The leaves of all plants eventually turn brown and die, but this problem seems to affect tomatoes more often than other plants. Several conditions can cause leaves to turn brown on tomato plants. If you give your plant too much fertilizer, it can cause leaf burn and scorch. Some insects cause browning leaves, as do several diseases. If you disturb the roots of your tomatoes when you cultivate around them, the leaves may turn brown.
Texas A&M University advises that excessive fertilization can result in leaf scorch or burn, which turns tomato leaves brown. Because heavy applications of fertilizer can burn the roots, it causes the ends and margins of leaves to become brown and then die back. After this happens, the tomato plant might recover or it might die. Avoid this problem by digging organic compost into your soil before you plant tomatoes, and then mulch with compost and other organic materials to give your plant continuous nutrients every time you water.
Spider mites can cause tomato leaves to turn yellow, then turn brown, and then die. The soil-dwelling microscopic worm called a nematode can cause injury to tomato roots, which results in leaves turning brown and then dropping. Spray your tomatoes with insecticidal soap to get rid of mites. The University of California at Davis reports that no chemical controls exist that are effective in controlling nematodes---cultural methods such as using organic compost in your soil and crop rotation can help to prevent nematodes from disturbing the roots of your tomato plants. Nematode-resistant varieties of tomatoes are available, so if you have this pest, search for appropriate types of tomatoes to grow.
Early blight and Septoria leaf spot are fungal diseases that afflict tomato leaves during the hottest part of the summer. Both diseases cause leaves to form black, "target-like" spots, according to Colorado State University. Other diseases that can cause tomato leaves to turn brown include bacterial canker, tobacco mosaic virus, Fusarium wilt and Verticillium wilt. If you plant disease-resistant varieties of tomatoes and pick fallen plant material from the ground, you can keep these plant diseases under control. Fungicides can be effective in stopping fungal diseases. Always prohibit tobacco from being used near your tomatoes.
Many gardeners use a hoe to loosen the soil around plants and also to take out nearby weeds. When you cultivate around tomatoes, you can damage the roots, which will cause browning of some leaves. Instead of disturbing the soil around your tomato plants, mulch with an organic mulch such as compost, grass clippings or fallen leaves of other plants. These materials will help keep weeds down and will nourish your plants. If you build raised beds for your tomato patch, the soil will remain light and fluffy---avoid walking on the bed to keep it aerated.