An Etymological Dictionary of the German Language/Annotated/Mehltau
Mehltau, masculine, ‘mildew, blight,’ corrupted from the equivalent Middle High German miltou, neuter, Old High German mili-tou, neuter; compare Anglo-Saxon meledeáw (miledeáw), English mildew. The opinions as to the origin of the first component are various. The most probable derivation is that from Gothic miliþ, neuter, ‘honey’ (compare Old Icelandic milska, feminine, ‘sweet drink’), to which in West Teutonic a substantive mili (Greek μέλι, stem μέλιτ-, Latin mel) is possibly akin; hence Mehltau, ‘honeydew’?. It is less probable that the word was borrowed from or was primitively allied to the equivalent Greek μίλτος, so that Tau may have been an explanatory addition (compare Lindwurm and Windhund). It is also possible that Old High German mili-, Anglo-Saxon mele-, mile-, is connected with Mehl, and a special formation from the root mel, ‘to grind.’ In that case the Modern High German change of Miltau into Mehltau is warranted by etymology; Mehltau is defined as ‘a greyish-white, meal-like coating on plants in summer.’