Cut-off shoots method for estimation of partial resistance in apple cultivars to fruit tree canker caused by Neonectria ditissima

Fruit tree canker, caused by the fungus Neonectria ditissima, is an economically devastating disease in apple orchards, especially in north-western Europe. Complete resistance has not yet been reported in apple, but variation in levels of partial resistance has been described and could be valuable in plant breeding programmes. A screening method based on spore inoculation of manually inflicted leaf scars on cut shoots was evaluated for reliability and repeatability in discrimination of 11 apple cultivars during two years - 2012 and 2013 - in two different facilities - biotron and standard greenhouse - and different time periods (February-May). The resulting cankers were counted (infection percentage), first appearance noted (incubation period) and lesion size measured at regular intervals during a period of 25-29 days for comparison of resistance levels. Results of two-way analysis of variance and correlation analyses showed that the area under the disease progress curve yielded the most robust data. This parameter was then applied for screening 30 apple cultivars using the same cut-shoot method. 'Elise' showed very high susceptibility, while the ornamental 'Prairifire' showed the highest level of partial resistance. Results suggested that cut shoots can be used to assess levels of resistance among cultivars, but great care must be taken to provide stable experimental conditions.