Leaves of different *Vitis vinifera* cultivars, susceptible (‘Chasselas’ and ‘2185’), less susceptible (‘2142’) or resistant to downy mildew ('Solaris’ and ‘2091’), were inoculated with four different concentrations of an aqueous sporangia suspension of *Plasmopara viticola* (5 × 105, 2 × 105, 6 × 104 and 2 × 104 sporangia/ml). The infection rate of these samples was then examined by light microscopy and synthesis of stilbenes was analysed at infection sites. Infection rate increased parallel with inoculum concentration, but there was no correlation between the infection rate and resistance to *P. viticola*. Moreover, at the lowest inoculum concentration, the infection rate is similar for susceptible and resistant grapevine varieties. Quantification of stilbenes at 72 hpi showed that at the lowest inoculum concentration, the most susceptible grape variety synthesized the largest amount of stilbenes, whose level remained however below the ED50 values defined for each of them. Conversely, at the highest inoculum concentration, the most resistant varieties produced the highest amounts of the most toxic stilbenes against *P. viticola*. The critical role of the inoculum concentration used for artificial inoculation to evaluate grapevine resistance to downy mildew is discussed.

The authors would like to thank Mr. ERIC REMOLIF for grapevine cuttings production, and we gratefully acknowledge the Juana de Vega Foundation (Spain) for its financial support.

Fundación Juana de Vega

Peer reviewed