SO4 | Acronym for Sélection Oppenheim No. 4 de Teleki, a V. berlandieri x V. riparia hybrid used as a rootstock. It was developed at the viticulture school at Oppenheim in Germany. Dr Richard Smart (2015, p627) says SO4 is a ‘popular rootstock, used widely in France and Germany, shows excellent phylloxera resistance, and tends to favour fruit set and slightly advanced maturity [aka high yields, early ripening]. Vigour is moderate, as is tolerance to nematodes. SO 4 is not, however, suited to dry conditions [flip side being it can handle compact, badly drained soils], and is prone to magnesium deficiency,’ Dr Smart concludes.
Some growers feel SO 4 induces excess vegetation, leaving the berries shaded and prone to grey rot, oidium and incomplete ripening. In Bordeaux at least, and especially with Cabernet Sauvignon, SO 4’s tolerance both to nematodes and active lime is outweighed by its susceptibility to eutypiose. SO 4 is not used in areas of high active lime, such as Jerez.
Dr Richard Smart in the Oxford Companion to Wine 4th edition ed. Jancis Robinson MW and Julia Harding MW (Oxford University Press, 2015, p627.