  We investigate the spontaneous emission rate of a two-level quantum emitter
near a graphene-coated substrate under the influence of an external magnetic
field or strain induced pseudo-magnetic field. We demonstrate that the
application of the magnetic field can substantially increase or decrease the
decay rate. We show that a suppression as large as 99$\%$ in the Purcell factor
is achieved even for moderate magnetic fields. The emitter's lifetime is a
discontinuous function of $|{\bf B}|$, which is a direct consequence of the
occurrence of discrete Landau levels in graphene. We demonstrate that, in the
near-field regime, the magnetic field enables an unprecedented control of the
decay pathways into which the photon/polariton can be emitted. Our findings
strongly suggest that a magnetic field could act as an efficient agent for
on-demand, active control of light-matter interactions in graphene at the
quantum level.
