Resistance to Candidatus Liberibacter solanacearum in tomato landraces from Mexico

Candidatus Liberibacter solanacearum (CLso) is an economically important plant-pathogen of tomato (Solanum lycopersicum) crops in the United States, Mexico, Central America, and New Zealand. Currently, there are no reports of resistance to CLso in tomato cultivars. Identification and development of CLso-resistant cultivars may offer the most efficient way to manage this tomato disease. Resistance of 46 tomato landraces collected in different regions of Mexico, representing a wide range of genetic variability from this country was evaluated. Two assays were done in consecutively years to assess the resistance to CLso under greenhouse conditions. Plants from both tests were inoculated with CLso through 20 Bactericera cockerelli insects per plant. In the first trial, landraces FC22 and FC44 showed a significantly higher proportion of resistant plants, less symptoms severity, and longer incubation time, followed by landraces FC40 and FC33 compared with the rest of the 42 landraces and 2 susceptible cultivars 60 days post inoculation (dpi). In the second assay, only landraces FC22 and FC44 had again significantly higher proportion of resistant plants, less symptoms severity, relative lower CLso titers, and longer incubation time in comparison with landraces FC40 and FC33 and the two susceptible cultivars 60 dpi, corroborating their resistance to CLso. Presence of CLso DNA in all resistant plants from both assays discards scape plants and indicates that the methodology used was adequate to discriminate between resistant and susceptible plants. These results confirm that landraces FC22 and FC44 are promising resistant sources for the development of CLso-resistant cultivars of tomato.