I grew this seedling and five others by starting a slice of tomato in a pot. It's about 6" tall. It's been growing perhaps about a month, and I embarrassingly just found out seed starting mix doesn't come with nutrients, and fertilized it yesterday. It and its siblings all have purple stems near the base, and leaves beginning to yellow. This evening, I found this and a couple others had white spots on them.
Could this be due to the nutrient deficiency? Or is this some type of fungus that needs a fungicide?