Citrus black spot (CBS), caused by Phyllosticta citricarpa, is an economically important disease, which is effectively controlled by repeated fungicide applications to protect fruit from infection. Systemic fungicides such as benzimidazoles are widely used for controlling CBS in South Africa, but the molecular mechanisms of benzimidazole resistance in P. citricarpa had not been investigated. Analysis of the nucleotide sequence of the β-tubulin gene in P. citricarpa revealed mutations inducing three amino acid replacements in benzimidazole-resistant isolates when compared with those of sensitive strains. Amino acid replacements in benzimidazole-resistant isolates included the change of glutamic acid to either alanine or lysine at codon 198 of the β-tubulin gene and the change from phenylalanine to tyrosine at codon 200. All three mutations were previously implicated in benzimidazole resistance in several fungal pathogens. A PCR assay was designed to amplify a portion of the β-tubulin gene, which is subsequently sequenced to identify benzimidazole resistance in P. citricarpa. This PCR and sequence assay was found to be a more rapid and reliable method for detecting resistance compared with the fungicide-amended plate tests and is valuable for monitoring the occurrence of benzimidazole-resistant P. citricarpa and for assessment of the need for alternative CBS management practices.
Keywords: DNA sequence; PCR; amino acid; benomyl; bioassays; citrus black spot; sensitivity.