I have successfully grown basil in this raised cedar planter (Peterborough) for the last five years, but this year my plants are in distress. When planted, the leaves were bushy and green, but the leaves have yellowed and have brown spots, and the stems are pale and weak. The soil is a moisture control container mix. The Italian parsley, rosemary, and chives I have planted in the same bed are thriving. The bed gets lots of afternoon sun (west facing). I had read that over watering might be the problem so I have been careful to check the soil before watering. I fertilized with a liquid 12-4-8 about one week ago. Can this batch of plants be saved, or should I start again? If I do need to start from scratch, is there something I should do differently?
Thanks in advance for any advice.
From your photograph, your basil appears to have a fungal disease, perhaps Fusarium wilt of basil, whose symptoms are consistent with your description of yellowing leaves and pale and weak stems. This is a soil-borne pathogen which can remain in the soil for many years, so it is a good idea to rotate your basil crop to different parts of your planter over the course of three or four years.
Since there is still plenty of time to grow basil this summer, our suggestion is to start over in a new container with new soil. Here is a good fact sheet with information on this fungal disease, as well as other diseases that can affect basil crops, and some good details regarding cultural practices, irrigation, fertilisation, etc.: https://hgic.clemson.edu/factsheet/basil/