Leaf rotting (bacterial, fungal or other?)

[Inspire91]

Hello everyone!

To be upfront I am very new to keeping palms and so it’s safe to assume I know zero about issues like these 😅

Whatever this is, it happens really quickly. The dark rotting color takes over the leaf in a matter of days. In some cases where I catch it early enough I chop that part off and that may or may not save it. It sometimes just keeps going down the leaf till it’s taken the whole thing.

My guesses are it’s either edema from overwatering, fungal or bacterial.

Here is some info on their location and care:
1. I douse them with daconil once a month. The whole plant, leaves and stalk.
2. I don’t think that I over water, I’m very mindful to keep the soil moist but not saturated. I don’t typically let the soil dry out 100%. Should I be?
3. They get good airflow as they’re outside in a spot where air constantly blows through.
4. They’re in a general palm soil amended with lots of cinder/perlite to improve drainage. 
5. I fertilize with 18-6-8 slow release granules.

Most of my issues are with the Thai mottleds but recently it’s taken a leaf on one of my Geonoma and Caplytrocalyx. I’ve never seen this issue with my Licuala, Rhapis, Areca, and a few other one off genus I have.

My growing conditions are as follows:

1. Temp as high as 85F and low at night as 70F.

2. Both high and low humidity but regularly low. My area is typically sunny, hot and dry with the occasional passing shower(s).

3. The palms with the worst affliction are in a heavily shaded area with only ambient light coming in from one side. They’re under a deck right under the edge of it.

I really appreciate your help! I’m really lose as to what this could be…
 

It’s taken a few of my seedlings before and I thought it were isolated issues. But it’s continuing and I don’t want to keep losing these babies 🙏🏽

*Don’t mind the burnt tips on the Calyptrocalyx, those came with the palm when I bought it a year ago. The issue is the dark green on the inside of the leaf. 

Thank you all!