Climatic Variables and Virulence May Contribute to the Population Dynamics of Pyricularia oryzae at Local Scale

Rice blast disease, caused by Pyricularia oryzae Cavara, is the most threatening rice pathogen in Italy. The development of resistant cultivars is a sustainable approach to mitigate yield losses. However, P. oryzae genomic plasticity often allows rapid adaptation to host resistance. Understanding the population structure of plant pathogens is crucial for assessing their genetic diversity and implementing durable management strategies. Despite first attempts from Piotti et al. (2005) to study the Italian P. oryzae population, a gap in current local P. oryzae genetic variability still needs to be addressed. Here, the population structure of a large set of P. oryzae isolates from diseased rice panicles, collected in five different Italian regions between 2011-2012 and 2020-2022, was characterized using SSR genotyping. Mating type was determined to investigate the occurrence of sexual reproduction in Italy. The integration of different cluster analyses of 200 unique multilocus genotypes allowed to identify five distinct genetic clusters. Analysis of molecular variance and of genetic divergence revealed a limited influence of geographic origin and time on population structure. A strong positive correlation was detected between climatic variables and allelic diversity in Piedmont, the most evenly sampled region in this study. The gradual disappearance over time of a genetic cluster could be linked to lower virulence on a susceptible rice cultivar. This study provides new insights into the genetic dynamics of Italian P. oryzae population, supporting the strategic deployment of resistance genes in rice breeding programs.