Resistance to thiophanate‐methyl and azoxystrobin in Colletotrichum species from Brazilian grapes

BACKGROUND: Grape ripe rot (GRR) caused by Colletotrichum species can cause yield loss in vineyards in Southern and Northeast Brazil. Thiophanate-methyl (FRAC 1) and azoxystrobin (FRAC 11) fungicides are frequently used to control GRR, but resistance to these fungicides in Colletotrichum species may impact their efficacy. RESULTS: Sensitivity to thiophanate-methyl was assessed for 89 isolates of the C. gloeosporioides species complex using mycelial growth inhibition. Isolates were classified as sensitive (n = 2), low-resistant (n = 70) or highly resistant (n = 17). Highly resistant isolates had nucleotide mutations at amino acid codons 198 and 200 in the β-tubulin gene. Sensitivity to azoxystrobin was assessed for 176 isolates belonging to the C. gloeosporioides and C. acutatum species complexes using conidial germination inhibition. Of these, 33 isolates were resistant, and salicylhydroxamic acid (SHAM) was required to detect those carrying the G143A amino acid substitution in the cytochrome b (cytb) gene. Detached fruit assays revealed that thiophanate-methyl and azoxystrobin applied at label rates were not effective against highly resistant and resistant isolates, respectively. Seven Colletotrichum spp. isolates were resistant to both fungicides. CONCLUSION: This study is the first to report Colletotrichum spp. isolates from Brazilian vineyards resistant to azoxystrobin, thiophanate-methyl, or both. Resistance was associated with β-tubulin (E198A, F200Y) and cytb (G143A) mutations. These results demonstrate the risk of control failure with FRAC 1 and 11 fungicides and highlight the need for resistance management strategies in vineyards.