Fungicide Sensitivity in the European Canker Fungus, Neonectria ditissima

Abstract

Neonectria ditissima is one of the most serious pathogens of apple trees in temperate climatic zones, causing bark canker as well as pre- and post-harvest fruit rots. Control of fruit rots in particular is hampered by uncertainty about the sensitivity to, and efficacy of, available fungicides. Using 43 N. ditissima isolates from Germany and South Tyrol, the effective concentrations causing a 50% inhibition of germ-tube growth (EC50) were determined for fludioxonil (0.009–0.046 ppm), trifloxystrobin (0.018–0.43 ppm), thiophanate-methyl (2.05–15.9 ppm), dodine (0.61–6.33 ppm), cyprodinil (5.04–83.7 ppm), pyrimethanil (22.0–500 ppm), fluopyram (1.35–140 ppm) and boscalid (> 500 ppm). In a detached-fruit assay based on artificial wound inoculation of apples previously dipped in fungicides at commercial dosages, fludioxonil and, to a lesser extent, fluopyram and dodine significantly reduced the fruit rot lesion size, whereas trifloxystrobin, cyprodinil and boscalid did not. Lesion size was also directly correlated to the amount of conidial inoculum, whereby wounds inoculated with 5000 conidia gave rise to larger lesions than those with 500 or 50 conidia. In a field trial of apple trees artificially inoculated with N. ditissima conidia at flowering, fludioxonil gave good control of blossom-end rot, and trifloxystrobin gave limited control, whereas cyprodinil was ineffective. Therefore, the efficacies of different fungicides were broadly in line with the EC50 values determined under laboratory conditions.