The answer to 'deck brown patches' is that there are several common causes, including fungus, moisture buildup, and improper staining or sealing. The symptoms are discolored, splotchy areas on the deck surface. Solutions can range from cleaning and re-sealing the deck to more extensive repairs, depending on the cause.
If the brown patches are caused by a fungus like mildew, you may be able to treat it with a deck cleaner or brightener. But if the issue is more serious, like rot or structural damage, it's best to call in a professional landscaper or deck contractor to assess and repair the problem.