Sounds like your basil may be suffering from a fungal infection on the leaves. To control immediately remove any leaves with spots and collect any that fall from the plant. To discourage fungi do not overwater or overfeed. Basil is not a heavy feeder. One application of slow release plant food such as Osmocote in June is enough. Make sure your basil plants have good drainage and full sun and that if potted the plants do not sit in their drainage water. Try to encourage better air circulation by providing room around the plants so air flows freely. You can cut back your basil plant by one third to encourage a fresh start. Keep the foliage as dry as possible. Good luck, Marianne Binetti