Postharvest fungicide sensitivity of South African Botrytis cinerea isolates causing grey mould on pears

Botryticides currently registered for postharvest use on pears are at risk for fungicide resistance development due to the nature of the pathogen and the site directed activity of the chemicals. Monitoring of fungicide sensitivity is a prerequisite for fungicide resistance management. Botrytis cinerea baseline fungicide sensitivities (N=45) were compared to fungal isolates from pear trees previously exposed to botryticides from three orchards. Fungicide sensitivity to iprodione and pyrimethanil at discriminatory doses (N=166) was tested. Pyrimethanil was found to reduce fungal growth of B. cinerea by 50% (EC₅₀) at concentrations of 0.18 mg L^‑1 (range 0.09-0.40 mg L^‑1) and iprodione at 0.21 mg L^‑1 (range 0.15-0.31 mg L^‑1). Significant shifts towards pyrimethanil resistance were identified in B. cinerea isolates when tested at the discriminatory dose (29% resistant). Mycelial growth was inhibited by iprodione at the discriminatory dose in 95% isolates. Furthermore, a microtitre-assay was optimised, which promises to be a less labour- and time intensive method to monitor B. cinerea fungicide sensitivity. In 96-well cell culture clusters, spore suspensions of B. cinerea were exposed to a range of pyrimethanil concentrations in the presence of rezazurin dye. The measured reduction of the rezazurin dye indicated the presence of cells with active respiration. Thus non-respiring fungal cells indicated toxicity and fungicide sensitivity could be measured by quantifying fungal biomass with the use of a microplate photometer. Results suggest that pyrimethanil sensitivity levels in B. cinerea have shifted towards resistance and that dicarboximide-group fungicides could effectively control grey mould on pears.