A mushroom brush is a soft-bristled brush which is designed to be used for cleaning mushrooms and other delicate vegetables. Since some vegetables suffer from vigorous washing, a mushroom brush can be used to remove encrusted dirt and other material, ensuring that the vegetable is safe to eat. Many kitchen supply stores stock mushroom brushes, and they can also be ordered through specialty purveyors. Typically mushroom brushes are quite inexpensive, and they can be incredibly useful kitchen tools to have around, even if you don't use a lot of mushrooms in your cooking.
The bristles of a mushroom brush are typically made from nylon or another synthetic, and they are designed to be extremely gentle. The brush may take the form of a round knob with bristles on one end, making it very easy to handle, or it can be a flat square or rectangle. As a general rule, mushroom brushes are designed to be washable; they can be made from either plastic or wood, and they should not be run through a dishwasher, as this can damage the bristles.
Cleaning mushrooms and other vegetables is important, as they can contain elements which could make you sick or add a strange flavor to a finished dish. Many mushrooms grow in sandy, rocky areas, and a good brushing can remove particles which could impact the texture of a dish; no one likes to bite down on a chunk of sand. You can also use a mushroom brush in the field while you are collecting, to determine whether or not a specimen is worth bringing home.
In many cases, a dry brushing with a mushroom brush can be enough to remove unwanted material from things like mushrooms, truffles, and other delicate vegetables. A mushroom brush can also be used under running water, although cooks should take care to dry vegetables after rinsing them and to use them immediately, as the water can encourage rot in delicate vegetables. The soft, dense bristles should pull away dirt and other unwanted particles without damaging the vegetable.
If you are in the habit of picking up fungi at farmers' markets, you may want to take a mushroom brush along. Most merchants will object to an inspection which could damage a mushroom, but a gentle brushing with a mushroom brush is acceptable, and it can reveal flaws and defects which would make the mushroom unsuitable for your use. Always ask before using a mushroom brush on merchandise, however, as some merchants may object.