Reduced sensitivity to fluazinam and tebuconazole in Colletotrichum nymphaeaefrom apple and investigation of target gene sequences

Apple bitter rot is caused by Colletotrichum nymphaeaeand other Colletotrichumspecies and management relies primarily on synthetic pesticides. Very few fungicides are effective against the disease and resistance has further limited their usefulness. A recent study indicated a range of EC50values (the concentration required to inhibit 50% of mycelial growth in vitro) of C. nymphaeaeisolates from Brazilian apples to fluazinam and tebuconazole, two fungicides that are not routinely used for bitter rot control. Isolates on opposite sides of the sensitivity range were designated sensitive (low EC50values) and reduced-sensitive (high EC50values). The objective of this research was to confirm stability of EC50values after storage, assess fluazinam and tebuconazole efficacy against reduced-sensitive isolates for apple bitter rot management in detached fruit studies, and to sequence the fungicide’s putative target genes. Our research confirmed stability of EC50values after 8 months of cold storage. Detached fruit studies revealed that protective treatment of cv. Gala apple fruit with fluazinam and tebuconazole controlled both sensitive and reduced-sensitive phenotypes but if used curatively (after infection) reduced-sensitive isolates were controlled less effectively. The complete OS-1, CYP51A, and CYP51Bgenes from C. nymphaeaeisolates sensitive or reduced-sensitive to fluazinam (potential target OS-1) and tebuconazole (CYP51Aand CYP51B) were sequenced. This study underscores the potential usefulness of fluazinam and tebuconazole for bitter rot control of apple but also indicates weaknesses when products were to be applied post infection.