Respectfully, additional water, but perhaps no additional fertilization?
You know I'm no one to talk, but I feel that most plant illnesses are caused by lack of nutrition rather than most other suspects, but to tell anyone that and have them concur? Not a chance! 'Sacrificing' one plant to see what works differently on it may show other-than-expected results. When one has his own good results, he/she wants others to try it and to have been the source of the information. I can see that the H2O level is appropriate - I have the same dark/light soil pattern on my AG test plants, which I'm about to fert-hydrate in a few minutes and they are doing great on just 10-52-10. I do not grow onions and yet, maybe some barely-giant pumpkins, lol---eg