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Abstract
Magnaporthe oryzae is a fungal pathogen with a broad host range that threatens major Poaceae species, including rice, wheat, and oats, causing significant yield and quality losses. While research has focused on rice, its adaptability to non-rice hosts remained a concern, particularly in the U.S., where the pathogen causes gray leaf spot (GLS) in turfgrasses. The emergence of virulent pathotypes necessitates a deeper understanding of resistance mechanisms and host specificity for a durable disease management. This study integrated genome-wide association studies (GWAS) and pathogen characterization to address these challenges. A GWAS was conducted on 134 elite IRRI-Green Super Rice cultivars, revealing 12 quantitative trait loci (QTL) associated with blast resistance against two virulent isolates, M101 and M64. Five QTLs were detected for M101 and seven for M64, with notable resistance loci identified within the Pib and Pi-ta/Ptr gene clusters. Additionally, novel resistance sources were discovered, underscoring the potential for breeding improved blast-resistant rice varieties. Expanding the investigation into tall fescue, a GWAS of 283 accessions identified three marker-trait associations (MTAs) for GLS resistance. Candidate genes associated with these MTAs included leucine-rich repeat (LRR) proteins, F-box domains, and protein kinases—key components of pathogen recognition and defense signaling. To characterize host specificity and genomic diversity, 15 M. oryzae isolates from turfgrasses and oats were used. Out of these, three isolates—Fa_GA_GLS and Fa_MO_GLS from tall fescue and As_GA_GLS from oats—were assessed for pathogenicity on 13 Poaceae species: ryegrass, tall fescue, oat, and barley exhibited the highest susceptibility, while millet and wheat showed moderate infection. Phylogenetic analysis based on whole-genome sequencing data from Nanopore technology placed As_GA_GLS within the Lolium clade, while Fa_GA_GLS and Fa_MO_GLS clustered within the Festuca clade, providing evidence of lineage-specific host adaptation. This was further supported by multi-locus molecular typing using 11 housekeeping genes from these three isolates, along with all other isolates, which revealed host specialization among the isolates. The findings exemplified in this dissertation offer insights into M. oryzae-host interactions, resistance loci in rice and tall fescue, and pathogen adaptability, aiding in the breeding of disease-resistant cereals and turfgrasses while enhancing our understanding of the genetic structure of emerging U.S. isolates.