Phytophthora Sojae Infecting Soybean: Pathotype Diversity, New Sources of Resistance and Interaction with the Soybean Cyst Nematode

Phytophthora root and stem rot, caused by Phytophthora sojae Kaufmann and Gerdemann, is an important disease of soybean (Glycine max L.) in South Dakota. To gain a better understanding of the importance of P. sojae in South Dakota, specifically pathotype diversity, identification of new resistance sources and the interaction with the soybean cyst nematode (Heterodera glycines Ichinohe, SCN), this research was undertaken with the following objectives - 1) to characterize the pathotype diversity of P. sojae causing Phytophthora root and stem rot on soybean in commercial fields in South Dakota; 2) to compare inoculation methods to evaluate for partial resistance to P. sojae on soybean and identify new sources of resistance to two virulence pathotypes of P. sojae in a recombinant inbred line (RILs) population derived from the cross between cultivated Glycine max (cv. Surge) and wild Glycine soja (PI 468916); and 3) to study the interaction between SCN and P. sojae on soybean. In order to achieve the objectives, a total of 114 isolates of P. sojae were recovered from soil samples covering 30 counties in South Dakota during a three year survey (2013 - 2015), of which 70 P. sojae isolates were pathotyped using 13 standard soybean differentials. Results suggest that mean complexity of the P. sojae pathotypes have increased over time and over 85% of the P. sojae isolates were able to defeat Rps1a, Rps1c and Rps1k that are commonly deployed Rps genes in the commercial cultivars of South Dakota. In order to find new sources of partial resistance to P. sojae, a qualitative comparison among three inoculation methods (inoculum layer test, tray test and rice grain inoculation) was accomplished in the greenhouse. Based on the recovery of P. sojae isolates (%), inoculum layer method was adopted to screen 100 recombinant inbred line (RIL) for partial resistance to two virulence pathotypes of P. sojae identified in South Dakota [PS-15-TF3 that is virulent on all 13 soybean differentials and PS-14-F14 that is virulent on only one differential (Rps7)]. As compared to the parents of the RIL population, [Glycine max (cv. Surge) and wild Glycine soja (PI 468916)] we found 9 RILs that had relatively shorter lesion length (0 to 5 mm) when inoculated with either of the P. sojae isolates. To study the interaction between SCN and P. sojae on soybean, a greenhouse experiment was set up in a completely randomized design in a factorial arrangement with four soybean cultivars (Jack, Surge, William 82 and Williams). Two isolates of P. sojae representing two different virulent pathotypes (PS-15-TF3 and PS-14-F14) and SCN HGtype 0 representing the most commonly found HG-type in South Dakota was used to perform inoculations. For all the cultivars, we observed that the lesion length was caused by P. sojae was increased in the presence of SCN relative to P. sojae treatment. However, SCN population was reduced in the presence of both the pathogens. The findings of our study highlight the high pathotype diversity of P. sojae and and increased lesion size when P. sojae co-infects with SCN. This information will help with the development of effective and improved strategies for managing Phytophthora root and stem rot through deployment of resistant genes in commercial soybean varieties that are likely to be more durable, managing SCN to reduce severity of Phytophthora root rot, and incorporation of identified resistance to P. sojae in RIL population for future breeding efforts

Common <i>Phytophthora sojae</i> Pathotypes Occurring in South Dakota

Phytophthora root and stem rot, caused by Phytophthora sojae, is an important disease of soybean (Glycine max L.) in South Dakota. Because P. sojae populations are highly diverse and resistance genes deployed in commercial soybean varieties often fail to protect against this pathogen, this study was initiated to determine P. sojae pathotypes occurring in South Dakota. A total of 216 P. sojae isolates were baited from soil collected from 422 soybean fields in 2013 to 2015 and 2017. The pathotype of each isolate was determined by inoculating 10 seedlings of 13 standard soybean P. sojae differential lines using the hypocotyl inoculation technique. Of the 171 pathotyped isolates, 47 unique pathotypes were identified. The virulence complexity of isolates ranged from virulence on one Rps gene (Rps7) to virulence on 13 Rps genes, and mean complexity was 5.2. Harosoy (Rps7), Harlon (Rps1a), Williams 79 (Rps1c), Williams 82 (Rps1k), and Harosoy 13XX (Rps1b) were susceptible to 98, 80, 78, 73, and 72% of the isolates, respectively. These results highlight the highly diverse P. sojae pathotypes in South Dakota and the likely Rps genes to be ineffective in commercial soybean varieties. </jats:p

Examining the Interaction between <i>Phytophthora sojae</i> and Soybean Cyst Nematode on Soybean (<i>Glycine max</i>)

Phytophthora sojae and soybean cyst nematode (SCN) are important pathogens of soybean. Although these pathogens infect soybean roots, there is limited evidence of any interaction between them. The objective of this study was to examine the interaction between SCN and P. sojae on soybean in the greenhouse. Seeds of four soybean cultivars (Jack, Surge, Williams 82, Williams) were pre-germinated and placed in cone-tainers (Stuewe and Sons Inc., Tangent, OR, USA), containing a steam pasteurized sand-clay mixture. The experiment was set up in a completely randomized design with five replications and performed twice. Two P. sojae isolates were used in this study that represented two different virulence pathotypes (simple and complex pathotypes). For each isolate, soybean plants were not inoculated, inoculated with one of the treatments—SCN, P. sojae, and combination of P. sojae and SCN. After 35 DOI, stem length, root length, plant weight, root weight, lesion length, and SCN population were recorded. On all soybean cultivars with different types of incomplete resistance, the complex pathotype (PS-15-TF3) influenced the lesion length (mm) in the presence of SCN. However, the SCN population was reduced by both complex and simple pathotypes of P. sojae. This suggests that use both SCN and P. sojae resistance cultivars, can manage the disease complex and reduce soybean yield loss

Examining the Interaction between Phytophthora sojae and Soybean Cyst Nematode on Soybean (Glycine max)

Phytophthora sojae and soybean cyst nematode (SCN) are important pathogens of soybean. Although these pathogens infect soybean roots, there is limited evidence of any interaction between them. The objective of this study was to examine the interaction between SCN and P. sojae on soybean in the greenhouse. Seeds of four soybean cultivars (Jack, Surge, Williams 82, Williams) were pre-germinated and placed in cone-tainers (Stuewe and Sons Inc., Tangent, OR, USA), containing a steam pasteurized sand-clay mixture. The experiment was set up in a completely randomized design with five replications and performed twice. Two P. sojae isolates were used in this study that represented two different virulence pathotypes (simple and complex pathotypes). For each isolate, soybean plants were not inoculated, inoculated with one of the treatments—SCN, P. sojae, and combination of P. sojae and SCN. After 35 DOI, stem length, root length, plant weight, root weight, lesion length, and SCN population were recorded. On all soybean cultivars with different types of incomplete resistance, the complex pathotype (PS-15-TF3) influenced the lesion length (mm) in the presence of SCN. However, the SCN population was reduced by both complex and simple pathotypes of P. sojae. This suggests that use both SCN and P. sojae resistance cultivars, can manage the disease complex and reduce soybean yield loss.</jats:p

HCPro Suppression of Callose Deposition Contributes to Strain-Specific Resistance Against Potato Virus Y

Potato virus Y (PVY; Potyviridae) is a continuing challenge for potato production owing to the increasing popularity of strain-specific resistant cultivars. Hypersensitive resistance (HR) is one type of plant defense responses to restrict virus spread. In many potato cultivars, such as cultivar Premier Russet (PR), local necrosis at the site of infection protects against the most common PVYO strain, but the HR often fails to restrain necrotic strains, which spread systemically. Here, we established the role of callose accumulation in the strain-specific resistance responses to PVY infection. We first uncovered that PVY, independent of the strain, is naturally capable of suppressing pathogenesis-related callose formation in a susceptible host. Such activity can be dissociated from viral replication by the transient expression of the viral-encoded helper component proteinase (HCPro) protein, identifying it as the pathogen elicitor. However, unlike the necrotic strain, PVYO and its corresponding HCPro are unable to block callose accumulation in resistant PR potatoes, in which we observed an abundance of callose deposition and the inability of the virus to spread. The substitution of eight amino acid residues within the HCPro C-terminal region that differ between PVYO and PVYN strains and were previously shown to be responsible for eliciting the HR response, are sufficient to restore the ability of HCProO to suppress callose accumulation, despite the resistant host background, in line with a new viral function in pathogenicity. </jats:p

Mapping crown rust resistance in the oat diploid accession PI 258731 (Avena strigosa).

Oat crown rust, caused by Puccinia coronata Corda f. sp. avenae Eriks. (Pca), is a major biotic impediment to global oat production. Crown rust resistance has been described in oat diploid species A. strigosa accession PI 258731 and resistance from this accession has been successfully introgressed into hexaploid A. sativa germplasm. The current study focuses on 1) mapping the location of QTL containing resistance and evaluating the number of quantitative trait loci (QTL) conditioning resistance in PI 258731; 2) understanding the relationship between the original genomic location in A. strigosa and the location of the introgression in the A. sativa genome; 3) identifying molecular markers tightly linked with PI 258731 resistance loci that could be used for marker assisted selection and detection of this resistance in diverse A. strigosa accessions. To achieve this, A. strigosa accessions, PI 258731 and PI 573582 were crossed to produce 168 F5:6 recombinant inbred lines (RILs) through single seed descent. Parents and RILs were genotyped with the 6K Illumina SNP array which generated 168 segregating SNPs. Seedling reactions to two isolates of Pca (races TTTG, QTRG) were conditioned by two genes (0.6 cM apart) in this population. Linkage mapping placed these two resistant loci to 7.7 (QTRG) to 8 (TTTG) cM region on LG7. Field reaction data was used for QTL analysis and the results of interval mapping (MIM) revealed a major QTL (QPc.FD-AS-AA4) for field resistance. SNP marker assays were developed and tested in 125 diverse A. strigosa accessions that were rated for crown rust resistance in Baton Rouge, LA and Gainesville, FL and as seedlings against races TTTG and QTRG. Our data proposed SNP marker GMI_ES17_c6425_188 as a candidate for use in marker-assisted selection, in addition to the marker GMI_ES02_c37788_255 suggested by Rine's group, which provides an additional tool in facilitating the utilization of this gene in oat breeding programs

Fig 1 -

Avena strigosa primary leaf infection type (IT) phenotypes inoculated with two Pca races TTTG and QTGB and shown 14 dpi; A. Susceptible parent, PI573582, with IT 4 B and C. PI 258731 carrying resistance with two different IT: “; N” (TTTG) and “0N” (QTRG).</p

Frequency of the resistant parent allele for three putative PACE markers in each phenotypic group.

Frequency of the resistant parent allele for three putative PACE markers in each phenotypic group.</p

Logarithm of odds (LOD) score profiles of quantitative trait loci (QTL) for field phenotype adjusted for PI 258731 seedling resistance allele, colored dotted lines on linkage group 07 based on multiple interval mapping.

A threshold of LOD = 3.2 was determined by a permutation test with 1000 iterations (P<0.05).</p