We previously demonstrated that providing potato plants with an optimum nitrogen supply (in the form of urea; 135 kg hm-2) produces high crop yields and increased resistance to late blight. However, the mechanisms underlying these responses have not been well characterized. In this study, we examined the effects of various levels of nitrogen fertilization (four levels: N1, 45 kg hm-2, N2: 90 kg hm-2, N3: 135 kg hm-2 and N4: 180 kg hm-2) on soil bacterial growth, community diversity and soil enzyme activity. We found that application of the optimum amount of nitrogen promoted maximum levels of bacterial growth and community diversity development compared with the other treatments. In addition, the highest activities were detected for soil enzymes such as urease, invertase and acid phosphatase (but not catalase) under N3 conditions but not under N1, N2 or N4 conditions. These results suggest that proper nitrogen application provides soil microbes with optimum conditions for development. Thus, the optimum growth of rhizobacteria conferred by N3 treatment appears to be responsible for achieving the highest yields and strongest pathogen resistance in potato plants exposed to Phytophthora infestans. Key words: Late blight, potato plant, rhizobacteria, soil enzyme, urea fertilizer.

Little is known about the effects of nitrogen supply on plant productivity and defensive response to pathogen exposure. Here, tuber yield and resistance to the fungal pathogen Phytophthora infestans were investigated in potato plants supplied with four different nitrogen levels (N1: 45 kg-hm-2, N2: 90 kg-hm-2, N3: 135 kg-hm-2, and N4: 180 kg-hm-2). The N2 level of nitrogen promoted the highest tuber yield and largest tubers under pathogen-free conditions. However, N3 promoted the strongest pathogen resistance and highest potato production in response to P. infestans exposure. Also, the highest level of chlorophyll accumulation was observed with N3 relative to the other treatment groups under pathogen-infected but not pathogen-free conditions. Further study showed that, in response to pathogen infection, the N3 level of nitrogen induced the highest activity levels for the defensive enzymes polyphenol oxidase and phenylalanine ammonia-lyase (but not chitinase and β-1,3-glucanase) and the most abundant accumulation of phenolic and flavonoid compounds. Our data demonstrated that modest increases in the nitrogen supply provided potato plants with optimum growth and defensive capability in response to P. infestans exposure. Key words: Nitrogen, tuber yield, pathogen resistance, Phytophthora infestans, Solanum tuberosum.