Hey all,
I'm just moving over to this forum from the Houzz/Gardenweb forum, so I hope this is the correct place to post questions about tomato issues. I just posted this in the general tomato section, but I'm realizing that this is probably the right place for this question:
I am growing 14 tomato plants in 30 gallon fabric containers and one of my plants, a Mortgage Lifter, started developing a yellowing pattern that I haven't seen before. Its sort of mottled with yellow uneven patches. At first I thought this may be a magnesium deficiency, but it's not looking like it. I'm am growing in new potting mix (Fox Farm Happy Frog potting mix) and have been feeding it with a small handful of Happy Frog granular fertilizer every 2 weeks. I water by hand approximately every 3 days with a hose at the base. Checking moisture levels regularly so not to dry and not too wet. Spray every 2 weeks with bonide copper fungicide. Any thoughts as to what this could be. Its only affecting one plant out of 14.

I think you were probably right, and it is magnesium deficiency.

Here is a web pic of mg deficiency that looks just like yours:
https://www.allotment-garden.org/wp-con ... Tomato.jpg

You could try epsom salt or cal mag. There are always other possibilities that a plant expresses a nutrient deficiency besides lacking that nutrient. Having too much of one nutrient can block out others, ph issues display as nutrient deficiencies, and so does damage to the roots, whether caused by root rot or pests. Based upon your other plants being fine, I would guess that variety might just be a heavy feeder, if it is the only one of that kind you have. If not, I would see if maybe the roots are getting mucky. Happy frog is good stuff, but a little dense. I usually dilute it with more perlite.

I think that this is a nutrient deficiency and not a disease. Some varieties of tomatoes just seem to need more of some things than others. I think that it is either magnesium or potassium that is lacking. Try a quick fix for magnesium by mixing 1 tablespoon of Epsom salts to a gallon of water. Drench the soil and you can even spray the plant too. If that does not work in a few days then try adding something high in potassium. Magnesium deficiency is very very common in plants grown in the greenhouse using a drip system.