Late blight caused by Oomycete Phytophthora infestans (Mont.) de Bary is a destructive disease on Solanaceous crops such as tomato (Solanum lycopersicum L.). Late blight is generally controlled by fungicides applications which quickly become ineffective due to the appearance of new P. infestans genotypes which can overcome the resistance of improved tomato cultivars and cause total production losses.

The aim of this study is to assess the resistance level of tomato cultivars. Inoculation were carried out on tomato leaflets (cv. Trakia, Saint Pierre and Marmande) using inoculums of the major P. infestans clonal lineages present in Algeria such as EU_13_A2 (n=1), EU_23_A1 (n=2) and EU_2_A1 (n=1) under controlled condition.

Aggressiveness components were assessed such as incubation (IP) and latency period (LP), lesion area (LA) and sporangia production (SP). The results showed that Marmande was the most susceptible cultivar to all P.infestans genotypes, whereas, Saint Pierre and Trakia showed a higher level of resistance. The EU_23_A1 was the most aggressive genotype on tomato cultivars in general.

The aim of this investigation is to show that the choice of resistant cultivars can help control late blight and provide economic and environmental advantages by reducing the use of inputs.