Experimental and statistical approaches to evaluate the reaction of soybean to Pratylenchus brachyurus

In Brazilian soybean production, the main annual yield losses are caused by nematodes. One of the principal plant-parasitic nematodes is the root-lesion nematode, Pratylenchus brachyurus. Resistance is among the major tools to manage nematodes, but the correct characterization of genotypes is a challenge. Several factors contribute to accurate phenotyping for nematode resistance, including the density of inoculum, the age of the plant at inoculation, and the timing of plant evaluation. Therefore, the objective of this study was to suggest standardizing the initial inoculum population and plant age at the time of P. brachyurus inoculation. We evaluated the population dynamics of P. brachyurus in a susceptible soybean cultivar until the 100th day after the inoculation based on the nematode reproduction factor (RF) weekly assessment. RF values were low at 30 DAI, and only after 51 DAI, the reproduction of P. brachyurus was sufficient to ensure more precise RF values in the susceptible soybean evaluated. Soybean plants inoculated at different plant ages (0, 10, and 20 days after planting) and inoculum densities (200, 400, 800, 1,600, and 3,200 nematodes per plant) were assessed 70 days after inoculation (DAI) to RF. Based on our data, we recommend that for an accurate classification of soybean genotypes in relation to resistance or susceptibility to P. brachyurus, inoculation should be conducted with 200 to 800 nematodes per plant within 7 to 15 days after sowing, and the evaluation should be done at least 70 days after inoculation.