Many inexperienced gardeners who are attempting to grow their own tomatoes for the first time provide too much water for the tomato plants. Signs of excessive water include wilting leaves, which novice gardeners often misinterpret as an indication that the plants require more water rather than less. Correcting your approach to watering and providing overly hydrated tomato plants with consistent, careful treatment thereafter might yet produce a good crop of tomatoes.
Confirm that your tomato plants' symptoms are related to excessive watering. Signs of overwatering include standing water and soil that is very wet and muddy when touched. The presence of wilting leaves and very wet soil indicates the plants have been overwatered.
Prune the plants---by pinching or with scissors---to increase airflow, which may in turn prevent mould or mildew. Remove only young leaves and shoots. Eliminate only shoots and suckers that are 1 inch long or shorter at the base. Do not cut or nick fully grown branches or the main stems.
Stop watering your tomato plants until the soil has dried out to an appropriate level of moisture. The soil should be damp when touched, but it should not be soaking wet.
Apply a layer of mulch 4 to 6 inches thick around the plants, leaving a 2-inch radius around the base of each plant free of mulch. Use either leaf mould, newspaper or straw for the best results. Water the plants after applying the mulch.
Create and maintain a regular watering schedule. Tomato plants benefit from regular moderate irrigation---a little bit of water every day rather than gross fluctuations in watering routine. Water the plants early in the morning rather than later in the day for the best results.
A regular irrigation schedule will also help prevent other diseases, such as blossom end rot.
A plant with mould on its roots must be discarded. Do not compost a tomato plant that has mouldy roots.