Date of Award


Document Type


Degree Name

Master of Science (MS)


Plant and Environmental Science

Committee Chair/Advisor

Guido Schnabel

Committee Member

Julia Kerrigan

Committee Member

James Faust


Apple bitter rot is caused by Colletotrichum nymphaeae and other Colletotrichum species 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 relatively low EC50 values (the concentration required to inhibit 50% of mycelial growth in vitro) of C. nymphaeae isolates 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 either sensitive (lowest EC50 values) and reduced-sensitive (highest EC50 values). The objective of this research was to confirm stability of EC50 values after storage, assess fluazinam and tebuconazole for apple bitter rot management in detached fruit studies, and to sequence the fungicide’s putative target genes. Our research confirmed stability of EC50 values 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 CYP51B genes from C. nymphaeae isolates sensitive or reduced-sensitive to fluazinam (potential target OS-1) and tebuconazole (CYP51A and CYP51B) were sequenced and three nucleotide changes located in two codons of OS-1 leading to three amino acid changes were identified in reduced-sensitive isolates. This study underscores the potential usefulness for fluazinam and tebuconazole for bitter rot control of apple and provides the genetic basis for studies on potential resistance mechanisms.



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