Faulty distribution: missed a spot with the fertilizer.
Elimination: fertilizer and water didn’t help. It must be insects.
Quadratic factoring of the variables: I can fertilize or treat for pests, but not both. I can apply weed treatment with fertilizer, but not with pest control. I must water after pest control but not before. I water before weed control, but not after. I can water before I fertilize, but definitely after. Unfortunately, guessing and checking is likely to kill the lawn.
Adding unlike terms: Do not combine unlike garden chemicals.
Extraneous solutions. The fungicide I used last year didn’t work this year…probably because the chemical froze over winter.
Order of operations: It’s not my fault the frost came after the blossom.
Clean up first: then the next steps will be more obvious.