Fludioxonil, Fenhexamid and Boscalid-Insensitive Botrytis Fabae Isolates Collected from Moroccan Faba Bean Fields

Chocolate spot, caused by Botrytis fabae, poses a significant threat to faba bean production in Morocco, leading to decreased crop quality and quantity. This study assesses the sensitivity of B. fabae isolates to three fungicides-fenhexamid, fludioxonil, and boscalid-commonly used in Moroccan faba bean cultivation. Fungicide sensitivity tests were conducted on 22 single-spore isolates of B. fabae from five faba bean-growing regions in Morocco. The Growth Inhibition Percentage (GIP) of each isolate was calculated using an automated quantitative (AQ) test to determine sensitivity profiles to the three fungicides. Six representative isolates from different sensitivity profiles were tested against a range of concentrations of fenhexamid, fludioxonil, and boscalid (0–1,000 mg/L) to determine EC50 values. In vivo greenhouse experiment evaluated the efficacy of two fungicides fludioxonil and boscalid against B. fabae isolates with distinct sensitivity profiles. Mass Disease Index (MDI) was used to evaluate fungicide effectiveness. Results indicated that 45% of the isolates demonstrated resistance to fludioxonil, and 14% exhibited resistance to boscalid. Two of the three boscalid-resistant isolates were also resistant to fludioxonil. In contrast, the majority (95%) of isolates were moderately resistant or sensitive to fenhexamid. The calculated EC50 values of fludioxonil, boscalid and fenhexamid against B. fabae isolates clearly distinguished sensitive from resistant isolates; for resistant isolates EC50 were ≥ 140 mg/L while the values for sensitive isolates were ≤ 30 mg/L. In vivo trials revealed inefficacy of fludioxonil and boscalid against resistant isolates. These findings highlight the emergence of fludioxonil and boscalid insensitivity in B. fabae in Morocco, indicating the need for continuous monitoring of fungicide resistance in chocolate spot management. Future research should develop methods to manage fungicide resistance, aiming for sustainable control of chocolate spot in faba bean.