Incidence of Colletotrichum acutatum on apple fruits and possible sources of inculum

Abstract: In order to quantify the importance of bitter rot caused by Colletotrichum acutatum, possible inoculum sources and stored fruits of cv. Aroma were followed over three years in eight commercial apple orchards located in southern Norway. Prior to bud burst, buds were collected and incubated for 3 weeks at 20 °C and in water saturated air. Leaves were collected in summer (July) and in autumn (September, before harvest), surface sterilized and frozen for five hours at -18 °C before incubation for six days at 25 °C. Apple fruits were harvested and stored for 3-4 months at 4 °C in normal air conditions followed by two weeks at 20 °C. Bud scales with conidial growth of C. acutatum were found only from one orchard, in the year following the most severe attack on the fruits. Conidial masses of C. acutatum were found on leaves, but only when assessed in September, not in July, in five of eight orchards. The highest incidence on leaves was in the orchard with infested buds. Bitter rot incidence after storage was from 0 to 27%, and apples with bitter rot was found in all orchards, but not every year.