We examine how viscoelasticity affects early stage spinodal decomposition in polymer solutions and blends when fluctuations of the stress and the composition are coupled in dynamics. The coupling is increased with increasing asymmetry between the two components. We introduce a long viscoelastic length xi(ve) within which the stress relaxation governs the composition relaxation. It can be of the order of the tube length in the reptation theory in strongly asymmetric polymer blends. For shallow quenching phase separation proceeds on time scales slower than the stress relaxation time tau and the kinetic coefficient depends on the wave number q as q(-2) for q xi(ve)> 1. On the other hand, for deep quenching phase separation takes place as in gels on time scales faster than tau. We describe the crossover between these two cases by assuming a single stress relaxation time.