Here's how I get through this without spraying. (Others can recommend spraying). As the seedling grows, I strip the lower leaves and side shoots gradually until there are no leaves 18 inches from the ground. The disease can come from the soil splashing up onto the plant. Keep the plant caged or tied up and off the ground.

I put a layer of compost around the plants as a mulch. I pick off any leaf that begins to have those yellow spots and throw them in the trash. Wash your hands to keep from spreading the disease. Meanwhile, the tomatoes are growing. This will get me through the summer with tomatoes to pick.

Also, they need airflow and watering from the bottom if possible. If the plants look like disease has taken over, you can pick the green tomatoes and ripen them on a plate indoors out of the sun.

We've had a lot of rain this month so it's hard to keep disease away. Just keep picking the bad leaves off. That will slow the spread (in my experience.)

Want to add that you should grow indeterminate varieties and not determinate. Indeterminate will keep growing taller as you pick off the damaged leaves, so you always have fresh growth.

Thanks, but, in the case of this particular plant, it was in a pot with new potting soil and a layer of (bagged) mulch, on our deck, so it shouldn't have been able to pick up a disease from the soil, unless the mulch or potting soil was infected. (Because of prior disease problems, I used new soil this year, in a clean pot.) This is also an unusually low-growing, viney plant, not more than 1 foot above the pot rim (and much of it hanging below the pot rim), so pruning off the low leaves wouldn't be an option!

I'll try removing the damaged leaves from the other plants, which are more upright and so far have less damage.

Can the disease spread through the air?

Look up tomato blight. It's a fungus spread by spores.