An excised twig assay was developed to evaluate cherry genotypes for their tolerance to Pseudomonas syringae pv. syringae. One-year-old wood was collected at monthly intervals from Oct. until Jan. of `Napoleon', `Corum', and a number of cherry rootstock. The rootstock included F/12-1, Giessen (GI), and M×M selections. Twigs were inoculated with one avirulent and three virulent strains. Evaluation of incision browning, callus, and gummosis production were made after incubation for 3 weeks. Based on gummosis is and browning ratings, all the rootstocks tested were found to be more tolerant than `Napoleon' and `Corum' to the 3 strains of Pseudomonas syringae used. No gummosis or browning was observed on twigs inoculated with water or the avirulent strain.
Plant signals extracted from cherry leaves have been shown to control expression of virulence genes in P. syringae. Crude aqueous extracts from `Napoleon' twigs induced the syrB::1acZ fusion in P. syringae strain B3AR132 Other rootstocks are currently being evaluated for their ability to induce virulence in P. syringae pv. syringae,