It could be micronutrient deficiency or fungal. I've used the Tomatoes Alive fertilizer from Gardens Alive with excellent results. I've provided the link to their web site. Their products are for the most part organic.

Here is a link that might be useful: Gardens Alive

How big are the plants and how long have they been planted out and what have you used for fertilizer, so far,if anything?

Has it been rainy and is the soil water saturated, b'c that can cause leaves to turn yellow quite rapidly b'c nutrients and oxygen cannot be absorbed thru the roots under those conditions.

Carolyn