Host Pathogen Interaction between Flax and Septoria linicola and the Genetics of Resistance

Pasmo disease in flax is caused by the fungus Septoria linicola (Speg.) Garassini (sexual state Mycosphaerella linorum Naumov. is a major disease affecting flax (Linum usitatissimum L.), and is prevalent in most flax growing areas worldwide. In Canada, the incidence and severity of pasmo have been on the rise causing major reductions in seed yield and quality. The objective of this research is to study the virulence in the pathogen population in western Canada. Diseases specimens have been collected from flax crops in western Canada and 100s of single spore isolates established. Seven representative isolates were tested on a set of 94 flax genotypes from diverse origins. Twenty-day old seedlings were inoculated with spore suspension and incubated for 48 h under controlled growth cabinet conditions equipped with a misting system. The disease incidence, severity, and defoliation were recorded at 10, 20, and 30 days after inoculation. The data showed some similarities among the interactions of the seven isolates on some of the 94 flax genotypes. The specific host-pathogen interactions were grouped into 42 virulence combinations indicating the complexity of the virulence genes in the pathogen and the matching resistance genes in the flax genotypes. Gene-pyramiding of the major race-specific genes will provide improved flax genotypes with wider genetic base for resistance to the diverse pathogen population in the flax crops in western Canada. Genetic analysis of pasmo resistance was performed. A total of 1,346 resistance gene analogs (RGAs) were identified in the flax genome. On these RGAs, 21,770 single nucleotide polymorphisms (SNPs) were detected from the flax core collection of 407 accessions, with an average of 16 SNPs per RGA. Association study revealed that 457 RGAs with a total of 2,879 SNPs were significantly correlated with pasmo resistance (p < 0.05/21,770 = 2.2967E-6), contributing averaged 9% (6-19%) of the total pasmo resistance variation. Those RGAs were located on all 15 chromosomes and some of them were clustered to 78 RGA clusters. Those RGAs will be candidates for further quantitative trait loci mapping and genomic selection