Effect of nitrogen supply rate on disease resistance in tomato depends on the pathogen

The aim of this study was to investigate the effect of tissue nitrogen conc entration, as a consequence of nitrogen supply rate, on the susceptibility of tomato plants to three pathogens. We varied tissue N concentration by su pplying N at different rates by adding nitrate in different, exponentially increasing amounts to the nutrient solution on which the tomato plants were grown. Separate experiments were carried out to test susceptibility of tom ato plants to the bacterial speck-causing Pseudomonas syringae pv tomato, t o the wilt agent Fusarium oxysporum f.sp. lycopersici and to tomato powdery mildew caused by Oidium lycopersicum. The effect of tissue N concentration appeared to be highly pathogen-dependent: there was no effect on susceptib ility to F. oxysporum, but susceptibility to P. syringae and O. lycopersicu m increased significantly with increasing N concentration. We have previous ly demonstrated the opposite for susceptibility to Botrytis cinerea: decrea sing susceptibility with increasing N concentration. The apparent contradic tory effects are discussed in relation to the effect of N supply on both th e nutritional value of the plant tissue to the pathogen and on the concentr ation of resistance-related compounds. We conclude that the effect of chang ing both characteristics on disease susceptibility is highly pathogen-speci fic and is probably dependent on differences in resource requirements of th e pathogen or the sensitivity of the pathogen to plant resistance reactions or on both these factors.