  We study the superradiant instability in scalar-tensor theories of
gravitation, where matter outside a black hole provides an effective mass to
the scalar degree of freedom of the gravitational sector. We discuss this
effect for arbitrarily spinning black holes and for realistic models of
truncated thin and thick accretion disks (where the perturbation equations are
nonseparable), paying particular attention to the role of hot coronal flows in
the vicinity of the black hole. The system qualitatively resembles the
phenomenology of plasma-driven superradiant instabilities in General
Relativity. Nevertheless, we show that the obstacles hampering the efficiency
of plasma-driven superradiant instabilities in General Relativity can be
circumvented in scalar-tensor theories. We find a wide range of parameter space
where superradiant instabilities can be triggered in realistic scenarios, and
discuss the constraints on scalar-tensor theories imposed by this effect. In
particular, we argue that the existence of highly spinning accreting black
holes is in tension with some scalar-tensor alternatives to the dark energy,
e.g. symmetron models with screening.
