Stressed plant - what is going on? Please help

I have a few years of experience with growing. This plant (Mango Sherbet) was a bit of an experiment. The soil is a blend of coco coir and worm castings. I also top dress and use liquid feeds to supply additional nutrients. My plant was looking great until around the transition time, at which point she began to drop her lower leaves. The coloring became pale and then yellow. Now many of her leaves have dry brown marks, often appearing in a similar fashion on her leaves (See pics). My feeling is that it is a ph and/or nutrient issue. Can anyone easily identify what the issue is from the pics? Thanks!