I am fairly sure that many of my tomato plants get Septoria (click) each year, Joan.
Notice that TAMU says "severe enough, the entire leaf will die."
They are not saying the entire plant - will die. Septoria is a problem that plants can live with. I would advise taking the leaves off.
About 10 years ago, I had many of the Beefy Boy tomatoes that were in the garden, sicken & some died. I have come to wonder if the seed was contaminated. I had one variety of very early determinate tomato from the Soviet Union back in the early 90's. It sickened the 1st year but I was able to save seed and tried it again the next year - the plants promptly died. Since this severe of disease problems was confined to only those 2 varieties, I think that disease brought in with the seed or just, an absence of resistance are the likely problems. However, some diseases like late blight have never shown up here, as best as I know.
What I'm saying is that not every disease is curtains for a tomato plant. Some can make for poor production and even damaged fruit. Others are things that the plant can just shrug off. I am hoping that this problem that you are showing us, Joan, falls into that category.
I'm afraid I'd have to pluck the entire plant - or plants as I have 3 that are infected and I'd say they are pretty severe. I'll give them a look tomorrow and decide if I need to chuck the lot. thank you guys!
In my climate it is not a problem. Pull off the leaves and remove from the garden. If you have a great variety of tomatoes you will probably see it only effects some of the varieties.
Remove all of the discolored leaves and try to open up the center of your plant by pruning out crowded suckers. This will allow for better air circulation which will allow the leaves to dry. I've noticed the condition gets worse as the plants start to come to the end of their bearing period.
I was just plucking off a couple of those leaves today. Yeap it's Septoria. I get it too, one thing that helps a little is pluck off the affected leaves and then sprinkle some garden lime onto the plant. My brother found this treatment. It's not a cure but helps to keep it at bay.