Epidemiology of Dickeya dianthicola and Dickeya solani in ornamental hosts and potato studied using variable number tandem repeat analysis

Two high-resolution strain discrimination methods based on variable number tandem repeat (VNTR) analysis were devised for the plant soft-rotting pathogens Dickeya dianthicola and Dickeya solani and used to investigate their diversity and epidemiology. Analysis of D. dianthicola identified 19 VNTR profiles (P1-P19). Diversity was observed in the earliest strains analysed (from 1957), which included four profiles from European strains isolated from Dianthus caryophyllus (P4, P7, P8 and P11) and one from Chrysanthemum morifolium (also P7) originating from the USA. Eleven D. dianthicola VNTR profiles were identified from potato dating from 1984. The time period between which specific D. dianthicola VNTR profiles were first isolated from ornamental hosts and then from potato, was between 8 and 16 years respectively. Identification of two profiles (P2 and P8) that occurred in both ornamental hosts and potato supported cross-infection between these hosts. Limited variation in VNTR profile was identified from strains of D. solani, from which only three VNTR profiles were discriminated by variation at only one of five loci, which may reflect the short time period and/or limited number of D. solani introductions since the pathogen's first emergence in Europe. Shared profiles between D. solani isolates from hyacinth and potato strains, is consistent with cross infection between hosts grown for horticultural production and potato.