Everything you need to know about Septoria, it's causes, symptoms and treatment.

http://vegetablemdonline.ppath.cornell.edu/factsheets/Tomato_Septoria.htm

As you will see it is an air borne fungus, and soil contamination is uncommon unless one leaves old plant debris to over-winter. Its prevention depends on a regular application of fungicides from the day of plant out when exposure to the fungus in the air becomes possible. It is one of the most common of the common tomato plant diseases and once it develops on the plant all you can do is slow it down.

Dave

Thank you Dave.

Would spraying the leaves from day 1 of transplant, and throughout the season, prevent the fungus altogether ?

Or am I over thinking this?

There are no absolute guarantees in gardening. There are far too many variables beyond our control. So while using a regular spay program, per the label directions, from the day of plant out will improve the odds of preventing it substantially, no one can claim 100% prevention.

Dave

Thanks as always for your help Dave.