Detection of G137S mutation conferring quinone outside inhibitor resistance in <i>Phyllosticta citricarpa</i> populations from Brazilian citrus orchards

Citrus black spot (CBS), caused by Phyllosticta citricarpa, is one of the most destructive citrus diseases worldwide, leading to fruit blemishes, premature drop, and trade restrictions. In Brazil, CBS control relies primarily on quinone outside inhibitor (QoI) and copper fungicides. During the 2024-2025 growing season, several commercial orchards in the São Paulo (SP) citrus belt exhibited increased CBS severity and reduced QoI effectiveness, suggesting shifts in pathogen sensitivity. Forty isolates of P. citricarpa were collected across the SP citrus belt and compared with historical wild-type isolates from 1998 and 2012. Sequencing of cytb gene fragments containing codons 129, 137, and 143 revealed a G137S substitution in isolates from multiple SP regions, while no mutations were found in wild-type isolates. Conidial germination assays with pyraclostrobin (1 µg/mL) showed rates from 12% to 96% among isolates with G137S mutation, compared with &lt;15% in the 2012 sensitive isolate without G137S mutation, confirming reduced QoI sensitivity. The presence of an intron downstream of codon 143 prevents full resistance (G143A). These findings provide the first molecular evidence of emerging QoI resistance in P. citricarpa populations in Brazil and reinforce the need for fungicide rotation and integrated antiresistance management strategies.