# A new method to measure EC50 reveals cultivar‐specific fungicide resistance and very high diversity within experimental field populations of Zymoseptoria tritici

## TL;DR

A new method to measure fungicide resistance in wheat disease reveals high diversity and cultivar-specific interactions.

## Contribution

A novel high-throughput method to measure EC50 values in Zymoseptoria tritici, revealing cultivar-specific resistance patterns.

## Key findings

- Field populations of Zymoseptoria tritici maintain high fungicide sensitivity diversity despite multiple fungicide treatments.
- Wheat cultivars resistant to Septoria tritici blotch are colonized by more fungicide-resistant Z. tritici strains.
- Host–fungicide–pathogen interactions are supported by specific cultivars selecting for resistance to specific fungicides.

## Abstract

Zymoseptoria tritici causes Septoria tritici blotch (STB), the most damaging wheat disease in Europe. In Europe, STB is controlled mainly by fungicides and fungicide resistance is frequently reported. Although fungicide resistance is thought to emerge mainly from standing genetic variation within field populations of Z. tritici, few studies have attempted to quantify the degree of fungicide resistance occurring at the field scale and to measure changes in the frequencies of resistant strains following fungicide applications during a single growing season. Even fewer studies have considered the effects of different wheat cultivars on the emergence of fungicide resistance. We developed a new high‐throughput method based on resazurin dye and image analysis to measure the effective concentration of a fungicide that reduces growth by 50% (EC50) values and applied it to 1005 strains of Z. tritici sampled at two time points from 17 different wheat cultivars growing in a replicated field experiment. The experimental field was treated with combinations of five different active ingredients at three times during the growing season.

We found that field populations of Z. tritici can maintain a very high diversity in fungicide sensitivity phenotypes despite three fungicide treatments, with as much diversity found within a single field during a single growing season as has been described across all of Europe over several years. We discovered that wheat cultivars that were more resistant to STB tended to be colonized by Z. tritici strains that exhibited higher fungicide resistance. We also found that specific wheat cultivars selected for resistance to specific active ingredients, providing the first direct support for the existence of significant host–fungicide–pathogen interactions.

Overall, our findings illustrate the many challenges associated with designing fungicide treatment programs that aim to reduce selection for fungicide resistance when confronted with a pathogen like Z. tritici that has a very high evolutionary potential. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

We isolated Zymoseptoria tritici strains from a diverse wheat field and quantified fungicide resistance using a novel plate assay. We found high diversity in fungicide sensitivity and host–fungicide–pathogen interactions.