Identification and inheritance of an Rdc gene resistance to soybean stem canker (Diaporthe phaseolorum var. caulivora)

Abstract

Soybean stem canker (SSC) is caused by two varieties of the fungus Diaporthe phaseolorum: var. meridionalis (Dpm) and var. caulivora (Dpc). The objective was to identify and characterize the mode of inheritance of Rdc genes through a classical Mendelian analysis. Resistant (R) and susceptible (S) genotypes were used to make 288 RxS and 132 RxR crosses, including their reciprocals. Segregating F2 generations were obtained by self-fertilization of the respective F1. The incorporation of codominant molecular markers (Single Nucleotide Polymorphism, SNP) allowed the molecular validation of 48.75% of F1 heterozygous individuals. Parents (R and S), F1 individuals, and F2:3 families (Progeny Test, PT) from COD 1–258-2 population were inoculated with an isolate of Dpc (Dpc16), previously identified morphologically and molecularly. The assay showed 21 F2:3 families categorized as R, 42 segregated R and S, and 11 as S. Particularly, this F3 population showed 466 individuals R and 274 S. The chi-square goodness of fit test verified that phenotypic segregation for individual plants in F3 adjusted to a 5:3 ratio (R:S) and the PT results corresponded to the genotypic ratios (1RR: 2Rr: 1rr) of F2 individuals. Results allowed the identification of a major resistance gene of simple Mendelian inheritance to SSC that was named Rdc1. Also, independent segregation 9:3:3:1 was verified between this Rdc1 gene and the gene that regulates flower colour, a typical phenotypic marker in soybean. Based on literature review, this is the first report on resistance genes (Rdc) identified for SSC caused by Dpc.