Q.Why do my healthy tomato plants always die off?
I live where volunteer tomatoes pop up in empty lots and flourish with no care and scarce natural watering (San Diego area). Yet my plants invariably end up like some version of this photo; I bought a 12″ healthy, disease resistant plant, and 2 months later it looks like this. Scrawny, yellow/grey mottled leaves with small fruit, that will soon die. In the past, I’ve tried more water, less water, fungicides, fertilizing, and even replacing all of garden with clean topsoil. What is my most likely problem, and solution? Thank you!
It's not possible to determine the disease that your tomatoes are suffering from at this late stage.
Are you practicing crop rotation? You should move tomato plants every 3 years to avoid soil born diseases.
Just removing topsoil will not be sufficient; you will need to sterilize soil.
https://www.gardeningknowhow.com/edible/vegetables/vgen/rotating-vegetables.htm
https://www.gardeningknowhow.com/garden-how-to/projects/how-to-solarize-garden-beds-to-eliminate-garden-pests-in-the-soil.htm
Growing in containers may be a better option for you.
https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-pots-containers.htm
