Effective post-harvest eradication of EBI tolerant Venturia inaequalis with EBI fungicides in South Africa
Abstract: During the 2006 and 2007 growing seasons four trials were conducted in apple orchards with severe scab infection to evaluate the eradicative potential of three ergosterol biosynthesis inhibitor (EBI) fungicides, viz. bitertanol (Baycor; DC 300 g/l, 80 ml/hl), difenoconazole (Score; EC 250 g/l, 12 ml/hl and flusilazole (Nustar; EW 100 g/l, 32 ml or 48 ml/hl). EBI tolerance was recorded in all four orchards. Experimental trees were caged. Single post-harvest applications were compared with double and triple applications. When leaf drop was completed samples were collected from each cage and discs with scab lesions were cut from the leaves, put in glass fibre bags, secured on racks between shade cloths and were allowed to spend the winter in the orchard. Ascospore production of samples was determined in the laboratory at fortnightly intervals during the full ascospore dispersal season. Ascospore survival was reduced by 62, 93 and 99% after single, double and triple fungicide applications on samples from the Grabouw orchard in 2006. At the Agter-Witzenberg orchard with similar applications the reduction of ascospores was 57, 76 and 94%, respectively. In the 2007 trials ascospore reduction in an Oak Valley orchard was 66, 86 and 92% after single, double and triple applications, while in the Koue Bokkeveld orchard ascospore reduction was 90, 97 and 99% for similar treatments. Significant differences of fungicide activity were obtained after only a single application.