Basil is a fast-growing herb that requires plenty of water and sunlight to grow to its full potential. This herb is hardy, withstanding hot temperatures and being equally well-suited to growing either in the ground or in containers. Like most other plants, basil is not immune to plant problems ranging from disease to environmental problems. If your basil plant's leaves become brown, it could be a sign of one or more of these problems.
Basil does not live forever, and as it grows older, it begins to brown. The stems of the plant become brown or purple in colour, also developing a woody texture. Leaves, especially lower leaves, become discoloured, often yellowing before browning, then falling off. This process is natural for the basil plant and cannot be prevented or treated. Simply plant new basil and allow it to grow into strong, healthy plants as your older plants finish up and begin dying off.
Water is very important to basil because it grows rapidly and requires lots of water to stay healthy and meet its nutritional needs. Over-watering basil may cause the leaves to turn yellow, grey or brown before the plant finally rots and dies due to the excess moisture. Under-watering basil also causes the leaves to turn brown or develop brown spots as it dehydrates and dies. In hot weather, basil should be watered daily. Basil grown in the ground should receive enough water to keep soil moist, never soggy or muddy. Basil in containers should be watered until the water flows from the drainage holes in the container.
Pests such as insects and worms are a big culprit behind brown basil leaves. These pests damage and destroy basil plants by eating the leaves and stems of basil as well as destroying the root systems or sucking out the fluids inside the plants. The final result of this damage is dry, withered, dead and brown leaves on the plant. Pests that attack basil and cause browning of the leaves include leaf miners, mites, thrips, aphids and white flies. Other symptoms of pests on basil include yellowing, spotting or striping of the leaves, rotting stems or wilt of the leaves or plant.
Basil may contract a disease of the plant or soil. If the plant itself is affected by disease, the browning of leaves occurs rapidly. Stems may also brown and wilt, or leaves may become marked with brown spots. Diseased soil may spread the disease to the plant's foliage, or the plant may pick up the disease through its roots and begin to decline, thus showing brown leaves. Fungal disease, bacterial disease and viral disease all cause browning of the leaves as well as other leaf discolouration, wilt, shrivelling, rotting, leaf drop or decreased plant growth.