Born in Koszalin, Ruhnke was appointed in Kiel where he gained his doctorate in 1954 with a thesis on Joachim Burmeister's music theory.
From 1960 to 2003 he was responsible for the Telemann edition of the Zentrum für Telemann-Pflege und -Forschung Magdeburg.
In 1984, 1992 and 1999 his three-volume Telemann-Werke-Verzeichnis was published, in which he completed the Telemann-Vokalwerke-Verzeichnis by Werner Menke (1982/1983) with the instrumental works.
Ruhnke "influenced [...] Telemann research as a whole, which he introduced into academic discourse and for which he set important standards" (Ute Poetzsch).
He also studied the music history of his home Pomerania and became a member of the Historische Kommission für Pommern [de].