are these plants indoors?

Hi Sonni,
No, the plants are on the balcony

bump - any tomato experts around?

How big are the pots?

Do you fertilize? If so, with what and how much, also how often?

I'm no expert. I grow heirloom variety tomatoes in South Florida and I never spray anything. I always have tomato disease. I cut away the ugly leaves and hope for the best. That works for me - but I have lots of room for lots of plants. If you google tomato disease, you'll find good sites with photos from Clemson U., Rutgers, Cornell, etc. To me, a lot of the problems look so similar, but you may identify the problem and find an organic solution to save your plants. I think Jean is asking about your pots and feeding because tomatoes can be sensitive to water and fertilizer needs. Too much, too little. They need good sunlight and adequate temperatures. So all that makes it a bit difficult to provide a solution from a photo. If you've successfully grown tomatoes on your balcony before - this may be a fluke. Definitely sanitize your pots before using them again. If this is your first or second time growing tomatoes - well, I'd say the fun is in the challenge.

Oh, I just noticed you are in Florida too. How has the humidity been? High humidity = wet leaves = disease. Don't water the leaves, maybe spread the plants out so air circulates better around them, is it possible to get morning sun to evaporate the dew? This probably won't help the plants you have now, but if you're in the southern end there's still time to replant this year.

It seems to be magnesium deficiency. You can try a CalMag fertilizer. But nutrient deficiencies sometimes can be due to how the soil mix is along with moisture level and if the plant is able to uptake the nutrients.

Does look like a mobile nutrient deficiency of magnesium. symptoms are Interveinal chlorosis (yellowing between veins but veins remain green) with dead spots. Maxjohnson is right and the deficiency could be caused by another problem. acid soil (low PH) will generally lower base saturation of Ca and Mg. Even if you put magnesium in a low PH soil the plant can't exchange (take in) the Mg, do a PH test. Or the soil mix is bad and Nutrients including Mg Leach out. Are you over watering and leaching out the Nutrients?