The first element in treating tomato plant disease is identifying the problem. Once you recognize the disease, you can deal with it properly. Most tomato diseases are management problems. Pests are the next most common trouble area followed by weather, virus or other uncontrollable circumstances. That means knowing how to plant and care for your tomato plants is the main way to assure a bumper crop of beautiful, ripe tomatoes.
Some things such as leaf roll are common tomato plant habits that gardeners think are problems. Overreacting to normal growth patterns results in over-pruning or the destruction of otherwise healthy plants.
Avoid overcrowding your tomato plants. Leave at least 2 feet between each seedling for proper adult plant spacing. Plants inserted too close together crowd out the sun and cause wilt, fungus or blight on the lower leaves.
Always provide good support for growing tomato plants. Most tomato plants grow tall and heavy. Wire baskets or wooden stake "cages" make the best climbing trellis for a tomato plant. Plants left on the ground as they grow will mold underneath, and the fruit on the ground will rot. This type of mismanagement almost solely causes blossom end rot.
Fungus occurs when water is splashed up on the underside of leaves, because that part of the leaf does not dry as fast and the water has a chance to mildew. Careful watering habits will prevent fungus. If you spot a leaf with mildew or mold, remove it immediately. If the fungus covers a large portion of the plant before you notice it, remove and destroy the plant.
Blight, another leaf spot problem, happens when water does not dry at the proper speed. This is more often a cause of weather-related water and temperature than human error, but improper watering techniques at the wrong time of day can cause it, too. Water plants at the very base. Use a trench to fill along the plant line instead of watering overhead. Do not water in the middle of the day when it is hottest, or at the end of the day when plants will not have time to dry.
Rotating the types of vegetables planted in each area every year greatly reduces the spread of virus or soil born contaminants. It also gives the soil a chance to recover from a specific draw on nutrients while providing a new set for a different plant. Plant different categories of vegetable in each spot, not just different types.
Do not work on plants when they or the ground around them are wet. This helps prevent the spread of any problems that may already exist. At the end of the growing season, completely remove any plant matter, including roots and stems. Burn or bury them deeply, far from the planting area. Avoid planting tomatoes in the same area for more than three or four years.