  We examine the influence of weak anisotropic interactions on the T=0 phase
diagram of the frustrated two-leg Heisenberg ladder, a well-studied spin model
exhibiting integer and fractional magnetization plateaux separated by gapless
incommensurate states. We find that the Dzyaloshinskii--Moriya coupling may
substantially modify the phase diagram so that the half-integer plateau and the
surrounding gapless phases merge into a single Ising-ordered phase breaking the
translational symmetry of the lattice. A different Ising order is found for a
weakly frustrated ladder. Implications for experimental ladder and dimer
systems are discussed.
