LAUSR

Emergence of pathogens with decreased sensitivity to succinate dehydrogenase inhibitor fungicides is a global agronomical issue. Analysis of Didymella tanaceti isolates (n = 173), which cause tan spot of pyrethrum (Tanacetum cinerariifolium), collected prior to (2004-2005) and after (2009, 2010, 2012 and 2014) the commercial implementation of boscalid in Tasmanian pyrethrum fields identified that insensitivity developed over time and has become widespread. To evaluate temporal change isolates were characterized for frequency of mutations in the succinate dehydrogenase (Sdh) B, C and D subunits associated with boscalid resistance, mating-type and SSR genotype. All isolates from 2004 and 2005 exhibited wild-type (WT) Sdh alleles. Seven known Sdh substitutions were identified in isolates collected from 2009 to 2014. In 2009 60.7% had Sdh substitutions associated with boscalid resistance in D. tanaceti. The frequency of WT isolates decreased over time, with no WT isolates identified in 2014. The frequency of the SdhB-H277Y genotype increased from 10.7 to 77.8% between 2009-2014. Genotypic evidence suggested a shift in the population structure occurred between 2005 and 2009 with decreases 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 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.