PRUINA

A waxy powdery substance which covers the surfaces of certain fruits (such as plums and grapes) and of different plant organs (leaf, stems etc.)

The pruina or epicuticular wax  is a fine more or less crystalline, farinaceous dust, easily removable, which one can usually wipe with the finger. Pruina has a protective functions it make skin impermeable to water, reflect intense sun radiation and protect against bacteria and insects attacks.

From Latin pruina: "hoarfrost."