Answer: With the on and off cold temperatures we have been experiencing, I'll bet those tomatoes are growing slowly. That could cause the yellowing, combined with nitrogen deficiency or severe iron deficiency. When large quantities of organic material are added to a planting area, and during the time it takes for the organic material to decompose, micro-organisms require nitrogen to accomplish the decomposition. In a marginally fertilized planting area, this nitrogen drain by micro-organisms can cause plant deficiencies. You can offset this by adding a slow-release fertilizer such as 19-5-9 before planting and sidedressing with two tablespoons of ammonium sulfate 21-0-0 sprinkled around each plant and watered in) every two weeks after plants have marble-size fruit. If the cause of this yellowing is a severe iron deficiency, use a liquid iron product and water around each plant with the appropiate mixture. The response may be slow in this cold weather since plants may be a bit stunted. Virus infection can cause a yellowed appearance but I don't think your plants are infected with virus unless there's a specific pattern to the yellowing.
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