TLDR: Citrus keeps turning black and oozing resin. But I can't find the problem. I thought it must be root rot, but they look perfectly healthy:
Long version: Because my first and second citrus trees fell victim to root rot, I started using a very airy substrate made of pine bark, perlite and some humus/worm castings in a 5:1:1 ratio for all my plants (figs, pineapple, cherimoya, monstera, etc.) with little adjustment. You may recognize this as 'aroid mix'. But it works surprisingly well in my indoor space with a west-facing window and terracotta planters (and my tendency to overwater).
But I can't wrap my head around citrus.
It always starts with rapid growth, followed by very suddenly dropping and crisping leaves, black stems and finally death.
I thought I must be root rot again, which I need to mind during winter. But today, when I dumped my fifth (!) tree, I found only perfectly healthy roots and nice smelling substrate.
I think it must be a pathogen… but what? I am at a loss. I keep killing my citrus trees and I don't know why. :'(
EDIT: replaced "5:1:1 mix" with "5:1:1 ratio" for clearification.
It would help if you had some pictures of the leaves before they fell to rule other things out, but it looks like a blight to me. That spreads by spores entering injured plant tissue. That's why you see the plant producing resin, it's trying to block off the sites where leaves fell so the disease doesn't spread to other areas.
It could definitely be injury from water stress, but could also be temperature stress if you have large fluctuations in anbient temp throughout the day. I kind of wonder if watering by placing the pot in a tub would be more beneficial? Plants will sometimes take up more water that way, but with the mix you have it won't be staying in the substrate long enough to cause rot
At first they stay green (even the fallen ones). But as the tree keeps declining, some will turn brown along the edges, in an inverted v-shape, starting from the tip.
Temperature stress is something I'll have to look into. Thank you!
Leaf tips turning brown is usually water stress, which definitely fits with the plant shedding green leaves. I would try watering a little more frequently for sure. Lots of luck with your next one!