Septoria is a tomato plague I've accepted as a chronic problem in my garden. Early Blight, to a lesser extent, the same. I realize both are gonna be with me for as long as I grow tomatoes at this location.
I have used Daconil in very limited applications. Once this year. None last year. Etc.
The one time this year was on the transplants when they were about 12 inches tall and still in their 16-oz. cups but in flats sitting out in the raised bed exposed to weather ... just to arrest the spread after I severely pruned off the damaged leaves ... so that I could get some more planted into the beds and mulch them down against further splash back.
I have a bad horse nettle problem 'cuz I use compost made from horse stall bedding, manure, old rotted straw, and other nearby agricultural products many from fields infested with invasive seeds. I also have some wild solanaceous weeds. Whatever.
So, what can you do right now?
Weed your garden thoroughly.
Rake up all the waste and debris that has fallen to the ground.
Cut all the badly affected leaves off the plants and prune some more of the excess foliage out of your cages, like get the plants cut back to where nothing is hanging outside the wire for you to brush up against and spread the fungus from one plant to another. Think out the inside foliage a bit to increase air flow.
Carefully spray the wire low down on the cages with a solution of 1 part Clorox or Purex with 10 parts water. Be very careful to just spray the wires and let the bleach solution fall to ground without hitting the stems. You might kill a few leaves, but when I've sprayed my house gutters with a stronger solution of bleach and it fell on tomatoes growing under the leaves, yeah, I got some leaf damage but didn't kill any tomato plants. Just sayin' ...
Re-apply the mulch around and between the cages, stuffing some fresh straw into the cages to prevent any more splash back.
After that, if you still have severe Septoria on certain plants, like spots on the fruit and wilted leaves covered with advanced spore growth, remove the entire plant and take the cage out of the garden for a thorough disinfection with the bleach solution.
Next year grow varieties known to be tolerant to or capable of outgrowing Septoria. I have not found a variety 100% resistant to Septoria, but some varieties will withstand or outgrow it.
Listed in order of ability to withstand Septoria in my heavily infested bed:
Mountain Magic - nearly bulletproof
Chello (a gold, open pollinated cherry) - no problems ever
Red Brandywine, Landis Strain - awesome tolerance
Druzba - almost as tolerant as was RB, Landis
Sun Gold - nearly bulletproof
Indian Stripe x Sun Gold - intermediately bulletproof (LOL)
West Virginia 63, also intermediately resistant to Late Blight
Indian Stripe - decent tolerance to Early Blight and Septoria but eventually will succumb to Septoria later in season, never seen it go down to Early Blight though
Bear Creek - Had lots of Septoria on lower leaves but vine persisted in nearly rampant growth, set and ripened fruit form palm-like upper fronds while lower stems were completely denuded. Last plant in garden in fall to produce and ripen decent tomatoes.
Daniels - fairly tolerant and reported to be tolerant to Late Blight, too. Yeah, it got Septoria but it went on to produce good, sound fruit and basically outgrow the plague.
Mozark - a determinate that may get Septoria but will still set and ripen a concentrated crop (40 - 50 tomatoes) before succumbing, the tomatoes will be red and ready.
Mozark x Sioux - good so far this year while others on both sides went down
Ananas Noir - completely free of Septoria until late in production cycle
Traveler 76 - after a severe bout early was able to outgrow and withstand
Cherokee Purple - same comment as Trav. 76
I'm sure there are others, but none I know of will be completely Septoria resistant.
If you don't want to use chemicals on your plants, and even "organic" fungicides are in fact chemicals ... just from organic rather than synthetic sources ... then you're gonna have to go with tolerant or intermediately resistant varieties and employ good cultural practices like ...
Spacing to enable air flow and movement thru the garden without carrying spores on your clothing
Pruning to remove all diseased foliage and create better air flow
Heavy mulching with successive applications of dry, clean staw
Complete removal of severely diseased plants
Sanitizing cages, tools, gloves and other clothing, etc.
Removing weeds and garden debris, tilling under the top layer, applying cover mulch over the winter.
Q