Incidence of potato tuber infection by Phytophthora infestans is low in Ecuador. Factors considered to potentially affect the incidence of tuber infection include pathogen aggressiveness, host resistance, direct suppression from biological and chemical characteristics of soil acting on pathogen propagules, and exclusion resulting from soil structure and high ridging. In this study, we tested the hypothesis that low incidence could be due to reduced pathogen aggressiveness and/or low host susceptibility by comparing several pathogen isolates and commonly grown potato cultivars from Ecuador with isolates and cultivars from Europe, where tuber blight is known to be a problem. Additionally, in Ecuador, whole tubers and slices of common varieties were inoculated with local isolates of P. infestans to test for potential infection under Ecuadorian conditions. All isolates, regardless of origin, caused tuber infection. The aggressiveness of isolates varied, but this was both between and among Ecuadorian and Swedish isolates and it was not possible to establish a clear difference in the degree of infection based on isolate origin, or origin of potato variety. In general, we found no evidence to suggest that low aggressiveness of the pathogen or extreme resistance of the host explains low incidence of tuber blight in Ecuador. Therefore, we conclude that low incidence of tuber blight in Ecuador is probably caused by soil factors. Furthermore, exclusion due to soil structure and high hilling may play an important role as a preliminary soil infectivity study demonstrated that P. infestans sporangia were infective in six Ecuadorian field soils for at least 15 days.
Late blight; soil suppressiveness; tuber resistance; tuber slice assay; whole tuber assay
