Blueberry stem blight is a fungal disease that affects the stems of blueberry plants, causing them to turn brown or black and eventually die. The disease can also cause the leaves to wilt and turn brown. To treat stem blight, it is recommended to prune out infected branches and remove any fallen debris from around the plant. Fungicides can also be used to treat the disease, but it is important to follow the instructions carefully and apply them at the right time.