Alocasia amazonica - brown/red spots

Question

I have been gifted this plant one month ago. According to plant.id it is an Alocasia Amazonica.

One or two weeks ago some of the leafs started to show a brown spot and some reddening on the side. The other leaves (especially the smaller ones) seem fine. I also think the affected leaves used to be flatter (less wrinkled) but I do not know for sure (no before photo).

There are many posts about such symptoms for that plant or similar plants on the internet; the trouble is, apparently the cause can be anything :

• underwatering, not enough humidity, and/or not enough light
• It's insects/fungi
• It's all normal, older leaves die off

What is it here? And how can I differentiate between the above causes? Note that this is the first plant I have on a full-time basis so I might be missing obvious stuff.

Background:

• it is in my kitchen, which is reasonably lit by natural light, but not directly under the window
• the kitchen may be a bit colder than 18°C which apparently can be a problem for that plant
• Watering: per the instructions I watered it once a week but I am not sure the quantity of water is right. I water near the base of the plant slowly until the soil stays wet after five seconds; I just measured, that's about 5-10cL.

Answer 1

The reddish brown spot seems too pronounced to be caused by under-watering or over-watering, less sunlight or sunburn.

Also, decline of older leaves would not create such a spot in the middle of the leaf. There, the leaf cells generally die off from the boundary of the leaf.

Which leaves the most likely option: that those leaves are infected.

Now, if the agents are insects including mites, you can see them if you observe closely. Fungii may not seen with naked eye.

However, Alocasia amazonica, as the name suggests, is originally a tropical plant, thriving in the warm humid environment of a rainforest. Temperature even lesser than 18C is just too cold for it too thrive. This would weaken the plant, and leave it more susceptible to further infection.

Provide it more warmth. Apart from the spot, the general discoloration is often a symptom of nutrient deficiency. It is required to ensure that the plant is properly fertilized (but not over fertilized). Using organic fertilizers, like compost of vermicompost, are good options for it.