I have one tomato plant (of only four) in the greenhouse exhibiting this symptom (although I believe several in the outdoor garden have similar symptoms - hard to tell as they are less than a tenth of the size). Here it is after I stripped it of the diseased leaves (it is quite pale in colour compared to the others).
Two years ago, I lost more than half of my tomato plants (all in containers). I suspected verticillium wilt but there are so many similar viruses. Wilt, blight ... arggh! It seems most of them are soil-borne, so once I have it, well ... I have it. I thought the only symptoms were seen in the picture below (so yellowish leaves and purple spots), but the main picture above was taken today showing more severe symptoms on that one leaf.
I have finally received my battery-operated fan to put in the greenhouse for improved air circulation (and to help with pollination). But it is still charging! I'll get it out as soon as I can.