The answer to 'why is my fence brown patches' is that the brown patches on your fence are likely caused by weathering, sun exposure, or fungal growth. Over time, the wood in your fence can start to discolor and break down, leading to unsightly brown spots or patches.
Some common causes of brown patches on fences include: UV damage from sunlight, moisture buildup and mold/mildew growth, insect or pest damage, and natural aging of the wood. If the patches are widespread, it may indicate a more serious problem like wood rot or structural damage to the fence.