Schwarzmann is a rootstock, a V. riparia x V. rupestris hybrid which, says Dr Richard Smart (2015, p.627), has high tolerance to phylloxera and nematodes but only moderate vigour. It is suited to deep, moist soils and is not as widely used as it might be, he adds (although it is found in New Zealand, at Domaine Rewa and Churton for example).

Other V. Riparia x V. Rupestris rootstocks: 101-14 Millardet et de Grasset. | 3306 Couderc. | 3309 Couderc. | 4010 Castel. | Schwarzmann. 

Bibliography

Dr Richard Smart in the Oxford Companion to Wine 4th edition ed. Jancis Robinson MW and Julia Harding MW (Oxford University Press, 2015, p.627.