Resistance sources to Valsa canker (Valsa ceratosperma) in a germplasm collection of diverse Malus species

Although Valsa canker caused by Valsa ceratosperma (Tode ex Fr.) Maire is one of the most destructive diseases in apple (Malus x domestica Borkh.) especially in eastern Asia, information available to help with breeding against Valsa canker in apples is limited. In this work, 53 accessions of diverse Malus species and their interspecific hybrids were tested for resistance to V. ceratosperma, using an excised shoot assay. Dormant shoots and succulent growing shoots from each accession were inoculated with a virulent isolate AVC-12 of V. ceratosperma, and the length of necrosis was measured at 10 days post-inoculation for the dormant shoots and at 7 days post-inoculation for the growing shoots. The lesion length relative to the susceptible control 'Fuji' in dormant shoots (RLD) and to that in growing shoots (RLG) were simple but useful parameters to differentiate between resistant and susceptible accessions. Fourteen accessions from M. baccata, M. florentina, M. halliana, M. micromalus, M. pratii, M. sieboldii, M. yunnanensis, M. x floribunda and M. x platycarpa gave low RLD and RLG values of less than 0.6 and were evaluated as resistant regardless of the difference in the stage of growth. The highest level of resistance was found in M. sieboldii. This high level of resistance in M. sieboldii was effective against different isolates of V. ceratosperma.