Aggressiveness and host specificity of Brazilian isolates of Phytophthora infestans

Abstract

The population of Phytophthora infestans in Brazil consists of two clonal lineages, US‐1 associated with tomatoes and BR‐1 associated with potatoes. To assess whether host specificity in these lineages resulted from differences in aggressiveness to potato and tomato, six aggressiveness‐related epidemiological components – infection frequency (IF), incubation period (IP), latent period (LP), lesion area (LA), lesion expansion rate (LER) and sporulation at several lesion ages (SSLA) – were measured on detached leaflets of late blight‐susceptible potato and tomato plants. Infection frequency of US‐1 was similar on potato and tomato leaflets, but IF of BR‐1 was somewhat reduced on tomato. Incubation period was longer on both hosts with US‐1, although this apparent lineage affect was not significant. Overall there was no host effect on IP. On potato, BR‐1 had a shorter LP (110·3 h) and a larger LA (6·5 cm2) than US‐1 (LP = 162·0 h; LA = 2·8 cm2). The highest LER resulted when isolates of BR‐1 (0·121 cm2 h−1) and US‐1 (0·053 cm2 h−1) were inoculated on potato and tomato leaflets, respectively. The highest values of the area under the sporulation capacity curve (AUSC) were obtained for isolates of US‐1 inoculated on tomato leaflets (6146) and for isolates of BR‐1 on potato leaflets (3775). In general, higher values of LA, LER, SSLA and AUSC, and shorter values of LP were measured when isolates of a clonal lineage were inoculated on their original host than with the opposite combinations. There is evidence that there are quantitative differences in aggressiveness components between isolates of US‐1 and BR‐1 clonal lineages that probably contribute to host specificity of P. infestans populations in Brazil.