I see episodes of tomato septoria often in summer, but don't see how disinfecting cages might help. No research has ever shown active contamination by equipment, and I've never noticed an association. Straw mulch has been found to suppress septoria, but since there is no resistance available, you d what you can. I cluster tomatoes in several spots in the garden -- 4 plants here and 6 there -- as a preventive measure. It's been my experience that when cool rain sets the stage, septoria will sweep through one bed but not another. The edges of the woods are full of horsenettle, and I think that's where the inoculum comes from.

The treatment of choice for most people is spraying with fungicides, usually copper based, but you won't get much backing for that idea in this forum. I don't know about septoria, but I understand that late blight needs host plant material all the time to survive (which is why crop rotation can work) and soil fungi are supposed to clobber late blight, so that seems to point to housekeeping and/or sanitation problems over the off season. I had this problem back when I lived in Exeter, but not here in Tucson, so I'm weather-lucky. In New Hampshire (damp and cool) we kept plants well separated and viciously nipped back. It seemed to work well.

Doesn't look like disease. More like an environmental glitch.

Septoria leaf spots are taking over my tomatoes this year too probably facilitated by the nonstop raining. The variety I observed so far (in my garden) that resists the virus are Cherokee Purple, Aunt Ruby German's Green, Cowlick's Brandywine and Kellog's Breakfast. They grow right next to the ones that suffered from it.