Growing tomatoes in large pots on the balcony- what to do?

Had that happen to mine. Old time gardner told me to put 3 tums or a tablspoon of epson salts in the planting hole before putting the plant in. Has worked fine, also use it in my garden when planting tomato plants, once plants are established I put some epsom salts around the plant about half way thro growing season

That is tomato blight. https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blight.html

What type of tomato are you growing? If they are determinate, they will die after setting and ripening their fruit. If it is blight that you are dealing with, I have used spoiled milk and mulch to prevent it, seems to work. I got slammed with it one year and lost every tomato plant. Not having space to move the crop, I treated with the milk and white corn meal and mulch and was fine the following year. I grow Amish Paste and Brandywine for a crop that produces until the first frost. I also save the seeds, since they are heritage and breed true, unlike the hybrids.

I bought cloth bags 3 gal from Amazon, they are great, I never have trouble with my tomatoes or other vegetables. They are also so much cleaner, can't over water either, which is very bad for tomatoes. Not expensive either.

Skip the plastic bags. The plants need to breathe, the plastic inhibits air flow.

Why do you put a trash bag into your pot? Won't that cause a problem with retaining too much water? I have never had a problem with diseases on my potted tomato.

Another thought with the plastic bag and container is the possibility of the plants having too much water. Does your container have drain holes and do you have rock or pebbles at the bottom? Very few plants can tolerate "wet feet.". Also, as others mentioned, you will need to feed the plants during the growing and fruiting times.