Decidability of the language of all TMs whose languages reduce to a fixed decidable language
Let $L_1$ be a decidable language. Consider the class of Turing-recognizable languages 

$$\mathcal{S} = \{ L \mid L \mbox{ reduces to } L_1 \}$$

This class is non-trivial in the sense of Rice's theorem, as no recognizable but undecidable language can be an element of $\mathcal{S}$, and since we obviously have that $L_1 \in \mathcal{S}$.

You can now apply Rice's theorem to get that $L_p$ is undecidable.