Date of Award


Document Type


Degree Name

Master of Science (MS)

Legacy Department

Plant and Environmental Science

Committee Chair/Advisor

Schnabel, Guido

Committee Member

Kerrigan , Julia

Committee Member

Agudelo , Paula


The United States is the largest producer of strawberries worldwide, generating approximately $2.4 billion worth of fruit each year. Strawberry production, however, is often threatened by pathogens. One of the most destructive fungal pathogens is Botrytis cinerea, the causal agent of gray mold disease. The control of gray mold in commercial fields is largely dependent on the application of fungicides, including the dicarboximide iprodione and the hydroxyanilide fenhexamid. Because both fungicides are prone to resistance development, a survey was conducted to determine the occurrence and prevalence of fungicide resistance. Single-spore isolates were collected from strawberry fields in Florida (for the dicarboximide study), North Carolina, and South Carolina and subjected to a spore-germination assay that distinguishes sensitive from resistant isolates. Of the isolates collected, 16.8% were resistant to fenhexamid, 2% were moderately resistant to iprodione, and 17.6% had low levels of resistance to iprodione. Resistance to the two fungicides was found in almost every location, indicating that resistance was widespread but only accounted for a low percentage of the population. Resistance to fenhexamid was associated with nucleotide mutations in the target gene erg27, resulting in aa changes T63I, F412S, F412C, or F412I. A rapid method was developed based on polymerase chain reaction to specifically detect these mutations. Low and moderate resistance to iprodione was associated with mutations in target gene bos1 resulting in I365N, I365S or a combination of Q369P and N373S, respectively. The I365N/S mutations were also present in five highly resistant isolates that were included to investigate the molecular mechanism of iprodione resistance, but no mutation or mutation combinations in bos1 were uniquely associated with the highly resistant phenotype. Detached strawberry fruit assays indicated that field rates of Elevate 50 WDG (fenhexamid) did not control fenhexamid-resistant isolates, and field rates of Rovral 4 Flowable (iprodione) did not control isolates moderately and highly resistant to iprodione. Fitness studies revealed that iprodione-resistant isolates did not differ from sensitive isolates in regard to radial growth rate on artificial medium, sporulation, and pathogenicity on strawberry fruit. The results obtained in this study contribute to our understanding of fungicide resistance development in the gray mold fungus and are useful for improving current resistance management practices.



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