I don't believe it's fusarium wilt because it starts from the stem base and this appears to be on the new growth.
Without seeing the whole plant, I will say new growth dying can be an indicator of Calcium deficiency. I'm not sure which Coast of Maine soil you're using but Jacks 20-20-20 doesn't have any calcium and cannabis is a calcium hungry plant. You didn't mention cal/mag in your nutes and it's a common deficiency...so I think it's a good chance it could be that.
An easy test is to foliar spray cal/mag and within a week you should notice a change in the dying off of your new growth. Then you'll know if that was it and you should start adding it to your nutes.