Determination of Fungicide Resistance in Botrytis cinerea from Strawberry in the Central Coast Region of California

Abstract

Grey mold, caused by Botryis cinerea, is one of the most important diseases of strawberry in California. Management of grey mold typically relies on repeated fungicide applications. The occurrence of fungicide resistance in B. cinerea was examined in the Central Coast strawberry production region of California. In mid-May 2013, 59 samples consisting of a single diseased fruit or plant part with gray mold symptoms were collected from six different strawberry fields. Single hyphal tip cultures were then used for mycelial growth assays to compare sensitivities to four different fungicides—boscalid, fenhexamid, iprodione, and pyraclostrobin. Each isolate was tested against discriminatory doses of each of the fungicides. In addition, representative highly sensitive and highly resistant isolates were tested against a range of fungicide concentrations to determine EC50 values. Although all of the 59 isolates were sensitive to iprodione, 37%, 31%, and 29% of the isolates were resistant to pyraclostrobin, fenhexamid, and boscalid, respectively. In some instances the isolates were dual and triple-resistant to these fungicides. EC50 values were often higher than 100 mg/liter, which was the highest concentration used. Therefore, appropriate fungicide resistance management measures should be employed in strawberry growing areas of the Central Coast region of California. Accepted for publication 19 February 2016. Published 2 March 2016.