A Method for Combining Isolates of Phytophthora sojae to Screen for Novel Sources of Resistance to Phytophthora Stem and Root Rot in Soybean

Abstract

Soybean cultivars with specific single resistance genes (Rps) are grown to reduce yield loss due to Phytophthora stem and root rot caused by the oomycete pathogen Phytophthora sojae. To identify novel Rps loci, soybean lines are often screened several times, each time with an isolate of P. sojae that differs in virulence on various Rps genes. The goal of this study was to determine whether several isolates of P. sojae that differ in virulence on Rps genes could be combined into a single source of inoculum and used to screen soybean lines for novel Rps genes. A set of 14 soybean differential lines, each carrying a specific Rps gene, was inoculated with three isolates of P. sojae, which differed in virulence on 6 to 10 Rps genes, individually or in a 1:1:1 mixture. Inoculum containing the 1:1:1 mixture of isolates was virulent on 13 Rps genes. The mixed-inoculum method was used to screen 1,019 soybean accessions in a blind assay for novel sources of resistance. In all, 17% of Glycine max accessions and 11% of G. soja accessions were resistant (≤30% dead plants), suggesting that these accessions may carry a novel Rps gene or genes. Advantages of combining isolates into a single source of inoculum include reduced cost, ability to screen soybean germplasm with inoculum virulent on all known Rps genes, and ease of identifying novel sources of resistance. This study is a precursor to identifying novel sources of resistance to P. sojae in soybean using RXLR effectors.

Quantitative Disease Resistance Loci towards Phytophthora sojae and Three Species of Pythium in Six Soybean Nested Association Mapping Populations

“…Resistance towards P. sojae has been identified in numerous soybean genotypes; >25 single Rps genes as well as numerous quantitative trait loci (QTL) have been mapped (Dorrance and Schmitthenner, 2000;Wang et al, 2010Wang et al, , 2012bLee et al, 2014;Matthiesen et al, 2016;Schneider et al, 2016;Dorrance, 2018). Although resistance towards Pythium has been identified in several soybean genotypes (Griffin, 1990;Bates et al, 2008;Ellis et al, 2013;Balk, 2014), it has only been mapped towards three species of Pythium to date (Rosso et al, 2008;Ellis et al, 2013;Stasko et al, 2016;Urrea et al, 2017;Lin et al, 2018;Klepadlo et al, 2018).…”

Modelling the key drivers of an aerial Phytophthora foliar disease epidemic, from the needles to the whole plant

“…All isolates were routinely passed through pine needles prior to each experiment to confirm their ability to infect. A mix of isolates was used to enable selection for the most virulent one [24] and favour infection, as commonly performed in epidemiological studies [25,26]. All isolates used as source of inoculum were identified using morphological and molecular techniques.…”

A Novel Phytophthora sojae Resistance Rps12 Gene Mapped to a Genomic Region That Contains Several Rps Genes

“…Seedlings were grown in the greenhouse for a week. Hypocotyls of seven-day old seedlings were inoculated using the wounded-hypocotyl inoculation technique [18–20, 59–63]. An approximately 1 cm long slit was made with the needle tip in each hypocotyl, 1 cm below the cotyledonary node, and 0.2 to 0.4 mL of the culture slurry was placed into the slit using the syringe.…”

“…The macerated culture was placed in a syringe for a second time and a #18 needle was used to further macerate the culture. Macerated R17 and Val 12–11 cultures were mixed in a 1:1 ratio to prepare the mixed inoculum [63], which is virulent to soybean cultivars carrying Rps genes mapped to any of the Rps1 to 7 loci (Fig 1, Table 1). P .…”