Hi Charles, thanks for the question.

While fertilizer won't prevent Septoria Leaf Spot, plants with proper nutrients may resist diseases better than those that are nutrient-deficient. Septoria spreads mainly via rain splash but can also be windborne, insect-transmitted, or seed-borne. It can survive the winter in plant debris. Therefore, fall cleanup and plant sanitation are crucial. Removing lower and infected leaves can be beneficial. Ensure good air circulation and avoid overhead watering. Mulching and crop rotation are advantageous. As a preventative measure, apply a fungicide listed for Septoria. You can also plant Septoria resistant varieties such as:

   Heirloom: Chiapas, Coyote Cherry, Green Zebra, 

   Plum: Invincible, Juliet F1, Katya    

   Slicer: Brandywise, Iron Lady F1, Seiger, Stellar F1, Wisconsin 55

If you are concerned about your fertilizer, you can get a Home | MSU Soil Test through MSU.

Check out the following links for additional information on Septoria leaf spot

Septoria Leaf Spot of Tomato | NC State Extension Publications (ncsu.edu)

Septoria Leaf Spot | Extension | West Virginia University (wvu.edu)

Disease-resistant tomato varieties | Cornell Vegetables

I hope this helps.