Detection of the F129L mutation in the cytochrome b gene of Phyllosticta citricarpa and reduced sensitivity to quinone-outside inhibitor fungicides

Fruit protection against infection by Phyllosticta citricarpa , the causal agent of citrus black spot (CBS), relies heavily on fungicides. Quinone-outside inhibitors (QoIs), which have a site-specific mode of action, have been in use since the early 2000s in South African citrus orchards. This study investigated the occurrence of the F129L, G137R or G143A mutations in codons 129, 137 and 143, respectively, in cytochrome b ( cytb ) of P. citricarpa . Molecular analysis detected the F129L mutation in cytb of 73 of 191 isolates tested, but the G143A and G137R mutations were not detected. A non-synonymous nucleotide substitution from TTC to TTA was predominantly detected in 69 isolates, while a codon change from TTC to CTC was detected in only four isolates. Both mutations result in an amino acid substitution from phenylalanine to leucine. To investigate the practical impact of this mutation on the field efficacy of QoI fungicides, trials were performed on a lemon orchard with a history of high CBS pressure and 100% detection rate of the F129L mutation. Results revealed a strong association between the occurrence of the F129L mutation and reduced efficacy in controlling CBS by pyraclostrobin and azoxystrobin, when applied outside the requirements of registration, i.e. not in a mixture with a fungicide from another FRAC group. This study is the first report of the F129L mutation and a shift in sensitivity to QoI fungicides in P. citricarpa populations.