| Summary: | In the era of sustainable grapevine production, there is a growing demand to define differences between <i>Vitis vinifera</i> varieties in susceptibility to downy mildew. Croatia, as a country with a long tradition of grapevine cultivation, preserves a large number of native grapevine varieties. A leaf disc bioassay has been conducted on 25 of them to define their response to downy mildew, according to the International Organisation of Vine and Wine (OIV) descriptor 452-1, together with the stress response of the leaf discs using chlorophyll fluorescence and multispectral imaging with 11 parameters included. Time points of measurement were as follows: before treatment (T<sub>0</sub>), one day post-inoculation (dpi) (T<sub>1</sub>), two dpi (T<sub>2</sub>), three dpi (T<sub>3</sub>), four dpi (T<sub>4</sub>), six dpi (T<sub>5</sub>), and eight dpi (T<sub>6</sub>). Visible changes in form of developed <i>Plasmopara viticola</i> (<i>P. viticola</i>) sporulation were evaluated on the seventh day upon inoculation. Results show that methods applied here distinguish varieties of different responses to downy mildew. Based on the results obtained, a phenotyping model in the absence of the pathogen is proposed, which is required to confirm by conducting more extensive research.
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