Site-specific field resistance of grapevine to <i>Plasmopara viticola</i> correlates to altered gene expression and was not modulated by the application of organic amendments

The influence of site on resistance of grapevine (cv. Chasselas) to <i>Plasmopara viticola</i> was evaluated. Grapevine leaves from three vineyards in the region of Lake Neuchâtel (Switzerland) were tested for their susceptibility to <i>P. viticola</i> in the lab in five successive years (2004–2008), and the expression levels of four selected defence-related genes (<i>Glucanase, Lipoxygenase 9, 9-cis epoxycarotenoid dioxygenase, Stilbene synthase</i>) were studied in 1 year. In all 5 years of examination, differences between sites were substantial. In four out of 5 years, plants from site Hauvernier were much less susceptible to P. viticola than plants from site Auvernier. In another year, differences were less pronounced but still significant for one leaf age. Susceptibility of plants from a third site (Concise) varied from year to year. Differences in the genetic background were excluded by microsatellite analysis. Differences in susceptibility were mirrored in the constitutive expression pattern of four defence-related genes, with samples from the Hauterive site clearly separated from samples of the other two sites in redundancy analysis. Furthermore, it was evaluated whether site-specific resistance can be modulated by agronomic practices such as the application of organic amendments. In two commercial vineyards (cv. Pinot noir), soils had either not (control) or yearly (compost) been amended with a compost for the last 9 years. Leaves from plants grown in any of the two treatments did not differ in their susceptibility to <i>P. viticola</i> in both years of examination. Additionally, under controlled conditions, none of 19 different composts amended to the substrate of grapevine seedlings or cuttings affected their susceptibility to <i>P. viticola</i>, but 8 out of 19 composts reduced severity in the control bioassay <i>Arabidopsis thaliana—Hyaloperonospora arabidopsidis</i>, indicating that a modulation of site-specific susceptibility of grapevine plants by organic amendments is at the very least, difficult.