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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #399567

Research Project: Breeding, Genomics, and Integrated Pest Management to Enhance Sustainability of U.S. Hop Production and Competitiveness in Global Markets

Location: Forage Seed and Cereal Research Unit

Title: Evolution of the genetic structure of the Didymella tanaceti population during development of succinate dehydrogenase inhibitor resistance

item PEARCE, TAMIEKA - University Of Tasmania
item SCOTT, JASON - University Of Tasmania
item WILSON, CALUM - University Of Tasmania
item Gent, David - Dave

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2023
Publication Date: 11/10/2023
Citation: Pearce, T., Scott, J., Wilson, C., Gent, D.H. 2023. Evolution of the genetic structure of the Didymella tanaceti population during development of succinate dehydrogenase inhibitor resistance. Phytopathology. 113:1946-1958.

Interpretive Summary: Fungicide resistance is a global concern in agriculture because of the potential for management systems to be disrupted and for plant pathogens to cause more damage to crops. In this research, we characterized genetic diversity of isolates of the fungus that causes tan spot of pyrethrum before or after resistance to the fungicide boscalid was detected in Tasmania, Australia. We sought to understand factors that may drive fungicide resistance in this plant pathogen. We found seven genetic mutations in a gene associated with resistance to the succinate dehydrogenase inhibitor class of fungicides. The frequency of certain mutations increased over time, coincident with decreases in genetic diversity of the pathogen. Resistance to the fungicide boscalid was widespread in diverse populations of the tan spot fungus, which suggests that the pathogen has a propensity to develop resistance to this fungicide, fungicide resistance is transmitted through mating in the fungus population, or both.

Technical Abstract: Emergence of pathogens with decreased sensitivity to succinate dehydrogenase inhibitor fungicides is a global agronomical issue. Isolates of Didymella tanaceti (n = 173), which cause tan spot of pyrethrum (Tanacetum cinerariifolium), collected prior to (2004 and 2005) and after (2009, 2010, 2012 and 2014) the commercial implementation of boscalid in Tasmanian pyrethrum fields identified that insensitivity has developed and become widespread. Isolates were characterized for mating-type, SSR genotype, and frequency of mutations in the succinate dehydrogenase (Sdh) B, C and D subunits associated with boscalid resistance. All isolates collected in 2004 and 2005 exhibited wild-type (WT) Sdh alleles. Seven known substitutions were identified in isolates collected from 2009 to 2014. In 2009 60.7% had a Sdh substitution associated with boscalid resistance. The frequency of WT isolates decreased over time, with no WT isolates identified in 2014. The SdhB-H277Y genotype increased in frequency from 10.7 to 77.8% between 2009 and 2014. Genotypic evidence suggested a shift in the population structure occurred between 2005 and 2009 with a decrease in gene diversity (uh; 0.51 to 0.34), genotypic evenness (E5; 0.96 to 0.67), genotypic diversity (G; 9.3 to 6.8) and allele frequencies. No evidence was obtained to support the rapid spread of the Sdh genotypes by clonal expansion of the population. Thus, insensitivity to boscalid has developed and become widespread within a diverse population within 4 years of usage. These results suggest D. tanaceti can disperse insensitivity either through repeated frequent mutation, sexual recombination or a combination of both.