How knowledge of pathogen population biology informs management of Septoria nodorum blotch on wheat

Septoria nodorum blotch (SNB) of wheat is caused by a complex of related fungi that infect both leaves and ears. The most intensively studied member of the species complex is Parastagonospora nodorum, which causes the majority of SNB. While P. nodorum is best known as a model pathogen for elucidating the inverse gene-for-gene interaction, knowledge of its population biology is also quite advanced. P. nodorum emerged as a wheat pathogen during the domestication of wheat in the Fertile Crescent. It then followed the spread of wheat agriculture around the world, likely moving on infected seed. Field populations around the world contain high levels of genetic and phenotypic diversity. This high diversity reflects the population biology of the fungus, which includes regular cycles of sexual recombination, high amounts of gene flow over regional scales, and large population sizes at the field scale. One hectare of a moderately infected field is estimated to contain about 1.8 million strains of P. nodorum that produce approximately 10 trillion spores that include about 70 million adapted mutant spores during a growing season. This standing diversity enables rapid adaptation to any selective agent, including deployments of fungicides and resistant cultivars. The key SNB management goal is to reduce the total pathogen population size, aiming to reduce the number of ascospores produced during and between growing seasons. One strategy that can be used to achieve this goal is to implement the practices of dynamic diversity across a regional scale.