How large and diverse are field populations of fungal plant pathogens? The case of Zymoseptoria tritici

Abstract

Pathogen populations differ in the amount of genetic diversity they contain. Populations carrying higher genetic diversity are thought to have a greater evolutionary potential than populations carrying less diversity. We used published studies to estimate the range of val-ues associated with two critical components of genetic diversity, the number of unique pathogen genotypes and the number of spores produced during an epidemic, for the septo-ria tritici blotch pathogen Zymoseptoria tritici. We found that wheat fields experiencing typi-cal levels of infection are likely to carry between 3.2 and 14.9 million pathogen genotypes per hectare and produce at least 2.3 to 10.5 trillion pycnidiospores per hectare. Given the experimentally derived mutation rate of 3 x 10-10 substitutions per site per cell division, we estimate that between 28 and 130 million pathogen spores carrying adaptive mutations to counteract fungicides and resistant cultivars will be produced per hectare during a growing season. This suggests that most of the adaptive mutations that have been observed in Z. tritici populations can emerge through local selection from standing genetic variation that already exists within each field. The consequences of these findings for disease management strategies are discussed.

The impact of wheat cultivar mixtures on virulence dynamics in Zymoseptoria tritici populations persists after interseason sexual reproduction

“…An understanding of the interactions between these factors will clearly require modelling approaches, but also further experimental fungal biology studies in semicontrolled conditions (e.g., Orellana-Torrejon et al, 2022b) to characterize the abovementioned mechanisms in greater detail. The potential impact of resistance deployment strategies, including cultivar mixtureswhich may be beneficial, but also detrimental over the years-remain to be fully assessed considering both the size and the genetic diversity of a pathogen population (McDonald & Linde, 2002;McDonald et al, 2022). Our conclusions were drawn at field scale, but it should clearly be borne in mind that the impact of mixtures is diluted at greater spatial scales when considering inoculum exchanges between fields planted with different cultivars (e.g., Fabre et al, 2015).…”

Preventing multiple resistance above all: new insights for managing fungal adaptation

“…However, delaying the emergence of single-resistant isolates and limiting the probability of recombination between them by ensuring that their frequencies remain low might be a more realistic option. It has been estimated that Z. tritici produces at least 2.1-9.9 trillion pycnidiospores per hectare (McDonald et al, 2022). The number of de novo mutation events associated with resistance occurring in a field may, therefore, be sufficiently high to prevent all of these mutations being lost by chance, even if the mutation rate is low.…”