mildew

noun

1. a thin, furry, usually whitish coating or discoloration, caused by a fungus, that forms on the surface of various plants, on organic matter, or on damp paper, cloth, bathroom tiles, etc.
2. any fungus that attacks various plants or that produces such a coating or discoloration
3. any plant disease, as powdery mildew, caused by such fungus

transitive verb, intransitive verb