Sensitivity of Venturia inaequalis Isolates to Fungicides Used in Lebanon

Apple scab, caused by Venturia inaequalis (Cooke) G. Wint. is one of the major apple diseases worldwide. In 1994, the disease caused severe losses particularly in apple orchards of the Akkar mountainous areas of Lebanon, although more than 10 fungicide sprays were applied. A total of 230 isolates of V. inaequalis were collected from 23 orchards and single-spore colonies were prepared. The orchards were selected to represent the major apple-growing areas in Lebanon: Akkar in the north, Kfardibian in northeast Beirut, and Barouk in southeast Beirut. In vitro tests were conducted to evaluate the level of resistance to benzimidazoles and sterol biosynthesis inhibitors (SBI). All isolates tested except one were resistant to benomyl and thiophanate-methyl at 1 μg/ml. The majority of isolates were not inhibited at concentrations as high as 50 μg/ml in mycelial growth. Similarly, the majority of the isolates were not inhibited by benomyl at 1, 5, and 50 μg/ml in spore germination tests. Preliminary in vivo tests with benomyl concerning resistance to benzimidazoles correlated well with in vitro results. All the isolates were inhibited by fenari-mol at 0.25 μg/ml (1) indicating that all the isolates can be considered sensitive to this fungicide. However, the sensitivity levels varied among the various isolates with the ED₅₀ for fenarimol ranging between 0.002 and 0.052 μg/ml. Two new SBIs, bromocunazol and fenbuconazol, not registered for use in Lebanon at the time this study was performed, were also tested. Their ED₅₀ ranged from 0.0089 to 0.025 μg/ml for bromocunazol and from 0.008 to 0.039 μg/ml for fenbuconazol, both indicating a good level of in vitro activity. These results indicate that resistance to benzimidazol is widespread in Lebanon while resistance to SBIs has not yet been detected. Reference: (1) W. Köller et al. Plant Dis. 75:726, 1991.