In 1990, new Phytophthora strains, later recognized as a new species, Phytophthora sansomeana, were found to cause Phytophthora root rot (PRR) in soybean in addition to P. sojae. The emergence and spread of a second pathogen causing PRR poses a significant threat to soybean production. While genetic resistance to P. sojae has been developed and widely deployed as a management tool, these varieties appear largely ineffective at controlling P. sansomeana, which has a broad host-range and can infect and survive on non-leguminous hosts including fir trees, Rosaceous fruit trees, maize, and several herbaceous weeds. This contributes potential for broad distributions worldwide across both agricultural and natural ecosystems. Despite having been studied since the 1980s under a variety of informal designations, little is known about the epidemiology, host-interactions, and management of this emergent pathogen. Due to the lack of management options, increased frequency of first reports in new geographic areas, and the overall limited body of knowledge surrounding this novel pathogen, P. sansomeana warrants more research attention from both biological and disease management perspectives. The aim of this review is to summarize the hosts, distribution, pathogenicity, and current management strategies of P. sansomeana and to provide a concise record of where it has been studied under other informal designations. Its role in PRR of soybean is emphasized due to the economic magnitude of PRR-associated losses and its well-documented aggressiveness as a soybean pathogen.

In the summer of 2020, 127 soybean [Glycine max (L.) Merr] seedlings (V1-V3 stage) with reduced growth vigor were sampled as part of a bulk collection of seedling pathogens from Purdue’s Agronomy C...

In summer 2020, 127 soybean (Glycine max (L.) Merr) seedlings (V1-V3 stage) showing reduced vigor or crown lesions were collected at Purdue's Agronomy Center for Research and Education in West Lafayette, Indiana. Root tissues from two seedlings with necrotic cotyledons and root rot were surface-sterilized and plated on dichloran-chloramphenicol-peptone agar (Andrews and Pitt 1986). Emerging hyphal tips were transferred to potato dextrose agar (PDA). Single-spore cultures were obtained and grown on PDA. Both isolates developed floccose white aerial mycelia with reddish-pink coloration in the media in 2 weeks on the benchtop. On carnation leaf agar, macroconidia formed on orange sporodochia within 2 weeks in darkness at 25C. Macroconidia were 3-5 septate, measuring 26 - 41 × 2.5 - 3.7 μm (avg. 34.8 × 3.2 μm, n=40). Microconidia were abundant in chains and false heads forming on both mono- and polyphialides, and measured 2.5 - 8.75 x 2.5 μm (avg. 5.9 × 2.5 μm, n=40). These characteristics were consistent with species descriptions of F. fujikuroi [Sawada] Wollenw. (teleomorph Gibberella fujikuroi) (Leslie and Summerell 2006). DNA was extracted from mycelium and the following genes were amplified and sequenced: the internal transcriber spacer (ITS) region using ITS1/ITS4 primers (White et al. 1990) (GenBank accessions MW463362/MW463363), the mitochondrial small subunit (mtSSU) rDNA using MS1/MS2 primers (White et al. 1990) (MW465310/MW465307), and the partial translation elongation factor 1-alpha (TEF1α) gene using 983F/1567R primers (Rehner and Buckley 205) (MW475297/MW475298). In GenBank BLAST searches, these sequences showed 100% identity to both F. proliferatum and F. fujikuroi. Species-specific forward primers Fuji1F and Proli1F were then used in combination with reverse primer TEF1R to amplify another region in the TEF1α gene (Amatulli et al. 2012). Proli1F/TEF1R primers failed under a variety of annealing temperatures while Fuji1F/TEF1R primers succeeded, and the products were sequenced (MW475299/MW475300). GenBank BLAST searches revealed 100% identity of both isolates to F. fujikuroi (MT448248.1). A pathogenicity test was conducted with isolate AC13 in the greenhouse following the protocol of (Ellis et al. 2013). Ten seeds (cv. Williams) each were used for inoculation and control, respectively, with one seed per cup. Root rot symptoms similar to those observed in the field were observed 14 days after planting on all inoculated plants but not on controls (VC stage). Infected plants showed symptoms of pre-emergence damping off, reddish-brown lesions on the tap and lateral roots, and root necrosis. Three plants also exhibited hyper-elongation of the stem (12.5, 11.1, and 18 cm, vs controls: avg. 6.8 cm, max. 8.5 cm, stdev 0.78 cm). F. fujikuroi was successfully reisolated from inoculated plants but not from controls and identified as described above. F. fujikuroi has been reported causing soybean root rot in China (Zhao et al. 2020), Korea (Choi et al. 2019), and the state of Kansas (Pedrozo et al. 2015). To our knowledge this is the first report of F. fujikuroi infecting soybeans in the state of Indiana. F. fujikuroi is known to cause elongated seedlings in rice (Leslie and Summerell 2006). Pedrozo et al. (2015) reported that F. fujikuroi isolated from soybean caused seedling elongation in rice but not in soybean. The increased distribution and new host symptomology observed here warrants heightened attention for the control of this pathogen.

Soybean ranks second by total production of all crops grown in the United States. From surveys of soybean production regions in the US and Canada, seedling diseases have been consistently identified as one of the top five biotic limitations on yield for over two decades. The role of Fusarium graminearum as an aggressive member of this complex was unknown until relatively recently and, consequently, publicly and commercially available varieties with resistance to this pathogen are unavailable. To address the need for resistant germplasm and to improve our understanding of the genetic basis underlying the resistance, we screened a set of 208 accessions of soybean from the United States Department of Agriculture Soybean Germplasm Collection (USDA-SGC) under controlled greenhouse conditions. A ratio of the root weight of inoculated plants compared to mock-inoculated controls was used to evaluate the degree of resistance. A linear mixed model identified eight resistant accessions (PI 548311, PI 438500, PI 561318 A, PI 547690, PI 391577, PI 157484, PI 632418, and PI 70466 -3) with significantly higher resistance than the population mean. Previous genotyping publicly available through the SoyBase database was used in a genome-wide association study (GWAS) to determine single nucleotide polymorphism (SNP) markers associated with resistant and susceptible phenotypes. A total of five significant marker-trait associations (MTAs) were discovered on chromosomes Gm02, Gm03, Gm06, Gm07, and Gm13, each accounting for 4.8, 4.3, 3.8, 4.1, and 3.0% of the phenotypic variance, respectively. This study, thus, lays a foundation for the better dissection of germplasm resistant to F. graminearum.

During the summers in 2019 and 2020, 137 soybean (Glycine max (L.) Merr) seedlings (V1-V3 stage) showing stunting, delayed emergence, and/or crown lesions were collected at Purdue’s Agronomy Center...

Soybean (Glycine max (L.) Merr.) ranks as the second-largest crop by total production in the United States, despite its production experiencing significant constraints from plant pathogens, including those causing seedling diseases. Pythium irregulare Buisman stands out as a predominant driver of yield loss associated with the seedling disease complex. There is currently a lack of public or commercial varieties available to growers with adequate genetic resistance to manage this pathogen. To address the pressing need for germplasm resources and molecular markers associated with P. irregulare resistance, we conducted a screening of 208 genetically diverse soybean accessions from the United States Department of Agriculture Soybean Germplasm Collection (USDA-SGC) against two geographically and temporally distinct isolates under controlled greenhouse conditions. Disease severity was assessed through comparisons of the root weight and stand count ratios of inoculated plants to mock-inoculated controls. Employing linear mixed modeling, we identified ten accessions (PI 548520, PI 548360, PI 548362, PI 490766, PI 547459, PI 591511, PI 547460, PI 84946-2, PI 578503, FC 29333) with resistance significantly above the population average to one or both of two isolates originating from Ohio or Indiana. Previously curated genotyping data, publicly accessible via the SoyBase database, was subsequently utilized for conducting a genome-wide association study. This analysis led to the discovery of two significant marker–trait associations (MTAs) located on chromosomes 10 and 15 and accounting for 9.3% and 17.2% of the phenotypic variance, respectively. The resistant germplasm and MTAs uncovered through this study provide additional resources and tools for the genetic improvement of soybean resistance to seedling disease caused by P. irregulare.