Mildew

UF/IFAS Extension does not treat mildew as a separate organism from mold; its household guidance states it directly: mildew is mold. Mold and mildew grow from spores which exist almost everywhere, and mildew requires moist, warm air.

EPA's mold guide describes the same biology without using the word mildew: molds reproduce by spores, and none of them will grow without water or moisture. Cambridge describes mildew by how it looks on a surface; UF/IFAS and EPA describe the fungus behind it.

U.S. Steel's guide to cleaning painted metal buildings uses mildew, or more generally biological growth, for what commonly appears on surfaces which remain moist for extended periods, such as north walls, under eaves or sheltered areas. It adds that dust carries enough organisms to start growth anywhere, that a visible growth of mildew holds moisture easily, and that eliminating the cause of the mildew prevents its return. What that means for a metal roof is its own question, answered at can mildew grow on a metal roof.

On a roof, whether a dark streak is mildew, mold or algae is taken up by the glossary entry for mold and the algae versus mold answer. The Tile Roofing Industry Alliance puts mildew on tile in its own frame: mildew, algae and moss grow on the dust and organic material that settles on tile, not on the tile itself. The black streaks on a Naples roof are described in the roof cleaning page's black streaks section, and why everything in Naples grows mildew is its own answer.

A product made to kill or suppress mildew is a mildewcide, a separate glossary entry from the growth itself.