Determine the occurrence of fungicide resistance against pyrimethanil and fludioxinol in Monilinia laxa and Botrytis cinerea populations from stone fruit orchards

Preharvest application of fungicides increases the risk of fungicide resistance build-up. Fludioxonil is one of the most effective postharvest fungicides for stone fruit that is now being introduced to preharvest environments. This study aims to determine the sensitivity of Monilinia laxa and Botrytis cinerea, isolated from stone fruit orchards, against pyrimethanil and fludioxonil.

Orchards with specific fungicide application backgrounds were identified. Samples were collected and isolated onto potato dextrose agar (PDA). Fungicide sensitivity of M. laxa and B. cinerea isolates were tested (in vitro and in vivo) using the discriminatory dose (DD) and recommended dose (RD) of pyrimethanil and fludioxonil.

Nearly all (2020) or most (2021) of the M. laxa isolates exhibited low sensitivity based on the DD of pyrimethanil. Most of the 2020 B. cinerea isolates (from one orchard) were sensitive to pyrimethanil whereas most from 2021 (multiple orchards) exhibited low sensitivity to pyrimethanil. No to little growth was observed from the RD of pyrimethanil, and DD and RD of fludioxonil. Trends were noted but the DD results from in vivo trials did not correspond with in vitro trials. No growth was observed for the RD. Optimized in vivo trials are suggested to build into a fungicide resistance monitoring protocol. Fungicide risk and fungicide resistance management recommendations were made. There is a need to start monitoring fungicide resistance within South African fruit industries. Protocols are available but require optimisation. Protocols must apply to FRAC standards. Fungicide resistance can only be claimed after multiple years of monitoring.

In some cases, low sensitivity against pyrimethanil in M. laxa and B. cinerea populations were observed, potentially due to prolonged exposure or use as preharvest application for multiple years. The RD of pyrimethanil remained effective. Both doses of fludioxonil were effective. There is a need to start monitoring fungicide resistance within South African fruit industries over several years.

Based on in vitro results a loss in sensitivity against pyrimethanil was noted although the RD remained effective. Isolates were sensitive to fludioxonil. Prolonged monitoring is suggested to effectively assess and manage fungicide resistance.