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

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

Supplementary data: Combined biotic and abiotic stress resistance in tomato

Abiotic and biotic stress factors are the major constrains for the realization of crop yield potential. As climate change progresses, the spread and intensity of abiotic as well as biotic stressors is expected to increase, with increased probability of crops being exposed to both types of stress. Shielding crops from combinatorial stress requires a better understanding of the plant’s response and its genetic architecture. In this study, we evaluated resistance to salt stress, powdery mildew and to both stresses combined in tomato, using the S. habrochaites LYC4 introgression line (IL) population. The IL population segregated for both salt stress tolerance and powdery mildew resistance. Using SNP array marker data, QTLs were identified for salt tolerance as well as Na+ and Cl- accumulation. Salt stress increased the susceptibility of the population to powdery mildew in an additive manner. Phenotypic variation for disease resistance was reduced under combined stress as indicated by the coefficient of variation (CV). No correlation was found between disease resistance and Na+ and Cl- accumulation under combined stress Most genetic loci were specific for either salt stress tolerance or powdery mildew resistance. These findings increase our understanding of the genetic regulation of responses to abiotic and biotic stress combinations and can provide leads to more efficiently breeding for tomatoes and other crops with a high level of disease resistance while maintaining their performance in combination with abiotic stress

Fig 3 -

a. Genetic linkage map of Pc (TTTG) resistance constructed with Illumina 6K SNPs and oat F5:6 RILs derived from a cross between PI 258731 and PI 573582. The flanking SNP markers are highlighted in red color. b. Genetic linkage map of Pc (QTRG) resistance constructed with Illumina 6K SNPs and oat F5:6 RILs derived from a cross between PI 258731 and PI 573582. The flanking SNP markers are highlighted in red color.</p

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

Phenotypic scores for disease severity (DS) at LSU, UFL and MN locations and phenotypic scores for infection reaction (IR) and coefficient of infection (CI) at LSU and UFL locations in the RIL population.

The y-axis in each plot represents the number of lines from the population that display scores shown in the x-axis, while locations are shown on the right y-axis; LSU = Louisiana State University Central Research Station in Baton Rouge, LA, UFL = University of Florida crown rust field nursery in Gainesville, FL, MN = University of Minnesota Matt Moore buckthorn nursery in Saint Paul, MN. Mean score of the parents is indicated in the legend.</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

Fig 5 -

Disease severity effect plots for major QTL (QPc.FD-AS-AA4), based on the allelic distributions determined at the peak SNP marker of the QTL (GMI_DS_LB_3547) at A. LSU location B. UFL location. The two possible alleles are indicated in each group (AA = alleles associated with resistant parent, BB = alleles associated with susceptible parent).</p