The planting materials used in this study comprised of twenty one (21) introduced soybean lines developed by IITA (Abuja, Nigeria) and three (3) soybean varieties (G175, G196 and G197) already used in soybean research programme in Burkina Faso used as local checks (Table 1). The seeds of the 21 soybean accessions were treated with an insecticide (Phostoxin) and a fungicide (Bendaco) before being transferred to Burkina Faso.

The experimental set-up was an alpha lattice design with 3 replicates separated from each other by 2 m. Each repetition consisted of 24 entries distributed in 4 blocks, each consisting of 6 varieties. The blocks were separated from each other

by 80 cm. The elementary plot was represented by 4 rows of 4 m with 50 cm of row spacing.

Nematodes were isolated in two stages from soil samples and transferred to the Nematology Laboratory of DRREA/West. The first sample was taken during the establishment of the trial and the second one after the harvest, on October 10, 2020 (111 day after sowing). For the second sampling, the composite sample was represented by 4 soil samples with the roots of each soybean variety and the root-soil mass was put in the same labeled plastic bag and kept cold until the moment of extraction.

The Elutriator method of [12] was used to extract nematodes from the soil during the first sampling stage. For the second sampling which was done after harvest, the modified Baerman tunnel method was used. It consisted of separating the soil roots from the sample and labeling them separately. Thus, a volume of 100 cubic centimeters (cc) of soil was taken and added to 300 mL volume of water. After 72 h of filtering on a tissue paper, the nematode suspension was collected in jars. Nematode counts were performed from a 25 mL volume suspension through a 5 Micron diameter sieve. Nematode population densities were expressed as number of nematodes/dm³ of soil.

The roots were lightly shaken to remove soil clumps and rinsed under a stream of water and then lightly dried on a paper towel. The roots were cut finely with scissors, weighed and placed in a coarse mesh sieve on two layers of tissue paper and placed in a cup. A volume of 200 mL of water was poured over the roots which were left to incubate for 72 hours at room temperature. Nematode counts of the roots were performed as previously described and nematode population densities were expressed as number of nematodes/g root.

Nematode frequencies were calculated from the formula

F = e n × 100

with e the number of samples containing the genus of interest, and n the total number of samples. Nematode abundance was determined by the formula

A = ∑ x i / e

with xi the number of individuals of the genus under consideration per liter of soil or per gram of dry root and e the number of samples in which the genus under consideration was present.

The data analysis was performed using Excel for the generation of the graphs. After the transformation of the raw data into Log₁₀(X + 1) where X is the observed data, an analysis of variance (ANOVA) was performed using GenStat Release 12.1 on all variables. All treatment means were compared using the Duncan multiple comparisons test at a 5% level of significance. The principal component analysis and Cluster analysis were performed with R software.

Nematological analysis of soil and root samples of soybean varieties revealed the presence of seven (7) genera of plant-parasitic nematodes which are the genera Meloidogyne, Pratylenchus, Helicotylenchus, Scutellonema, Xiphinema, Tylenchorhynchus and Paratrichodorus. The inventory reveals that the genera Pratylenchus, Helicotylenchus and Scutellonema are the most important in terms of frequency and abundance. The specie Pratylenchus brachyurus represented the highest density with 7805 nematodes/dm³ followed by Helicotylenchus dihystera with 362 nematodes/dm³ and Scutellonema cavenessi with 251 nematodes/dm³. The lowest density was found in the specie Paratrichodorus minor with 1.39 nematodes/dm³. The species of this genus Paratrichodorus are polyphagous and favor the transmission of viral diseases. Table 2 presents the different genera encountered and their frequencies.

The results of the analyses of variance (ANOVA) indicated no significant differences among soybean genotypes for the population densities of soil and root nematodes belonging to the genera Pratylenchus, Helicotylenchus and Scutellonema. Table 3 gives the mean values of population densities of the main soil and root parasitic nematodes according to soybean genotypes.

For the Pratylenchus brachyurus communities present in the soil, the mean values of densities ranged from 2233 to 17,000 N/dm³ of soil with a mean of 7805 N/dm³ of soil. Genotype TGX1989-19F and TGX2015-1E were the least infested with 2233 and 2250 N/dm³ of soil respectively. Genotype TGX2027-1E was the most attacked with 17,000 N/dm³ of soil.

Prat: Pratylenchus; Hel: Helicotylenchus; Scu: Scutellonema; NS: No Significant. Means followed by the same letter in a column are not significantly different at P ≤ 0.05.

For the Helicotylenchus dihystera communities present in the soil, the average values of population densities varied between 150 and 800 N/dm³ of soil. Genotypes G175, TGX2016-3E, TGX2017-6E and TGX2025-6E showed the lowest levels of infestation while the highest densities were obtained with the check variety G197 at 800 nematodes/dm³ of soil.

For the Scutellonema cavenessi communities present in the soil, the average values of population densities were significantly low compared to the first two genera. Population densities ranged from 100 to 450 N/dm³ of soil. Genotypes TGX2008-4F and TGX2019-1E had the lowest levels of infestation.

At the root level, population densities were very low compared to those present in the soil. The genera Pratylenchus brachyurus and Helicotylenchus dihystera were the most representative with an average of 61.68 and 2.73 N/g of roots, respectively.

For the Pratylenchus brachyurus community, genotype TGX2025-10E showed the lowest level of infestation with 16.56 N/g of root. It was followed by genotypes TGX2025-6E and TGX2027-1E with 21.18 and 21.78 N/g of root, respectively. Genotype TGX2011-6F recorded the highest density value of 202.81 N/g of root. For the nematode community of the specie Scutellonema cavenessi, the average values of population densities were very low and ranged from 0 to 7.58 N/g of root with an average of 2.73 N/g of root. Genotypes TGX2017-5E, TGX2016-3E and TGX1989-19F with each 0 N/g of root were free of infestation while variety TGX2016-4E showed the highest infestation with 7.58 N/g of root.

Figure 1 shows the results of the principal component analysis (PCA) of different agronomic and parasitic nematode distribution parameters as well as the

distribution of individuals in the biplot. Analysis distinguished two axes that explain 42.9% of the total genetic variability within the soybean genotypes and parasitic nematode densities. All variables are somewhat well represented in the biplot whereas SW100, soils nematodes Pratylenchus brachyurus (SNPra), Scutellonema cavenessi (SNScu) and Helicotylenchus dihystera (SNHel), Mat50 and GY weakly represented in the biplot (Figure 1), they do not strongly contribute to the formation of either axe 1 or axe 2. However, we can distinguish correlated variables and groups of variables. Soils nematodes SNHel, SNPra, SNScu and root nematode RNScu communities constitutes a group of correlated variables and were positively correlated to axis 1 (26.7% of the total variability). Among the parasitic nematodes, Scutellonema cavenessi from root (RNScu) contributed around 12% of the total variability according to the contribution diagram in Figure 1 and individual TGX2016-4E is highlighted to be well performing for that feature. Scutellonema cavenessi (SNScu) and Pratylenchus brachyurus (SNPra) from soil contributed each between 9% to 12% to the variability. Helicotylenchus dihystera from soil (SNHel) contributed very little to the variability, i.e., 6% of the total variability. Flo50 and NbNod are also well represented, correlated and contributed each up to 8% of the total variability. Individuals are widely dispersed in the diagram and show how much variability is within the plant materials studied.

The cluster analysis resulting from the hierarchical ascending classification (HAC) grouped the genotypes of soybean into three clusters (Figure 2). Cluster 1 consisted of 11 genotypes including genotypes TGX2011-6F, TGX1993-4FN and TGX2025-9E. The genotypes in this group are characterized by their low soil densities to species of the specie Helicotylenchusdihystera. Cluster 2 consisted of 3 genotypes TGX2016-4E, TGX1987-14F, and TGX2027-1E that exhibited the highest root population densities to the genus Pratylenchus. Genotypes of this group also exhibited the highest soil population densities to the species Pratylenchus

brachyurus, Helicotylenchus dihystera and Scutellonema. Cluster 3 contains 10 genotypes including the three checks varieties (G175, G196 and G197) that showed the lowest root and soil population densities to the specie Pratylenchus brachyurus. These genotypes can be used as sources of resistance in varietal control against species of the three major nematode genera identified including the genus Pratylenchus.