Performance Story: Fusarium head blight resistance to triazole fungicides in Western Canada
Fusarium head blight (FHB) is the most serious fungal disease affecting wheat production in Canada. Given the susceptibility of most commercial cultivars, application of triazole fungicides is the primary method of FHB management in durum wheat. These fungicides have been used in areas at high risk of FHB in southeast Saskatchewan and Manitoba for over two decades. Analogous to the emergence of antibiotic resistance in human pathogens, long-term application of fungicides enhances insensitivity in populations of plant pathogens. Strains of Fusarium graminearum, which are insensitive to triazole fungicides, have been found in the north eastern United States (Spolti et al., 2014), but prior to this study, the population of F. graminearum in Western Canada had not been studied to determine the presence and prevalence of triazole fungicide insensitivity.
The current project did not identify triazole insensitive F. graminearum isolates in the Prairie Provinces. This key information indicates that stakeholders do not need to significantly change fungicide use in the region. Although insensitive isolates were not identified, a range of sensitivity was observed, suggesting that the sensitivity to triazole fungicides could shift over time. Therefore, continued monitoring will be important.
Monitoring the evolution of the fungicide sensitivity currently requires phenotypic evaluation, done by first isolating the pathogen from infected samples followed by extensive culturing and measuring, which is a costly and time consuming process. The F. graminearum population is highly variable, with a broad range in fungicide sensitivity, aggressiveness, and type of deoxynivalenol (DON) mycotoxin production. The fungicide sensitivity and aggressiveness affect the yield, as well as accumulation of DON mycotoxin, which makes the grain unsafe for consumption. Monitoring based on genetic markers that track genetic variation in the population using rapid DNA tests would be advantageous.
The development of these genetic markers requires a better understanding of the genetic basis for resistance to triazole fungicides. Early on in this study, it was determined that the chemotype groups (3ADON vs. 15ADON) were not a source of variation in fungicide resistance. Subsequently, global patterns of gene expression were analyzed in four isolates with the most extreme sensitivity (highest and lowest) to the triazoles, before and after fungicide treatment. Triazole fungicides work through blocking ergosterol biosynthesis genes (ERG), and many of the differentially expressed genes identified were involved in various steps of the ERG pathway, or regulation of the pathway. By comparing the protein coding portion of these differentially expressed genes across the nine Fusarium genome assemblies developed through this project, including isolates used for gene expression analyses, a list of candidate genes that could lead to lower sensitivity to the triazole fungicide metconazole in the population of F. graminearum was generated.