Phal bellina dropping green leaves clean
 

This bellina seedling dropped a leaf clean pretty much out of nowhere. I don't see clear signs of rot, although some roots seem to have some blackening and there's some drying on the stem but it feels more like a normal small localized infection which I've seen in other plants and never turns into anything. Can't remember seeing a leaf drop clean without yellowing first.

Anyone has had a similar experience or can offer suggestions as to what to do?

Might it be low humidity or too low light? Maybe it got too cold?

Could it have gotten a chill (by bellina standards) recently?

Was it an old leaf or a new one? Losing an old one can just be force of nature (maybe it got a bit dry and it was trying to cut down on leaf area as a defense) Losing a new one would be of more concern.

I had a phal species (the equestrian one, I can never remember the spelling) drop a green leaf like that. It was an older, but still very healthy, leaf. Nothing else happened for weeks, so I assumed it was a fluke. It eventually died of other causes, unrelated to the leaf.

I'm attaching closeups of where the leaf detached.


It was an older leaf, and it did grow during a time in which I was moving, and it never got very big. I attributed it to subpar lighting and stress during that time.

Maybe? That was my first thought. It's placed on a heat mat, but there's a thermo-hygrometer placed about a foot away, not on the heat mat, and the temperatures read min 67, max 78F. Humidity 47-72%. I was hoping the heat mat to keep the pot's temperature in the 77-88ºF range.

Some of the newer roots have what I suspect are salt burns, and some of the grow cubes and LECA have some salt buildup. I do flush thoroughly and fertilize weakly.

Might it be a bit of a perfect storm going on? Since I keep the plant on a heat mat to combat evaporative cooling, that might actually be speeding up the evaporation process making ~50% RH too low for the top layer of LECA to keep up, and thus the salt buildups happened. Faster evaporation = faster cooling, plus the heat mat will heat up the base but not necessarily the temperature around the leaves. So not sure if the heat from the mat is being neutralized by the faster evaporation around the root zone.

My final concern is that I perceive a bit of a foul odor coming from the pot, plus there's some black looking tissue in the crevices of the lower part of the stem. It makes me wonder if keeping the bottom water really warm (it's potted in S/H) might've created a perfect breeding ground for water molds like Pythium.