Tomatoes need water to survive, but it is possible to overwater a tomato plant. Overwatering a tomato plant can cause it to die or not be as productive as you want. In some cases, overwatering can kill the plant faster than insufficient watering. The signs of overwatering are similar to underwatering, but you can tell the difference. Examine tomato plants frequently to detect signs of overwatering.
The leaves around the tomato plant where the tomatoes should eventually appear should be a healthy green colour. Wilted leaves can be a symptom of problems other than overwatering, so look closely at the colour of the leaves. Yellowish leaves usually indicate the plant has been overwatered. Wilting, brown leaves indicates either underwatering or some other problem.
Mold or Algae
Examine the bottom end of your plants. Look for mould or algae growing around the base or on the ground around the tomato plant. If mould or algae is present, then the plant is not using all of the water provided. Excess water provides a breeding ground for the mould or algae, which will then snack on the tomato plant.
The roots of the tomato plant are the support system for the rest of the plant. Overwatering can cause an iron deficiency, which eventually causes the roots to start to decompose. The plant will start to droop in places as the leaves and stems become yellow in colour. This symptom is often blamed on insects, but the decomposition of the roots is what actually attracts pests to the plants. An iron deficiency in a tomato plant often explains why insects attack one plant heavily and leave another in close proximity alone.
Water a tomato plant with a hose attachment that soaks it gently but thoroughly. Make sure plenty of water goes to the roots. Give the top of the plant a dousing but focus most of the attention on the root area. Don't leave standing water when you are done. Provide enough water so the ground soaks it up and remains moist. Unless you live in an especially harsh environment, most tomato plants need to be watered only every two or three days. Total watering should equal about 1 inch per week in mild summer months and 2 inches during the hottest months.