Mildew refers to certain kinds of mold or fungus. In Old English, it meant honeydew (a substance secreted by aphids on leaves, formerly thought to distill from the air like dew), and later came to mean mildew in the modern senses.[1]
Powdery mildew growing on a leaf. The ladybird is an Orange Ladybird (Halyzia sedecimguttata) which feeds on the mildew.
The term mildew is often used generically to refer to mold growth, usually with a flat growth habit. Molds can thrive on any organic matter, including clothing, leather, paper, and the ceilings, walls and floors of homes with moisture management problems. Mildew often lives on shower walls, windowsills, and other places where moisture levels are high. There are many species of molds. In unaired places, such as basements, they can produce a strong musty odor.
What most horticulturalists and gardeners call mildew is more precisely called powdery mildew. It is caused by many different species of fungi in the order Erysiphales. Most species are specific to a narrow range of hosts, and all are obligate parasites of flowering plants. The species that affects roses is Sphaerotheca pannosa var. rosa.
Another plant-associated type of mildew is downy mildew. Downy mildews are caused by fungus-like organisms in the family Peronosporaceae (Oomycota). They are obligate plant pathogens, and the many species are each parasitic on a narrow range of hosts. In agriculture, downy mildews are a particular problem for growers of potatoes, grapes, tobacco and cucurbits.
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