'Organic' viticulture, as it has been formally defined, authorises only traditional forms of disease, pest and weed control, using products deemed to be natural and eschewing all that have been artificially synthesized. The contradiction lies in the fact that chemically the chief sprays and dusts allowed (copper compounds and sulphur, for fungal disease control) are inorganic, whereas the prohibited synthetic compounds are all or principally organic, i.e. carbon-containing. Nor is distinction possible on the ground of toxicity. For instance toxic build-up of copper in the soil has long been reckoned a problem in Bordeaux, while sulphur applications can be toxic to useful microfauna and can hasten soil acidification.It seems to me the definition of organic requires a bit of tidying up. While I am not a copper or toxicity expert, from what I've read it's not such a good thing in quantity. A French winemaker quoted in this article from UK wine magazine Decanter has a similar idea. The article reports that the winemaker is dropping a French organic certification to reduce copper build-up in his soil, and considers that some synthetics instead give a better overall environmental outcome. A sensible debate appears underway.
An interesting anomaly of organic viticulture is its treatment of copper. Gladstones in Wine, Terroir and Climate Change (2011, Wakefield Press) summarises it well at p89: