15 C might be too low. I don't actually have this, I have a luteo-alba. IOSPE has it as a hot grower but having been around/near its natural habitat (I did NOT see it in-situ ), I'd say intermediate.
I see dark or even black in that bald patch, also seeing some discoloration starting at the juncture of some leaves.
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Anon Y Mouse
"Never attribute to malice that which is adequately explained by stupidity." Hanlon’s Razor
I am not being argumentative. I am correcting you!