Hello Garf. Ken here in the Chicago area. The vascular tissue of your
tomato does look brown. Did you have dieback of any leaves and/or
stems. The leaves would go from yellow to brown and die still attached to plant.
If so, you might have fusarium wilt. It enters the plant through the roots and
blocks the vascular tissue. If this is true, then provide proper cultural
care and use resistant varieties. Good luck and take care.
Ken here with The Home Depot in the Chicago area