There are a few diseases that cause basil plants to turn brown, such as downy mildew, fusarium wilt and bacterial leaf spot. These diseases cause brown spots and streaks to appear on the plant, including the leaves and stems.
One of the most serious diseases for basil plants is downy mildew, which is caused by a fungal disease. Leaves turn brown or gray, starting with the yellowing of the top leaf on the plant. Next, the undersides of the leaves turn purple or gray because that is where the fungal spores propagate. Finally, the leaves turn brown and die. Those gardeners who catch it early can remove the diseased leaves, but they have to exercise caution because if the spores get knocked off, they can spread.
Another disease that causes a basil plant to start turning brown is fusarium wilt, which is a mold caused by fungal infection. This mold usually infects plants because it is in the soil. It causes leaves to wilt, turn yellow and fall off, and the stems to become twisted, stunted and get brown spots or streaks.
The third disease that causes brown basil plants is bacterial leaf spot, which is caused by the bacteria Pseudomonas cichorii. This disease is also carried in the soil and turns leaves and stems brown.