mildew

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various fungi or oomycetes that form a superficial, usually whitish growth on plants and various organic materials.
• noun A plant disease caused by such organisms.
• noun A superficial coating or discoloration of organic materials, such as cloth, paper, or leather, caused by fungi, especially under damp conditions.
• transitive & intransitive verb To affect or become affected with mildew.

from The Century Dictionary.

• To taint with mildew.
• To become affected with mildew.
• noun A disease of cucumbers, melons, pumpkins, and similar plants, caused by Plasmopara Cubensis.
• noun A minute parasitic fungus which frequently appears on the leaves, stems, and various other parts of plants or other decaying organic substances as a white frost-like down, or in spots or with various discolorations.
• noun A state of decay produced in living and dead vegetable matter, and in some manufactured products of vegetable matter, such as cloth and paper, by the ravages of very minute parasitical fungi.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A growth of minute powdery or webby fungi, whitish or of different colors, found on various diseased or decaying substances.
• noun a fungal disease of plants caused by an ascomycete of the order Erysiphales, characterized by a powdery white film on the surface of the affected plants. It is damaging to, e.g., roses and lilacs. Also, a fungus that causes such a disease.
• intransitive verb To become tainted with mildew.
• transitive verb To taint with mildew.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A growth of minute powdery or webby fungi, whitish or of different colors, found on various diseased or decaying substances.
• verb transitive To taint with mildew.
• verb intransitive To become tainted with mildew.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the process of becoming mildewed
• noun a fungus that produces a superficial (usually white) growth on organic matter
• verb become moldy; spoil due to humidity