Revisiting the intron hypothesis of QoI resistance in Phyllosticta ampelicida, the causal agent of grape black rot, and other Phyllosticta species

Abstract

Chemical control of grape black rot, caused by Phyllosticta ampelicida, relies mainly on the use of demethylation inhibitors (DMIs) and quinone outside inhibitors (QoIs). The effectiveness of QoI fungicides is influenced by alternative respiration activity, and the exon/intron structure and point mutations in the target protein's gene, the cytochrome b (cytb) gene. Our study aims to investigate the QoI fungicide sensitivity of 48 P. ampelicida isolates in vitro by measuring EC50 and the molecular characteristics of the cytb gene and its mRNA in P. ampelicida and other Phyllosticta species. Mycelial growth tests revealed that the P. ampelicida isolates were sensitive to both azoxystrobin and trifloxystrobin; baseline EC50 values were 0.029 and 0.022 μg/mL, respectively. Addition of salicylhydroxamic acid (SHAM) resulted in lower EC50 values (0.024 and 0.017 μg/mL, respectively). None of the typical point mutations conferring resistance to QoIs in some fungi were detected. A group I intron was present right after the 143rd codon in the cytb gene in four of the six Phyllosticta species examined. The sequence and exon/intron structure of the cytb gene of P. ampelicida isolated from Vitis vinifera is studied in detail and published here. Our results indicate a low risk of QoI resistance development via the G143A mutation in P. ampelicida.