Evaluation of apple (Malus domestica Borkh.) genetic resources for European canker (Nectria galligena Bres.) resistance Methodological study | Evaluation de la resistance au chancre europeen (Malus domestica Borkh.) de ressources genetiques du pommier (Nectria galligena Bres.) : etude methodologique

Very little work has been done to evaluate resistance to Nectria canker in the European collections of apple genetic resources. There is also a lack of methodological tools adapted for the assessment of Nectria canker resistance in large collections, the management and analysis of the evaluation data and, the validation of the evaluation data. Because of this, there are only a limited number of resistance sources available for breeding. Nevertheless, in Western Europe, Nectria canker constitutes one of the most important apple diseases. This work is based on a ten-year evaluation of a collection including 271 apple cultivars (cvs) distributed in three orchards. Results show that some conditions need to be met when integrating data coming from different orchards : means of disease incidence in the orchards must be comparable; disease incidence must be within a well-defined range of optimal values and, disease dispersion in the orchards must be homogeneous. The susceptibility ranking of our cvs is based on their relative value compared with the mean disease intensity present in the orchards as a common reference. Using this system, evaluation data results from various collections could be integrated into international databases. This method of grouping cvs into susceptibility classes, clearly underlines the most resistant cvs. Correlation between this cvs classification and the data available in the literature is very good. Of the 50 least susceptible cvs surveyed, nearly 90 / are new potential sources of resistance. No case of immunity to Nectria canker was observed. The study shows that cvs express a differential susceptibility according to the entry sites of the disease and some landraces cvs show an extreme tolerance to the disease by producing wound callus that stops the progression of the disease. Finally, as a practical conclusion, standard protocols are explained that define an adapted methodology for the primary evaluation of Nectria canker resistance for Malus genetic resources collections