I have 5 Ailsa Craig tomatoes growing in my unheated greenhouse. They were doing very well and had six trusses each. I read that the top should be pinched out after the sixth truss appears so that the tomatoes would develop. Soon after I pinched out the top, the tomatoes started to wilt and now look very sad.They have been watered and fed regularly but the greenhouse is quite cold because of the poor weather and the greenhouse has a few gaps between the glass and walls.
The removal of the growing point after 6 trusses is not bad advice, although it is not usually done until August. It has nothing to do with wilting, which is down to drying out at the root, over-feeding which causes problems with water uptake, or root rotting which prevents water uptake.
Tomato root rot diseases are soil borne and will re-occur unless the soil is changed or grow bags used.