Many modern F1 varieties e.g. Sakura F1 are resistant to the most common leaf diseases such as Leaf Mould (Cladosporium) and Tomato Mosaic Virus (TMV) (Gardener’s Delight is NOT resistant to TMV). Powdery mildew can also be a problem in some years.
If you have a small greenhouse and always grow your tomatoes in the same place, then there can be a build up of soil-borne diseases such as Fusarium (wilt; crown and root rot), Verticillium wilt and Corky root rot. You can get round this either by not growing in the soil or by using grafted plants: see To graft or not graft?
Did You Know?
In the UK, we eat 6oz (160g) of fresh tomatoes per person per week. This is the equivalent of two classic British tomatoes per week, or more than 100 per year - very low compared with other European countries, especially those in the Mediterranean region.