Identifying New Sources of Resistance to Brown Stem Rot in Soybean

Abstract

Brown stem rot (BSR), caused by the fungus Phialophora gregata f. sp. sojae (Allington & D.W. Chamberlain) W. Gams (syn. Cadophora gregata), causes yield losses up to 38%. Three dominant BSR‐resistant genes have been identified: Rbs1, Rbs2, and Rbs3. Additional BSR resistance loci will complement breeding efforts by expanding the soybean [Glycine max (L.) Merr.] genetic base. The objective of this research was to determine if PI 594637, PI 594638B, PI 594650A, and PI 594858B contained novel BSR resistance genes. The accessions were crossed to three genotypes with known BSR resistance genes and populations were developed for allelism studies. A minimum of 60 F2:3 families tracing to individual F2 plants in each population were used, and six seeds from each F2:3 family were tested. Resistant and susceptible controls and parents were also included. The BSR symptoms were assessed under growth chamber conditions 5 wk after inoculation by measuring foliar and stem severities and recovery of P. gregata from stem sections. Allelism tests of F2:3 plants from crosses of PI 594638B, PI 594858B, and PI 594650A with the resistant sources fit a 15:1 ratio, indicating that the resistant gene possessed by each of the PIs was nonallelic to Rbs1, Rbs2, and Rbs3. The three PIs contain at least one novel BSR resistance gene and have the potential to serve as donors to elite germplasm, increasing stability of host resistance to P. gregata. Allelism tests of PI 594637 segregated in a 3:1 ratio and no significant difference was found between PI 594637 and the susceptible controls, indicating that PI 594637 is susceptible to BSR.

Leveraging RNA-Seq to Characterize Resistance to Brown Stem Rot and the Rbs3 Locus in Soybean

“…The P. gregata spore suspension was prepared using isolate Oh 2-3 (Lewers et al 1999), a single-spore isolate of Oh 2 (Eathington et al 1995) originally provided by Cecil Nickell at the University of Illinois. Cultures were grown on green bean extract medium and incubated for 60 days at room temperature with ambient light, as described by McCabe et al (2016). P. gregata spores were suspended in a 1.2% water agar paste and adjusted to a final spore concentration of 2.7 × 10 7 spores/ml using a hemocytometer.…”

“…Because tissues were collected prior to disease symptom development, additional seedlings of each genotype-treatment combination were allowed to grow for 5 weeks to confirm expected BSR symptom development using three standard screening methods (McCabe et al 2016). Appropriate BSR phenotypes were observed for each genotype by treatment combination using all three screening methods ( Supplementary Table S1.…”

“…Eight plants of each genotype (Corsoy 79 and PI 437970)treatment (infected and mock) combination were phenotyped 5 weeks after infection to verify expected BSR symptoms. BSR severity was assessed using three measurements: foliar severity, stem severity, and stem recovery of P. gregata, as described by McCabe et al (2016). BSR foliar severity was evaluated on each plant using a scale of 1 to 7, with 1 representing the most severe symptoms.…”

“…Typically, screening for BSR resistance is conducted in growth chambers and greenhouses 5 weeks after inoculation. Screening methods are time consuming, requiring both foliar and internal stem severity measurements (McCabe et al 2016). Although P. gregata is an economically important pathogen, few genomic tools have been used to characterize resistance.…”

Registration of ‘IAR1902 SCN’ Cultivar Resistant to Soybean Cyst Nematode and Brown Stem Rot

“…Brown stem rot resistance screenings were performed in the Cianzio laboratory during August 2013 following the protocol described by McCabe et al (2016) and Tabor et al (2003). Seed of IAR1902 SCN (as experimental line AR09‐191018) was germinated on germination paper, rolled, and placed in a clear plastic container with 1.5 cm of water.…”