hi Boulder and welcome to goy. If it is a mould it is fungus not a virus. Have a close look as at first glance root aphid can look 'mouldy'. There are always fungal spores in the soil so you will be able to grow other things.

thank you. but why are they flopping and the mould was only under the affected lettuce
it is strange that it was in two different raised beds, quite a way away from each other. I have never had lettuces die like this

they flop because the roots are unable to take up water. perhaps two plants were infected prior to planting out.

I would also check the composition of the soil. Too much of the wrong kind of organic matter (grass clippings, fruit & veggie scraps, green leaves, etc.) can encourage that fuzzy gray mold.