Pestilential, a.
Etymology: [Cf. F. pestilentiel.]
1. Having the nature or qualities of a pestilence. "Sends the pestilential vapors." Longfellow.
2. Hence: Mischievous; noxious; pernicious; morally destructive. So pestilential, so infectious a thing is sin. Jer. Taylor.
Shades of meaning
Fernald has no entry of its own for pestilential; he lists it under pernicious.
He lists it without comment; the entries above explain its neighbors.