Long-term deep-freeze storage of Neonectria ditissima conidium suspensions does not reduce their ability to infect apple trees

Fresh, wild-type conidium suspensions of Neonectria ditissima prepared from sporulating European canker lesions for use in field inoculation trials are not always readily available. The effect of freezing inoculum on the viability of spores and their ability to cause infection in situ was assessed using 15—19 frozen N. ditissima conidium suspensions during 2015—2017. Freshly prepared inoculum and water only were used to inoculate rasp wounds on ‘Braeburn’ (2015), ‘Scilate’/Envyâ„¢ (2016) and ‘Royal Gala’ and ‘Braeburn’ (2017) along with frozen inoculum ranging in age from 1—462 days at concentrations >104 conidia/mL. Lesion development was assessed at approximately monthly intervals. In all experiments, there was no substantial effect of storage time on lesion development although germination in droplets may have decreased during storage. As expected, there was a cultivar effect (2017 data only), with fewer lesions on ‘Braeburn’ compared with ‘Royal Gala’. Long-term deep-freeze storage up to 462 days of N. ditissima conidium suspensions (>104 conidia/mL) did not affect the ability of spores to infect apple trees. Therefore, spore suspensions can be batch-prepared and stored frozen for future experimental use.