Phytophthora infestans remains a major threat to global potato production. This study focused on characterizing and assessing the pathogenicity of P. infestans isolates on detached potato leaves and in greenhouse trials across four cultivars. Seven isolates were obtained from high potato-producing regions in the department of Nariño, Colombia. The isolates were analyzed using 12 microsatellite markers to determine genetic distances. Two genetically distinct isolates showed markedly different pathogenicity on detached leaves: isolate P00921 caused complete infection by day five, whereas P00321 showed no symptoms. These two isolates (P00921 and P00321) selected for having the greatest genetic distance and highest pathogenicity among the seven analyzed were further tested in a greenhouse setup on four potato cultivars using a randomized block design. Disease progression was monitored over nine days. The results indicated significant variations in pathogenicity linked to genetic diversity among isolates. Notably, Capiro and Margarita cultivars were more prone to severe disease than Suprema and Única. These findings highlight the complex nature of host–pathogen interactions and suggest the need for tailored approaches in disease management and cultivar selection.

Phytophthora infestans sensu lato is a phytopathogenic oomycete that causes late blight in Solanaceae and has a significant global impact. Its rapid evolution requires population monitoring using polymorphic markers to detect new genotypes with greater biological efficacy. In this context, this study aimed to evaluate the genetic variability of isolates of Phytophthora infestans sensu lato obtained from different species of Solanaceae cultivated in southern Colombia. A total of 40 pathogen isolates were collected from different hosts, including Solanum tuberosum, Solanum lycopersicum, Solanum muricatum, and Solanum betaceum. Genomic DNA was extracted and amplified using 12 microsatellite markers, and the resulting fragments were visualized on 3% agarose gels. Amplification band sizes were measured using ImageJ, and data analysis was performed using the Poppr package in R. High marker polymorphism was observed, with more than four alleles amplified per locus, evidence of triploidy, and high genetic diversity compared to other regions of Colombia. Populations were structured according to their host, with genetic differentiation (Gst) values ranging from 0.24 to 0.65, likely influenced by host preference and the presence of alternative hosts. Linkage disequilibrium analysis suggested asexual reproduction in Phytophthora betacei populations and possible sexual reproduction in P. infestans populations from potato, sweet cucumber, and tomato, emphasizing the need for further investigation of these populations. The markers were highly informative in both the diversity and the reproductive form of the oomycete.

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.