Different levels of resistance to cyprodinil and iprodione and lack of fludioxonil resistance in Botrytis cinerea isolates collected from pistachio, grape, and pomegranate fields in California

Botrytis cinerea is the causal agent of gray mold disease and can infect fruits of important crops like grape, pistachio, and pomegranate, causing significant yield losses worldwide. In this study, mycelial growth assays in vitro were used to evaluate the sensitivities of 160 single-spore isolates of Botrytis cinerea collected from commercial grape (n = 58), pistachio (n = 74), and pomegranate (n = 28) fields in California to cyprodinil, fludioxonil, and iprodione. Based on the effective concentration at which mycelial growth was inhibited by 50% (EC50 values), 100, 83, and 38% of the isolates from the 3 hosts were sensitive (EC50 < 1 μg/mL) to fludioxonil, cyprodinil, and iprodione, respectively. Thirteen, 3, and 1% of the isolates showed low (CyLR), weak (CyWR), or high (CyHR) resistance to cyprodinil, respectively; Sixty-two percent of the isolates tested showed low resistance (1 < EC50 < 10 μg/mL) toward iprodione (IpLR) and were detected in all three hosts. Absence of cross-resistance was not observed between cyprodinil, iprodione, and fludioxonil. Sequence analysis of the two-component histidine kinase target gene (Bos1) revealed that some of the IpLR phenotype isolates were associated with either I365N/S, Q369P, and/or N373S mutations. Laboratory studies showed that IpLR, CyLR, and CyWR phenotypes were not associated with fitness penalties for mycelial growth rate on non-amended media and pathogenicity on detached grape berries. Furthermore, preventative applications of commercial formulations of iprodione (Rovral) and cyprodinil (Vangard) were effective against gray mold caused by Ip- and Cy-sensitive isolates, but failed to control disease caused by IpLR and, CyLR and CyWR isolates, respectively. The generated information will serve as a basis for implementing fungicide use recommendations against bunch rot of grapes, blossom and shoot blight of pistachio, and gray mold of pomegranate.