It's always prudent to sow more tomato seed than are actually required in case some seed fails to germinate. This means I'm usually left with more tomato plants than I actually want. However I can never bear to dispose of my spare plants, so I plant them out at the bottom of the garden in an area which is not ideal. I don't give these spare plants as much attention as my 'main' plants.
I noticed today that one of my spare plants is looking a bit ill. I was worried that this might be tomato blight, which can spread rapidly and also infect potato plants, so I pulled up two of my spare tomato plants which seemed to be affected. The fruit looks ok, although not yet ripe, so I kept the tomatoes and put them on a sunny windowsill. I put the affected tomato plants in the recycling wheely bin, not in my compost (I'm sure the council ensure their compost is sterilised).