Field pea (Pisum sativum L.) 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 stems, leaves and pods of field pea 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 have screened 1,276 Pisum spp. germplasm accessions in seedling disease assays with P. pinodes and 641 accessions with A. koolunga (513 isolates 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 to Australian field pea varieties PBA Gunyah and PBA Oura, and recombinant inbred line populations 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.