As you can see, the leaves are gradually eaten by some gray/dark dry blight. I have absolutely no idea what it is.
1 Answer
That might well be slug or snail damage - given there's lots of leaf litter laying around your plant, you've created a perfect environment for slugs to hide beneath during the day, and then appear at night and snack on your basil. If you want to confirm that's the problem, go out with a torch at night, especially a damp night, and see what's around.
Clear away the plant debris and try using whatever slug/snail bait you prefer to protect the plant.
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Yup, not a blight (fungal disease). Earwigs would be my guess as I don't see any slime trails. Eliminate the Earwig and slug CONDOMINIUMS. Clean up debris, get rid of old lumber, rocks, river rock edging. I'd put a barrier around my basil at night, a milk jug with the bottom cut out and shoved down into the soil an inch or two. Careful leaving this jug over your plants during the day. This plastic jug could easily cook your basil in the sunlight. If it is slug damage, go out at night with scissors and chop them in half. They'll be all over your lawn and you can reduce the population...– stormyDec 19, 2016 at 21:48