Is that a K-deficiency? Boron-def matches the interveinal look below. Mg-def could match too. Phosphorous excess could cause cal/mg deficincies, so that could be it too. manganese tox too
What have you done lately? heavy feeding? light feeding? do you know what pH you add each time? can be rhetorical... then act appropriately, if you want to be proactive about it. Rule out the ones above that don't match.
From the diary, looks like it's near the end. It'll be relatively fine even if you just continue as normal.
there are 2 different leaf charts with symptoms out there. One looks like it's from a 1970s text book, one is a bit better depiction. find the latter and save it somewhere. As you use it, it becomes familiar... Then, as you avoid all problems, you forget it again and need the chart.