The terms mold and mildew are both common names for various species of fungi that frequently grow in the home and garden. In that both terms refer to fungal growth and are sometimes used interchangeably, there is no difference between mold and mildew, but some types of mold are not commonly called mildew.
Mildew in the Home
The fungi that flourish in the home and are most often called mildew feed on organic materials, and they can grow on cotton fabric, leather, linen, wood, paper and many other household surfaces. The fungi, which appear as whitish, pink or black growth, can stain and damage surfaces, and they often emit a musty, unappealing odor. The organisms thrive in dark, damp locations with poor air circulation, so the best way to control them is to improve ventilation in mildew-prone areas and eliminate the consistent presence of moisture.
Mildew in the Garden
In the garden, mildew can be a significant problem because many different species of fungus infect plants and cause diseases that are broadly known as mildews, and the infections can weaken or kill plants. Powdery mildew shows up as a dusty white layer on the surface of leaves, stems and fruit; the powder consists of the spores produced by the fungus. Downy mildew is caused by entirely different kinds of fungi, and it presents itself as fuzzy growth, usually on the underside of leaves. As is the case with mildew inside the house, garden mildews are best controlled by ensuring that there's ample air circulation around plant foliage.
Some fungi produce visible growth that's not commonly called mildew. These molds include Penicillium, the green fuzzy mold that grows on bread, and Stachybotrys chartarum, a greenish black mold that often grows on damp wallboard and is commonly called black mold. Some molds, such as Alternaria, are problematic not so much for their visible growth but because of their microscopic spores that float in the air and cause allergic reactions in some people.
Slime Molds and Water Molds
Some organisms that are commonly called molds are quite different from fungal molds and mildews. Slime molds such as Lycogala epidendrum and Fuligo septica are cohesive communities of single-celled organisms similar to amoeba; these colonies sometimes resemble fungal mold growth, but they are essentially unrelated to fungal molds. Water molds, too, are sometimes confused with fungal molds, but they are actually Oomycetes, fungilike organisms that often cause diseases in plants.