Southern Blight

Southern blight shrivels and wilts a tomato plant quickly, eventually killing the entire plant, according to the University of Kentucky Cooperative Extension. The fungus, Sclerotium rolfsii, attacks the stem near the soil, forming a white mould. The bacteria rises through the root system, entering the stem and roots. If you have other vegetable plants in the garden, they will likely shrivel and die because the bacteria affects numerous edible crops.

Walnut Toxicity

Walnut toxicity, also called walnut wilt, occurs in tomato plants nearby walnut or butternut trees. Juglone, a toxic substance that these nut trees produce, will cause tomato plants to display shrivelled, wilted, drooping leaves. If the plants produce any crops, the fruit will have an undesirable, nutty-residue taste. The plant will also turn yellow and then brown. To prevent shrivelled leaves and infected tomatoes, transplant the plant at least 20 to 25 feet from the walnut tree's drip-line, the furthest reach of its canopy of branches. The toxic juglone will exist in the soil for a few years, even if you decide to remove the tree altogether.

Watering

A tomato fruit consists of 95 per cent water, meaning tomatoes require a lot of water to create fruit and healthy leaves. A tomato plant with drooping, shrivelled leaves may indicate too much, or too little, water. Tomato plants are extremely sensitive to moisture, and moisture deprivation, in hot and dry seasons. The plants require at minimum of 1 inch of water every week. If under-watering is a likely cause for your plant's shrivelled leaves, increase the watering schedule. Water the plants at the base, nearest to the soil and roots, every day, saturating the soil until it starts to pool on top. If after one week the plant still looks shrivelled, watering is likely not the problem but a bacterial or viral infection instead.

If over-watering is more likely the culprit, decrease your watering schedule. Ensure the tomato plant receives no more than 1 inch of water per week. Re-pot the plant and bring it inside if your area is experiencing flooding conditions.

Insects

Tiny insects cause tomato plant leaves to shrivel. While there are numerous insects that cause tomato problems, a dry and curling leaf is the result of psyllids, a small insect that sucks plant juices. According to the Colorado State Cooperative Extension, the insects suck enough juice out of the leaf to turn it yellow and cause it to shrivel. Psyllids permanently stunt plant growth, and the tomato plant will create no fruit. Spraying insecticidal soap on the plants is an effective method against psyllids.