Fengping Chen, and
Guido Schnabel, School of Agricultural, Forestry, and Life Sciences, Clemson University, Clemson SC 29634
Botrytis cinerea, the causal agent of gray mold disease, is one of the most important plant-pathogenic fungi affecting strawberry. During the last decade, control of gray mold disease in the southeastern United States has largely been dependent on captan and the use of at-risk fungicides with single-site modes of action, including a combination of the quinone outside inhibitor (QoI) fungicide pyraclostrobin and succinate dehydrogenase inhibitor (SDHI) fungicide boscalid formulated as Pristine 38WG. Reports about loss of efficacy of Pristine in experimental fields in North Carolina prompted us to collect and examine 216 single-spore isolates from 10 conventional fields and 1 organic field in North Carolina and South Carolina in early summer 2011. Sensitivity to pyraclostrobin or boscalid was determined using a conidial germination assay with previously published discriminatory doses. Pyraclostrobin- and pyraclostrobin+boscalid-resistant isolates were found in all conventional fields (with some populations revealing no sensitive isolates) and in the organic field. Among the isolates collected, 66.7% were resistant to pyraclostrobin and 61.5% were resistant to both pyraclostrobin and boscalid. No isolates were identified that were resistant to boscalid but sensitive to pyraclostrobin, indicating that dual resistance may have derived from a QoI-resistant population. The molecular basis of QoI and SDHI fungicide resistance was determined in a subset of isolates. Polymerase chain reaction–restriction fragment length polymorphism analysis of the partial cytochrome b (CYTB) gene showed that pyraclostrobin-resistant isolates possessed the G143A mutation known to confer high levels of QoI fungicide resistance in fungi. Boscalid-resistant isolates revealed point mutations at codon 272 leading to the substitution of histidine to arginine (H272R) or tyrosine (H272Y), affecting the third Fe-S cluster region of the iron-sulfur protein (SdhB) target of SDHIs. The results of the study show that resistance to QoI fungicides and dual resistance to QoI and SDHI fungicides is common in B. cinerea from strawberry fields in the Carolinas. Resistant strains were more frequent in locations heavily sprayed with QoI and SDHI fungicides. However, resistance to both fungicides was also found in the unsprayed, organic field, indicating that some resistant strains may have been introduced from the nursery.