance was best determined from the final population density of P. thornei in the soil and roots under each single plant when grown at a density of 4 plants/m2 in 2021 (R2 = 0.73, p < 0.001) and 1 plant/m2 in 2022 (R2 = 0.54, p = 0.001), when compared to published resistance indices derived from multiple glasshouse resistance experiments. This study demonstrated that LD can be used to effectively identify individual plants with both tolerance and resistance to P. thornei, with single-plant ultra-low densities (ULD) between 1 and 4 plants/m2 being the most suitable. The advantage of using ULD sowing in the field for segregating populations of wheat over single plant glasshouse resistance screening experiments is the ability to simultaneously screen plants for tolerance to P. thornei.