Hi Jack and welcome to GoY, any chance of a photo? The bottom leaves on tomato plants do start to die off but a photo would help to rule out any problems.

Powdery mildew perhaps? Though it is a bit early i think for that.
How tall are your plants, any trusses showing?

My bottom leaves do not die off, but i always remove them up to the level of the first truss anyway when that has set nicely, to allow better circulation and easier watering.

They are tall with three trusses. I will take leaves off upto first truss. Thank you very much for answer