Stem Rot Of Soybean Research Articles

Phytophthora root and stem rot of soybean: the importance of the disease and host resistance

Phytophthora root and stem rot of soybean: the importance of the disease and host resistance

A MYB-related transcription factor regulates effector gene expression in an oomycete pathogen.

Phytophthora pathogens possess hundreds of effector genes that exhibit diverse expression patterns during infection, yet how the expression of effector genes is precisely regulated remains largely elusive. Previous studies have identified a few potential conserved transcription factor binding sites (TFBSs) in the promoters of Phytophthora effector genes. Here, we report a MYB-related protein, PsMyb37, in Phytophthora sojae, the major causal agent of root and stem rot in soybean. Yeast one-hybrid and electrophoretic mobility shift assays showed that PsMyb37 binds to the TACATGTA motif, the most prevalent TFBS in effector gene promoters. The knockout mutant of PsMyb37 exhibited significantly reduced virulence on soybean and was more sensitive to oxidative stress. Consistently, transcriptome analysis showed that numerous effector genes associated with suppressing plant immunity or scavenging reactive oxygen species were down-regulated in the PsMyb37 knockout mutant during infection compared to the wild-type P. sojae. Several promoters of effector genes were confirmed to drive the expression of luciferase in a reporter assay. These results demonstrate that a MYB-related transcription factor contributes to the expression of effector genes in P. sojae.

The EIN3 transcription factor GmEIL1 improves soybean resistance to Phytophthora sojae.

Phytophthora root and stem rot of soybean (Glycine max), caused by the oomycete Phytophthora sojae, is an extremely destructive disease worldwide. In this study, we identified GmEIL1, which encodes an ethylene-insensitive3 (EIN3) transcription factor. GmEIL1 was significantly induced following P. sojae infection of soybean plants. Compared to wild-type soybean plants, transgenic soybean plants overexpressing GmEIL1 showed enhanced resistance to P. sojae and GmEIL1-silenced RNA-interference lines showed more severe symptoms when infected with P. sojae. We screened for target genes of GmEIL1 and confirmed that GmEIL1 bound directly to the GmERF113 promoter and regulated GmERF113 expression. Moreover, GmEIL1 positively regulated the expression of the pathogenesis-related gene GmPR1. The GmEIL1-regulated defence response to P. sojae involved both ethylene biosynthesis and the ethylene signalling pathway. These findings suggest that the GmEIL1-GmERF113 module plays an important role in P. sojae resistance via the ethylene signalling pathway.

Life and death: the destiny of Phytophthora sojae determined by a receptor-like kinase

Leucine-rich repeat receptor-like kinases (LRR-RLKs) are well known to act in plant growth, development, and defense responses. Plant LRR-RLKs locate on cell surface to sense and initiate responsive signals to a variety of extracellular stimuli, such as microbe-associated molecular patterns (MAMPs) released from microorganisms. LRR-RLKs are also present in microbes and function in microbial growth and development, but their roles in communicating with hosts are largely unknown. A recent study published in Nature Communications uncovered that a microbial LRR-RLK, PsRLK6, is required for oospore development in the sexual reproduction of Phytophthora sojae, an oomycete pathogen that causes root and stem rot in soybean. Meanwhile, PsRLK6 is recognized as a novel type of MAMP by an unknown plant LRR receptor-like protein and triggers immune responses in soybean, tomato, and Nicotiana benthamiana. The findings reveal dual roles of a pathogen LRR-RLK in determining both life through sexual reproduction and death through triggering plant immunity.

Phytophthora sojae boosts host trehalose accumulation to acquire carbon and initiate infection.

Successful infection by pathogenic microbes requires effective acquisition of nutrients from their hosts. Root and stem rot caused by Phytophthora sojae is one of the most important diseases of soybean (Glycine max). However, the specific form and regulatory mechanisms of carbon acquired by P. sojae during infection remain unknown. In the present study, we show that P. sojae boosts trehalose biosynthesis in soybean through the virulence activity of an effector PsAvh413. PsAvh413 interacts with soybean trehalose-6-phosphate synthase 6 (GmTPS6) and increases its enzymatic activity to promote trehalose accumulation. P. sojae directly acquires trehalose from the host and exploits it as a carbon source to support primary infection and development in plant tissue. Importantly, GmTPS6 overexpression promoted P. sojae infection, whereas its knockdown inhibited the disease, suggesting that trehalose biosynthesis is a susceptibility factor that can be engineered to manage root and stem rot in soybean.

Effectiveness of some Bio-control Agents and Chemical Resistance Inducers Against Brown Stem Rot in Soybean (Glycine max (L.) Merrill)

Effectiveness of some Bio-control Agents and Chemical Resistance Inducers Against Brown Stem Rot in Soybean (Glycine max (L.) Merrill)

First Report of Neocosmospora Stem Rot of Soybean Caused by Fusarium neocosmosporiellum in Mississippi.

In September 2021, diseased soybean [Glycine max (L.) Merr.] plants were observed in a commercial field in Lamar County, Mississippi (MS). Foliar symptoms included mild interveinal chlorosis and necrosis in the affected plants (Fig. 1a). Stems and roots exhibited orange to black lesions and pith discoloration (Fig. 1b). Signs of the fungus included orange-to-red spherical perithecia, observed in clusters on crowns and roots. Perithecia were surface-sterilized with 70% ethanol for 1 min. Subsequently they were gently pressed in a droplet of 5 μl of sterilized reverse osmosis (RO) water and the suspension was streaked onto potato dextrose agar (PDA) amended with chloramphenicol (75 mg/liter) and streptomycin sulfate (125 mg/liter). Plates were incubated in the dark at 25°C. White to light pink and floccose mycelia developed after four days of incubation and hyaline, one celled, cylindrical to oblong-ellipsoidal microconidia were observed (Fig. 2b; ×400). Conidia measured 8.8 to 19 × 2.8 to 5 μm (n=15). Abundant orange to brown perithecia developed on PDA after three weeks of incubation (Fig. 2a). The asci within the perithecia were cylindrical, eight-spored, thin walled and measured 94.6 to 123.6 μm × 10 to 14 μm (n=10). Ascospores were uniseriate, globose to ellipsoidal, hyaline to brown and measured 11.6 to 16 μm × 7 to 10 μm (n=10; Fig. 2c). The morphological characteristics were consistent with those of Fusarium neocosmosporiellum O'Donnell & Geiser (≅ Neocosmospora vasinfecta E. F. Sm.; Geiser et al. 2013). Genomic DNA was extracted from isolate TW068-21 from a 3-week-old culture plate. The internal transcribed spacer region (ITS), elongation factor 1-alpha (EF1-alpha) and calmodulin (cmdA) gene were amplified, and consensus sequences deposited in GenBank (OM640625, OM681343, OM681344). ITS and EF1-alpha sequence comparison using NCBI BLAST, showed > 99.2% similarity with N. vasinfecta JL2210 while cmdA sequence was 99.8% similar to strain CBS 517.71. A pathogenicity test was performed on 2-week-old Asgrow 46X6 soybean seedlings grown in 10.2 cm pots in a growth chamber. Isolate TW068-21 was grown on antibiotic-amended PDA for 4-weeks and inoculum suspension was prepared with sterilized RO water and adjusted to 2 × 105 ascospores/ml. Soybean seedlings (n=8) were removed from pots and roots were dipped into the inoculum suspension for 20 min. Four control plants were dipped in autoclaved RO water. Plants were re-planted in potting mix, and the inoculated plants were immediately drenched with 20 ml of the inoculum and placed in a growth chamber (25°C; 14 h light). The experiment was repeated once. Inoculated plants presented dark brown discoloration at the base of the stem after 3 weeks of incubation, but no foliar lesions were observed. Control plants remained asymptomatic. Symptomatic stems were placed in a moist chamber (≈23°C; 12 h:h light:dark) and light brown perithecia developed after 1 week. Fusarium neocosmosporiellum was re-isolated from perithecia and stems and colony and spore morphology were similar as described above. To our knowledge, this is the first report of F. neocosmosporiellum in MS. Signs and symptoms of this disease resemble red crown rot. Consequently, careful morphological and molecular assessments should be used for confirmation. Neocosmospora stem rot has been previously reported in Alabama (Gray et al. 1980), Arkansas (Greer et al. 2015), and Georgia (Phillips 1972). Yield losses due to this pathogen in MS are currently unknown.

An updated assessment of the soybean–Phytophthora sojae pathosystem

AbstractFilamentous disease‐causing agents (e.g., fungi and oomycetes) constitute a diverse, widely distributed group of plant pathogens that continually threaten crop production. A prominent example is Phytophthora sojae, a pathogenic oomycete that renders root and stem rot in soybean (Glycine max). The expansion of diverse/complex P. sojae pathotypes limits traditional methods of disease prevention and is exacerbated by climate variability. Therefore, a deeper understanding of the molecular features governing soybean–P. sojae interaction is required to preserve soybean yield. This review summarizes current knowledge surrounding the soybean–P. sojae pathosystem with an emphasis on host–pathogen coevolution. We describe innate immunity, novel and well‐defined mechanisms of P. sojae virulence and the molecular basis of host resistance. We then provide rationales for disease management through the deployment of gene regulatory networks and a deeper understanding of the biotic growing environment. Insight herein will help unravel molecular and biological mechanisms of plant–pathogen conflict and has benign implications for sustainable agriculture.

Field Application of Wuyiencin Against Sclerotinia Stem Rot in Soybean

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a devastating disease of soybean. Biological control is a potential alternative to chemical fungicides for disease management, and provides broad benefits to the environment, farmers and consumers. Herein, we established a field application technique for biocontrol of Sclerotinia stem rot in soybean using wuyiencin, expanding on a previous study showing biocontrol potential. We used wuyiencin to reduce sclerotia in soybean seed, and disease incidence analysis by seed bioassay revealed an optimal wuyiencin seed soaking concentration of 12.5 μg/mL. We found that different application methods had different effects on soybean plant growth. Soybean pot experiments showed that 100 μg/mL wuyiencin was obtained a significant disease protection effect and promote soybean growth through root irrigation, and the optimal concentration for wuyiencin spraying was 100–200 μg/mL. We tested the efficacy of applying wuyiencin under field conditions, and the protection effect of 200 μg/mL wuyiencin sprayed three times was the best (64.0%), but this was slightly inferior to the protection effect of 200 μg/mL dimethachlon (77.6%).

Antifungal Mechanism and Efficacy of Kojic Acid for the Control of Sclerotinia sclerotiorum in Soybean.

Sclerotinia stem rot, which is caused by the fungal pathogen Sclerotinia sclerotiorum, is a soybean disease that results in enormous economic losses worldwide. The control of S. sclerotiorum is a difficult task due to the pathogen’s wide host range and its persistent structures, called sclerotia. In addition, there is lack of soybean cultivars with medium to high levels of resistance to S. sclerotiorum. In this work, kojic acid (KA), a natural bioactive compound commonly used in cosmetic industry, was evaluated for the management of Sclerotinia stem rot. Interestingly, KA showed strong antifungal activity against S. sclerotiorum by inhibiting chitin and melanin syntheses and, subsequently, sclerotia formation. The antifungal activity of KA was not obviously affected by pH, but was reduced in the presence of metal ions. Treatment with KA reduced the content of virulence factor oxalic acid in S. sclerotiorum secretions. Preventive applications of 50 mM KA (7.1 mg/ml) completely inhibited S. sclerotiorum symptoms in soybean; whereas, in curative applications, the combination of KA with prochloraz and carbendazim improved the efficacy of these commercial fungicides. Taken together, the antifungal activity of KA against S. sclerotiorum was studied for the first time, revealing new insights on the potential application of KA for the control of Sclerotinia stem rot in soybean.

Pathotype Complexity and Genetic Characterization of Phytophthora sojae Populations in Illinois, Indiana, Kentucky, and Ohio.

Phytophthora sojae, the causal agent of Phytophthora root and stem rot of soybean, has been managed with single Rps genes since the 1960s but has subsequently adapted to many of these resistance genes, rendering them ineffective. The objective of this study was to examine the pathotype and genetic diversity of P. sojae from soil samples across Illinois, Indiana, Kentucky, and Ohio by assessing which Rps genes were still effective and identifying possible population clusters. There were 218 pathotypes identified from 473 P. sojae isolates with an average of 6.7 out of 15 differential soybean lines exhibiting a susceptible response for each isolate. Genetic characterization of 103 P. sojae isolates from across Illinois, Indiana, Kentucky, and Ohio with 19 simple sequence repeat markers identified 92 multilocus genotypes. There was a moderate level of population differentiation between these four states, with pairwise FST values ranging from 0.026 to 0.246. There were also moderate to high levels of differentiation between fields, with pairwise FST values ranging from 0.071 to 0.537. Additionally, cluster analysis detected the presence of P. sojae population structure across neighboring states. The level of pathotype and genetic diversity, in addition to the identification of population clusters, supports the hypothesis of occasional outcrossing events that allow an increase in diversity and the potential to select for a loss in avirulence to specific resistance genes within regions. The trend of suspected gene flow among neighboring fields is expected to be an ongoing issue with current agricultural practices.

Changes in biochemistry and cellular ultrastructure support different resistance mechanisms to Phytophthora sojae in nonhost common bean and host soybean

AbstractPhytophthora root and stem rot of soybean caused by Phytophthora sojae is a destructive disease affecting soybean production worldwide. In nature, soybean is the only economically important cultivated host of P. sojae. The aim of this study was to explain different resistance mechanisms to P. sojae in nonhost common bean and host soybean as a basis for the control of Phytophthora root and stem rot of soybean via nonhost resistance. Observations and measurements of disease resistance‐related variables showed slight differences in structural and biochemical resistance mechanisms between common bean and soybean. P. sojae infection induced a stronger hypersensitive response in nonhost common bean than in host resistant soybean. Moreover, phytoalexin phaseollidin synthesis‐related vestitone reductase gene was extremely highly up‐regulated, and phytoalexin glyceollin synthesis‐related isoflavone reductase gene was slightly less up‐regulated in common bean than in soybean, which resulted in a higher level of phaseollidin and a lower level of glyceollin in common bean. Phaseollidin had stronger inhibitory effects on mycelial growth and oospore formation of P. sojae than glyceollin, and more cell wall depositions and callose accumulated in common bean, which are probably related to the stronger resistance of nonhost common bean to P. sojae.

The Plant-Beneficial Rhizobacterium Bacillus velezensis FZB42 Controls the Soybean Pathogen Phytophthora sojae Due to Bacilysin Production.

Soybean root rot caused by the oomycete Phytophthora sojae is a serious soilborne disease threatening soybean production in China. Bacillus velezensis FZB42 is a model strain for Gram-positive plant growth-promoting rhizobacteria and is able to produce multiple antibiotics. In this study, we demonstrated that B. velezensis FZB42 can efficiently antagonize P. sojae. The underlying mechanism for the inhibition was then investigated. The FZB42 mutants deficient in the synthesis of lipopeptides (bacillomycin D and fengycin), known to have antifungal activities, and polyketides (bacillaene, difficidin, and macrolactin), known to have antibacterial activities, were not impaired in their antagonism toward P. sojae; in contrast, mutants deficient in bacilysin biosynthesis completely lost their antagonistic activities toward P. sojae, indicating that bacilysin was responsible for the activity. Isolated pure bacilysin confirmed this inference. Bacilysin was previously shown to be antagonistic mainly toward prokaryotic bacteria rather than eukaryotes. Here, we found that bacilysin could severely damage the hyphal structures of P. sojae and lead to the loss of its intracellular contents. A device was invented allowing interactions between P. sojae and B. velezensis FZB42 on nutrient agar. In this manner, the effect of FZB42 on P. sojae was studied by transcriptomics. FZB42 significantly inhibited the expression of P. sojae genes related to growth, macromolecule biosynthesis, pathogenicity, and ribosomes. Among them, the genes for pectate lyase were the most significantly downregulated. Additionally, we showed that bacilysin effectively prevents soybean sprouts from being infected by P. sojae and could antagonize diverse Phytophthora species, such as Phytophthora palmivora, P. melonis, P. capsici, P. litchi, and, most importantly, P. infestans. IMPORTANCEPhytophthora spp. are widespread eukaryotic phytopathogens and often extremely harmful. Phytophthora can infect many types of plants important to agriculture and forestry and thus cause large economic losses. Perhaps due to inappropriate recognition of Phytophthora as a common pathogen in history, research on the biological control of Phytophthora is limited. This study shows that B. velezensis FZB42 can antagonize various Phytophthora species and prevent the infection of soybean seedlings by P. sojae. The antibiotic produced by FZB42, bacilysin, which was already known to have antibacterial effectiveness, is responsible for the inhibitory action against Phytophthora. We further showed that some Phytophthora genes and pathways may be targeted in future biocontrol studies. Therefore, our data provide a basis for the development of new tools for the prevention and control of root and stem rot in soybean and other plant diseases caused by Phytophthora.

The BTB/POZ domain protein GmBTB/POZ promotes the ubiquitination and degradation of the soybean AP2/ERF-like transcription factor GmAP2 to regulate the defense response to Phytophthora sojae.

Phytophthora root and stem rot in soybean (Glycine max) is a destructive disease worldwide, and hence improving crop resistance to the causal pathogen, P. sojae, is a major target for breeders. However, it remains largely unclear how the pathogen regulates the various affected signaling pathways in the host, which consist of complex networks including key transcription factors and their targets. We have previously demonstrated that GmBTB/POZ enhances soybean resistance to P. sojae and the associated defense response. Here, we demonstrate that GmBTB/POZ interacts with the transcription factor GmAP2 and promotes its ubiquitination. GmAP2-RNAi transgenic soybean hairy roots exhibited enhanced resistance to P. sojae, whereas roots overexpressing GmAP2 showed hypersensitivity. GmWRKY33 was identified as a target of GmAP2, which represses its expression by directly binding to the promoter. GmWRKY33 acts as a positive regulator in the response of soybean to P. sojae. Overexpression of GmBTB/POZ released the GmAP2-regulated suppression of GmWRKY33 in hairy roots overexpressing GmAP2 and increased their resistance to P. sojae. Taken together, our results indicate that GmBTB/POZ-GmAP2 modulation of the P. sojae resistance response forms a novel regulatory mechanism, which putatively regulates the downstream target gene GmWRKY33 in soybean.

Gene-Based Allele Specific Marker for Resistance to Phytophthora sojae in Soybean (Glycine max L.)

Phytophthora sojae is a soil-borne oomycete that causes both pre- and post-emergence damping-off disease in soybean that are present in poorly drained soils. Phytophthora root and stem rot of soybean has become an emerging threat to soybean production in South Korea as soybean cultivation in paddy fields has increased recently. The cultivar Daewon was identified as a genetic source for resistance to P. sojae isolate 2457; moreover, a 573 kb resistance locus was previously mapped on chromosome 3 via linkage analysis using Daepung × Daewon recombinant inbred line (RIL) population. This study aimed to develop a gene-based molecular marker associated with P. sojae resistance using single nucleotide polymorphisms (SNPs) at this locus. Three sets of single nucleotide amplified polymorphism (SNAP) markers were initially designed based on genic SNPs in the identified genomic region. Of these, the marker SNAP-Set2 successfully worked for allele-specific amplification for the respective Daepung and Daewon, as well as 20 RILs derived from crosses of the two cultivars. To validate this marker, 11 soybean germplasms were randomly selected and genotyped, which resulted in reliable allele-specific amplification that agreed with the 180 K Axiom® SoyaSNP array data. Phenotypic evaluation of the 20 RILs and the 11 germplasms subsequently demonstrated that Daepung-type and Daewon-type for the SNAP-Set2 are both associated with susceptibility and resistance to P. sojae isolate 2457. The availability of a molecular marker linked to this resistance locus would expedite the use of this valuable resistance allele in soybean breeding programs for increased resistance to P. sojae.

Identification of glutathione transferase gene associated with partial resistance to Sclerotinia stem rot of soybean using genome-wide association and linkage mapping.

Association and linkage mapping techniques were used to identify and verify single nucleotide polymorphisms (SNPs) associated with Sclerotinia sclerotiorum resistance. A novel resistant gene, GmGST , was cloned and shown to be involved in soybean resistance to SSR. Sclerotinia stem rot (SSR), caused by the fungus Sclerotinia sclerotiorum, is one of the most devastating diseases in soybean (Glycine max (Linn.) Merr.) However, the genetic architecture underlying soybean resistance to SSR is poorly understood, despite several mapping and gene mining studies. In the present study, the identification of quantitative trait loci (QTLs) involved in the resistance to S. sclerotiorum was conducted in two segregating populations: an association population that consisted of 261 diverse soybean germplasms, and the MH population, derived from a cross between a partially resistant cultivar (Maple arrow) and a susceptible cultivar (Hefeng25). Three and five genomic regions affecting resistance were detected by genome-wide association study to control the lesion length of stems (LLS) and the death rate of seedling (DRS), respectively. Four QTLs were detected to underlie LLS, and one QTL controlled DRS after SSR infection. A major locus on chromosome (Chr.) 13 (qDRS13-1), which affected both DRS and LLS, was detected in both the natural population and the MH population. GmGST, encoding a glutathione S-transferase, was cloned as a candidate gene in qDRS13-1. GmGST was upregulated by the induction of the partially resistant cultivar Maple arrow. Transgenic experiments showed that the overexpression of GmGST in soybean increased resistance to S. sclerotiorum and the content of soluble pigment in stems of soybean. The results increase our understanding of the genetic architecture of soybean resistance to SSR and provide a framework for the future marker-assisted breeding of resistant soybean cultivars.

Correction to: Determination of the protection period of fungicides used for control of Sclerotinia stem rot in soybean through bioassay and chromatography

Correction to: Determination of the protection period of fungicides used for control of Sclerotinia stem rot in soybean through bioassay and chromatography

Determination of the protection period of fungicides used for control of Sclerotinia stem rot in soybean through bioassay and chromatography

In Brazil, Sclerotinia stem rot (SSR) control in soybean depends primarily on application of the fungicides carbendazim, fluazinam, and procymidone. While these fungicides can provide effective control of the disease, their protection period, which is crucial for timing their optimal application, is poorly understood. In the present study, the protection period of carbendazim, fluazinam and procymidone was determined using a detached leaf bioassay (DLB) and chromatography. For the DLB, soybean plants were treated twice with each fungicide with or without mineral oil as adjuvant; leaves were collected at two or three-day intervals from 11 to 23 days, inoculated with Sclerotinia sclerotiorum mycelium, and the necrotic area was assessed 48 h after inoculation. For the chromatography test, each fungicide was applied and the leaves were collected every two days after treatment for 14 days. Fungicide residue concentration in tissue was determined according to a modified version of the quick, easy, cheap, effective, rugged and safe (QuEChERS) method. At 5, 7, and 9 days after the first application, fluazinam, carbendazim and procymidone, respectively, had no protective effect even when applied with mineral oil. Greater control efficacy was achieved after a second application. Fluazinam showed protective activity up to 4 days while carbendazim and procymidone showed about 10 days. The mineral oil only enhanced protection activity of procymidone in the second application. The analytical chemical separation indicated that the relative concentration of fluazinam decreased at a faster rate than that of carbendazim and procymidone. At day 14, the concentration of carbendazim and procymidone decreased from 34.97 to 9.39 mg kg−1, and 58.98 to 11.22 mg kg−1, respectively, suggesting that 10–14 days represent the approximate timing in spacing two consecutive applications. The knowledge of the protection period of these fungicides sets the basis to improving their timing of applications.

Mining germplasm panels and phenotypic datasets to identify loci for resistance to Phytophthora sojae in soybean.

Phytophthora sojae causes Phytophthora root and stem rot of soybean and has been primarily managed through deployment of qualitative Resistance to P. sojae genes (Rps genes). The effectiveness of each individual or combination of Rps gene(s) depends on the diversity and pathotypes of the P. sojae populations present. Due to the complex nature of P. sojae populations, identification of more novel Rps genes is needed. In this study, phenotypic data from previous studies of 16 panels of plant introductions (PIs) were analyzed. Panels 1 and 2 consisted of 448 Glycine max and 520 G. soja, which had been evaluated for Rps gene response with a combination of P. sojae isolates. Panels 3 and 4 consisted of 429 and 460 G. max PIs, respectively, which had been evaluated using individual P. sojae isolates with complex virulence pathotypes. Finally, Panels 5-16 (376 G. max PIs) consisted of data deposited in the USDA Soybean Germplasm Collection from evaluations with 12 races of P. sojae. Using these panels, genome-wide association (GWA) analyses were carried out by combining phenotypic and SoySNP50K genotypic data. GWA models identified two, two, six, and seven novel Rps loci with Panels 1, 2, 3, and 4, respectively. A total of 58 novel Rps loci were identified using Panels 5-16. Genetic and phenotypic dissection of these loci may lead to the characterization of novel Rps genes that can be effectively deployed in new soybean cultivars against diverse P. sojae populations.

First report on the pathotype diversity of Phytophthora sojae in Manitoba, Canada

Phytophthora root and stem rot of soybean is caused by the oomycete pathogen Phytophthora sojae, which can infect plants at all stages of growth when soil conditions favour pathogen development. In Manitoba, the acreage of commercial soybean has increased dramatically in recent years. Root and stem rot caused by P. sojae was first detected in commercial soybean in Manitoba in 2011. The objectives of this study were to characterize the pathotype diversity of P. sojae infecting common soybean varieties in Manitoba and identify Rps genes effective against this population. Forty-four, 77 and 89 soybean fields located in the major production areas in Manitoba were surveyed in 2014, 2016 and 2017, respectively. Pathogen collections of 32 (2014), 34 (2016) and 35 (2017) isolates were made each year and evaluated for pathotype classification. Pathotypes of P. sojae were identified based on the pattern of their reactions on a differential set, which included the Rps genes 1a, 1b, 1c, 1d, 1k, 3a, 6 and 7. The most common pathotype was 1a, 1c, 7 (race 4), which comprised 45% of the 101 isolates examined, followed by the pathotypes 1a, 1b, 1c, 1k, 7 (race 25) and 1a, 1b, 1k, 7 (race 28), comprising 34% and 11% of the isolates, respectively. The resistance genes Rps1c and 1k are the most common genes deployed in Manitoba. Based on the results of this study, stacking of resistance genes Rps1d, Rps3a and Rps6 in soybean cultivars targeted for deployment in Manitoba would prove useful in reducing the severity of Phytophthora root and stem rot.