There’s really not enough shown here for us to tell whats happening. It may be basil downy mildew, but we would need to see two photos of those browned or yellowing leaves – one from the top and one of the underside of the leaf. If it’s downy mildew you’ll see gray mold-like growth underneath and the tops will be yellowing turning to brown.
No matter which fungal or downy mildew condition might be causing the problem you could 1) Be sure not to water too frequently or get the foliage wet when watering 2) Use Actinovate, a bacterial fungicide labeled for both downy mildew and assorted other fungal problems, being sure to spray not only tops but the underside of stems and leaves. Mix according to directions and spray weekly. 3) Fertilize with the liquid fast-food synthetic fertilizer (no time for organics to kick in this late in the season) of your choice to stimulate new growth that you can pick and use the new growth even as the disease-infected leaves continue to decline. (No harm to humans from eating leaves infected with any of these problems.)
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