The tomato is a very popular crop and is grown by almost all gardeners. Unfortunately, sometimes there are tomato seedling diseases, from which she may die, and then the work that lasted several months will go down the drain, and you will have to buy seedlings for planting. I would compare buying seedlings to buying a pig in a poke, you never know what will grow out of it.
The most unpleasant and dangerous seedling disease is blackleg. The source of infection is contaminated soil. As soon as it becomes waterlogged, or the temperature rises, the black leg is activated. You can reduce the risk of disease by adding wood ash to the soil. Make sure that the soil is not waterlogged or overheated, add some dry sand to the soil after each watering. This disease affects not only tomato seedlings, but in general any seedlings that you grow at home or in a greenhouse.
Diseases of tomato seedlings also include such an unpleasant thing as brown leaf spot... It is a consequence of high air humidity (especially often if the seedlings grow in a greenhouse). First, yellowish spots appear on the surface of the lower leaves, and on the lower side of the leaf in this place - an olive bloom. Then the spots turn brown and spread over the entire leaf, and below it becomes dark brown. Soon these leaves die off. Diseased leaves and completely diseased plants must be destroyed. Measures should be taken to reduce air humidity, for example, often ventilate the room.