Effect of freezing of Neonectria ditissima inoculum on its pathogenicity

Abstract

The fungus Neonectria ditissima causes European canker of apple. To determine the pathogenicity of different isolates, conidial inoculum of each isolate needs to be prepared. Freezing inoculum ensures that conidia do not germinate before inoculation, and facilitates screening of large numbers of isolates. In this study, conidial suspensions of three different isolates and 'field conidia' collected from apple cankers were used to inoculate dormant potted 1-year-old 'Royal Gala' trees in a glasshouse. Each conidial suspension (2×105 conidia/ml) was used either fresh or defrosted after 3 days at-20°C. Five buds per shoot were removed and inoculated, with four replicates of two shoots per treatment. Significant differences in disease incidence and lesion size were observed between the different isolates at each assessment date, 5 to 15 weeks after inoculation (P<0.003), but freezing the inoculum had no effect on disease incidence or lesion size. Frozen conidial suspensions can be used for pathogenicity studies and may also be a long-term storage option for cultures.