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.