Pathogenicity of Phytophthora infestans and Phytophthora betacei in Four Solanaceous Species from Southern Colombia

Late blight caused by Phytophthora infestans sensu lato represents a major threat to solanaceous crops in the Andean region. This study evaluated the aggressiveness of 30 P. infestans isolates and 10 P. betacei isolates obtained from different solanaceous hosts on four host species using detached-leaf assays. The incubation period and infection growth rate were determined over nine days after inoculation. The incubation period was significantly influenced by the origin of the isolate; whereas, the infection growth rate depended on both the inoculated host and the host–pathogen interaction. Phytophthora infestans from S. tuberosum showed the highest aggressiveness on its original host, reaching infection rates of up to 20.21% day−1. However, several isolates exhibited cross-infection capacity and wide variation in aggressiveness ranges on alternative hosts. In contrast, P. betacei showed lower pathogenic fitness and no evident host preference, with a mean infection rate of 1.71% day−1. These results reveal a complex pathogenicity structure influenced by the interaction between pathogen origin and host, with implications for breeding programs and late blight management strategies. Several isolates exhibited cross-infection capacity and wide variation in aggressiveness.