Baseline Sensitivity of Botrytis cinerea to Pyraclostrobin and Boscalid and Control of Anilinopyrimidine- and Benzimidazole-Resistant Strains by These Fungicides

Fifty-five isolates of Botrytis cinerea collected from vegetable crops were used to determine the pathogen's baseline sensitivity to two new fungicides: boscalid, which inhibits the enzyme succinate dehydrogenase in the electron transport chain, and pyraclostrobin, which blocks electron transport between cytochrome b and cytochrome c₁. Measurement of sensitivity to boscalid was based on both inhibition of mycelial growth and spore germination, while measurement of sensitivity to pyraclostrobin was based only on inhibition of spore germination. For both fungicides, the sensitivity distribution was a unimodal curve, with a mean EC₅₀ value (effective concentration that reduces mycelial growth or spore germination by 50%) of 0.033 μg ml⁻¹ for pyraclostrobin and 2.09 and 2.14 μg ml⁻¹ for boscalid based on the inhibition of mycelial growth and spore germination, respectively. No cross-sensitivity relationship was observed between the two fungicides (r = 0.09). In addition, no cross-resistance relationship was observed between these two fungicides with other botryticides: cyprodinil, pyrimethanil, fenhexamid, fludioxonil, and iprodione. Moreover, the control efficacy of the two fungicides was tested against two anilinopyrimidine-resistant and two benzimidazole-resistant isolates, and two of wild-type sensitivity. Both pyraclostrobin and boscalid provided satisfactory control of all six isolates that was independent of the isolate sensitivity to benzimidazoles and anilinopyrimidines. In contrast, carbendazim failed to control sufficiently the benzimidazole-resistant isolates, while cyprodinil failed to provide satisfactory control of the anilinopyrimidine-resistant isolates.