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My basil plants have recently started spotting on the leaves with a couple yellow leaf drops and streaking on stems. Is this a disease or some other issues?
A few details:
In light of your dry climate, it could be copper deficiency. Watering with a blue water soluble fertilizer should help to fix that, though, if true. Kelp is an option for extra copper (and other minerals).
Too much phosphorus could have caused the copper not to be available to your plant. I see that you fertilized with 10-15-10 fertilizer, and that you said your plant is trying hard to bloom (phosphorus is supposed to contribute to blossoms and plant maturity).
However, since you've already fertilized, you might want to go a different route than adding more blue fertilizer, and just balance your N-P-K ratios by adding more potassium and maybe also nitrogen, but not more phosphorus; a soil test should help. Balancing the N-P-K levels might make the copper more available, but I could be wrong—you might have to deplete some phosphorus first (but I don't think so).
I've read someone say that phosphorus should never be higher than potassium in a fertilizer (assuming the soil isn't high in potassium and low in phosphorus). I would have to research this to know how true it is, though.
It could be both copper deficiency and a fungus, because copper is anti-fungal and I postulate that a lack of it might make a plant more susceptible to fungi.
Old answer:
It could be downy mildew. I wouldn't have known to suggest that, but it looks a lot like the picture at the bottom here (they say that's what it is there).
I wouldn't be surprised if it were pythium, either (if that infects leaves; I know it infects stems). It can cause damping off disease in seedlings, too.
Septoria leaf spot is another candidate.
