Field Pea (Pisum sativum) Germplasm Screening for Seedling Ascochyta Blight Resistance and Genome-Wide Association Studies Reveal Loci Associated with Resistance to Peyronellaea pinodes and Ascochyta koolunga

Ascochyta blight is a damaging disease that affects the stems, leaves, and pods of field pea (Pisum sativum) and impacts yield and grain quality. In Australia, field pea Ascochyta blight is primarily caused by the necrotrophic fungal species Peyronellaea pinodes and Ascochyta koolunga. In this study, we screened 1,276 Pisum spp. germplasm accessions in seedling disease assays with a mix of three isolates of P. pinodes and 641 accessions with three mixed isolates of A. koolunga (513 accessions were screened with both species). A selection of three P. sativum accessions with low disease scores for either pathogen, or in some cases both, were crossed with Australian field pea varieties PBA Gunyah and PBA Oura, and recombinant inbred line populations were made. Populations at the F3:4 and F4:5 generation were phenotyped for their disease response to P. pinodes and A. koolunga, and genotypes were determined using the diversity arrays technology genotyping method. Marker-trait associations were identified using a genome-wide association study approach. Trait-associated loci were mapped to the published P. sativum genome assembly, and candidate resistance gene analogues were identified in the corresponding genomic regions. One locus on chromosome 2 (LG1) was associated with resistance to P. pinodes, and the 8 Mb genomic region contains 156 genes, two of which are serine/threonine protein kinases, putatively contributing to the resistance trait. A second locus on chromosome 5 (LG3) was associated with resistance to A. koolunga, and the 35 Mb region contains 488 genes, of which five are potential candidate resistance genes, including protein kinases, a mitogen-activated protein kinase, and an ethylene-responsive protein kinase homolog.

Assessment of the Effect of Seed Infection with Ascochyta pisi on Pea in Western Canada

The role of seed infection with Ascochyta pisi using naturally infected seeds with an incidence from 0.5 to 14.5% was studied in field pea experiments in western Canada at locations with historically low inoculum pressure. A significant effect of A. pisi seed infection on the emergence of seedlings was observed in one experiment and when all data were pooled, but emergence was only reduced minimally, and symptoms of A. pisi on the aerial parts of the seedlings were rarely observed. The level of seed infection at planting had no impact on A. pisi disease severity on mature plants, on seed yield and size, or on the incidence of A. pisi infection of harvested seeds although A. pisi was the dominant species recovered from seeds. Results suggest that the disease did not progress significantly from seeds to seedlings, hence did not contribute to infection of aerial parts of the plants, and therefore infected seeds cannot be regarded as a source of inoculum in the epidemiology of this pathogen under western Canadian growing conditions. Assessing seed components of seeds with varying levels of A. pisi infection and seed staining revealed that the pathogen was present in all components of the seed, regardless of the severity of seed staining. This indicates that infected seeds may be an important way for the pathogen to survive in nature.