Brown Spots on Leaves

Hello. I have been growing some plants in large tree containers. Things seemed to be going well right up until around when they started to bud. I noticed small brown spots on the lower leaves at first and now they have spread throughout the entire plant. Some plants are worse than others. I have been using liquid fertilizer once a week roughly. Recently I have picked up some CalMag. Does anyone know the problem here (I was thinking cal or mag deficiency)? Thanks in advance.

Good reference chart and starting point.

That symptom is generally calcium and/or magnesium deficiency. If you haven’t fed with cal/mag yet I’d start there.