Date of Award


Degree Name

MS in Agriculture - Plant Protection Science


Horticulture and Crop Science


Kelly Ivors


Gray mold of strawberry, caused by Botrytis cinerea, is a very destructive pre- and post-harvest fruit rot. Outside of California, fungicide resistance in B. cinerea has been reported to every site-specific chemical class labeled for use against gray mold. One objective of this study was to characterize the resistance of 888 isolates of B. cinerea from California strawberry fields to ten active ingredients. Isolates were collected from the same planting block in 47 fields during the early-season (0 to 8 fungicide applications) and late-season (16 to 26 fungicide applications) of 2016. Sensitivity of each isolate was determined using the following active ingredients at a discriminatory dosage (μg/ml): boscalid (75), cyprodinil (4), fenhexamid (50), fludioxonil (0.5), fluopyram (10), iprodione (10), isofetamid (5), penthiopyrad (5), pyraclostrobin (10), and thiophanate-methyl (100). Resistance to each active ingredient was observed at varied frequencies (early-season %, late-season %): boscalid (12, 35), cyprodinil (12, 46), fenhexamid (53, 91), fludioxonil (1, 4), fluopyram (2, 7), iprodione (25, 8), isofetamid (0, 1), penthiopyrad (8, 25), pyraclostrobin (77, 98), and thiophanate-methyl (81, 96). Captan, boscalid, cyprodinil, fenhexamid, and fludioxonil were the most commonly used fungicides in surveyed strawberry fields. A selection of 100 isolates was identified to the species level. All isolates were B. cinerea, excluding one isolate of Botrytis mali. A fungicide resistance trial was conducted v to observe resistance responses in populations of B. cinerea. Frequencies of resistance to boscalid and fludioxonil remained unchanged despite consecutive applications of these fungicides. Frequency of resistance to fenhexamid increased when this fungicide was applied and decreased when it was not. This occurred in fungicide treatments including fungicide rotation, tank-mixing with captan, and consecutive applications of fenhexamid. Multi-fungicide resistance was widespread in California strawberries; isolates resistant to fenhexamid, thiophanate-methyl and pyraclostrobin were the most common phenotype. The frequency of resistance increased from the early-season to late-season for multiple active ingredients tested. This within-season change in frequency of resistance was tested and confirmed in a field trial, where common resistance management strategies failed to prevent the buildup of fenhexamid resistance. New and improved methods of resistance management may need to be enacted to ensure the future efficacy of site-specific fungicides.