Blighted Tomato Plant

Once the yellowed leaves are removed and carefully discarded the plant will look hale and hardy---for a week or so. Then the fungus that has infected it will move further up the plant and new leaves will wither.
Treating it is labor intensive, expensive and mostly unnecessary. By the time the invader kills it's hoast the frost will have probably have already done the job.
But cutting it out where it's present does slow the progression.