Faculty of Agricultural and Food Sciences, American University of Beirut, Beirut, Lebanon

Background and objectives

Late blight, caused by Phytophthora infestans, is the most important disease of potatoes in the coastal plains of Northern Lebanon. Recent field observations indicated an increase in the threat of the disease with possible appearance of more aggressive and/or resistant populations of the pathogen to the commonly used fungicides. Several recent studies reported the migration of exotic strains of P. infestans that have caused a world-wide resurgence of the disease after decades of steady progress in controlling it. This investigation was conducted to survey the disease in the potato growing areas of Lebanon, isolate Phytophthora infestans, determine the mating type of the isolates and their sensitivity to the commonly used fungicides.

Materials and methods
Isolates of P. infestans were collected from Northern Lebanon, isolated on Rye B agar medium, from one or two sporangia on a single sporangiophore, and maintained in pure cultures on Rye A agar at 18C. The mating type of each isolate was determined by pairing it with US tester strains of known mating types on Rye A agar amended with sitosterol and evaluated after 7-9 ;days incubation at 18C in the dark. Sensitivity to metalaxyl, benalaxyi, cymoxanil and dimethomorph fungicides was determined in vitro by plating the isolates on Rye A media amended with each of the fungicides separately, at concentrations of 5 or 10 and 100 ;g/m], and incubated for 5 ;days in the dark at 18C. Isolates that had a radial growth less than 40% of the control on media containing 5 ;g/ml of metalaxyl, cymoxanil or dimethomorph or 10 ;g/ml benalaxyl, were considered sensitive; all other isolates were marked resistant.

Results and conclusions
All of the 38 isolates of P. infestans collected from Lebanon were of the Al Mating Type. Resistance of P. infestans to benalaxyl and metalaxyl fungicides was found to be widely spread. All isolates were resistant to benalaxyl and 34 out of the 38 isolates were resistant to metalaxyl. However, the degree of growth inhibition caused by metalaxyl or benalaxyl did vary among isolates. All of the 38 isolates were sensitive to cymoxanil and dimethomorph. The isolates of P. infestans differed also in their linear growth rate on the unamended Rye A agar medium. Integrated disease management, including alternation of fungicides with different modes of action and mixed application schedules, is recommended to minimize the damage by the disease and lower the risk of resistance development.