Apple blossom-end rot due to Neonectria ditissima is initiated by infections at full flowering and incipient petal fall

Neonectria ditissima is a major cause of blossom-end rot (eye rot, calyx-end rot) as a pre-harvest fruit rot of apple in Northwestern Europe. Artificial inoculation of trees was undertaken at close intervals under natural conditions during flowering in order to obtain a higher-resolution definition of the most susceptible phenological stage of apple for blossom-end rot infections by N. ditissima. Similar trials were conducted for another blossom-end rot fungus, Botrytis pseudocinerea. Results for N. ditissima showed full bloom to be the most susceptible stage for infection in cultivar ‘Pinova’, whereas with cultivar ‘Nicoter’ the highest infection rates were obtained at full bloom to petal fall, reflecting temporal differences in flower development between one-year-old and older wood. Few infections were observed by inoculation at the beginning of flowering or about one week after petal fall when no petals were left on the flowers, and the fruit knots were enlarging. This finding shortens the time-span for infections as compared to previous knowledge. Floral inoculations with conidia of B. pseudocinerea were unsuccessful. Options to control blossom-end rot in integrated pest management include a timing of scab or powdery mildew fungicide sprays to full bloom, and canker pruning just ahead of flowering in orchards strongly affected by N. ditissima.