The aim of this study was to investigate the effect of nitrogen availabilit
y on susceptibility of tomato leaves to the fungal pathogen Botrytis cinere
a. Plants with varying nitrogen availability were grown by adding N daily i
n exponentially increasing amounts to a nutrient solution at different rate
s. Leaves of plants grown at low nitrogen availability had a high leaf C/N
ratio (21 g g(-1)) and were about 2.5 times more susceptible to primary les
ion formation by B. cinerea compared to plant grown at high nitrogen availa
bility, which had a low leaf C/N ratio (11 g g(-1)). Leaf C/N ratio account
ed for 95% of variation in susceptibility. This relationship between C/N ra
tio and susceptibility persisted when plants were grown with exponential P
addition and optimal N supply, and was thus independent of plant growth rat
e or related factors. We could not explain the effect of nitrogen availabil
ity by variation in the most obvious N-based resistance compound alpha-toma
tine because more susceptible leaves with a high C/N ratio contained more a
lpha-tomatine. These leaves also contained more soluble carbohydrates. The
level of soluble carbohydrates correlated positively with susceptibility, i
ndependent of the growth method. We therefore suggest that the effect of N
availability on susceptibility must be explained by variation in levels of
soluble carbohydrates and speculate about the role of these carbohydrates i
n the infection process.