G-torsors and universal torsors over nonsplit del Pezzo surfaces

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Let S be a smooth del Pezzo surface that is defined over a field K and splits over a Galois extension L. Let G be either the split reductive group given by the root system of SL in PicSL, or a form of it containing the Néron–Severi torus. Let G be the G-torsor over SL obtained by extension of structure group from a universal torsor T over SL. We prove that G does not descend to S unless T does. This is in contrast to a result of Friedman and Morgan that such G always descend to singular del Pezzo surfaces over C from their desingularizations.