Histological and biochemical criteria for objective and early selection of grapevine cultivars resistant to Plasmopara viticola

Grapevine breeding is the most effective way to create cultivars resistant to downy mildew (Plasmopara viticola), and to reduce the number of fungicide applications. Four criteria, including histological and biochemical analyses, based on the level of different mechanisms of resistance to grapevine downy mildew, were tested on 42 different cultivars. Plantlets were artificially inoculated with downy mildew and the sporangia density was measured spectrophotometrically 6 d after infection. Callose synthesis in stomata and δ- and ε-viniferin levels at the site of infection were recorded 48 h after inoculation. These observations have allowed the 42 cultivars to be divided into 5 groups: very resistant (VR), resistant (R), less susceptible (LS), susceptible (S) and highly susceptible (HS). All 4 criteria have to be applied to assign the resistance level closer to field conditions. This method allows to rapidly evaluate the level of resistance of seedlings to downy mildew thereby leading to a reduction in duration of the breeding program by several years.