From the photo you sent I see yellowing leaves and some dark spots on the foliage. My best guess is that this plant has  too much water causing the dark leaf spots and yellowing foliage due to root rot. Make sure the plant has good drainage and does not sit in a saucer of water. Another cause of yellow leaves is sunburn, and a third cause is lack of nutrients. Have you fed this plant? One application of Osmocote plant food in the spring will feed for six months.  You may want to try cutting the plant back to half it's size right after you fertilize. This will encourage fresh new growth and more basil shoots for pesto. Keep growing, Marianne Binetti