Delayed Development of Resistance to QoI Fungicide in Venturia inaequalis in Israeli Apple Orchards and Improved Apple Scab Management Using Fungicide Mixtures

Quinone outside inhibitors (QoI) fungicides group were introduced for commercial use against apple scab (Venturia inaequalis) in Israel in 1997. Unlike other regions in the world, in which resistance of V. inaequalis to QoI fungicides was observed within 3–5 years of use, in Israel it only occurred after 14 years of use. Field trials conducted between 2007 and 2017 showed a significant reduction in susceptibility to QoIs in northern Israel only since 2011. The delay in the development of resistance is related to limited fungicidal sprays resulting from unfavorable conditions for the pathogen. Of the 28 isolates collected from infected leaves or fruits of commercial orchards in northern Israel, 27 were resistant to the QoI fungicide Kresoxim-methyl. Amplification of the CYTB gene and sequencing of the G143A mutation region confirmed the resistance of all 27 isolates to QoIs. Resistance is demonstrated in the orchard, in vitro and molecular-based study, which forced the growers to avoid using QoIs against apple scab. We show that foliar applications of tank mixtures of systemic fungicides plus captan or prepacked fungicidal mixtures improved efficacy and can be used as a strategic approach in fungicide resistance management, including in orchards in which resistance to QoIs has been detected.