What are these yellow/brown spots on the leaves of this Alocasia?

Question

Last year this alocasia 'giant taro' was very healthy growing foot long leaves shooting up every couple of weeks. At some point the leaves started to decay and while it's still alive, the leaves are small (4 inches long max) and they develop yellow spots which eventually turn brown.

I wonder if it's a pest as the underside of the leaves have some little black dots on and occasionally I have seen little black bugs crawling around. The soil is kept damp but not waterlogged.

The plant

A new leaf that has developed the problem almost immediately

Older leaves

Answer 1

One thing to keep in mind about Alocasia and other tuber rooted species is that the size of the root can displace a lot of soil, and in doing so reduces the water capacity and ability to hold nutrient. In the wild, the taro family is consistently rained on by nutrient rich solutions from the upper story of rainforest.

Moist rootball is good, but the pot as pictured is tapered and only contains half the soil mass of a cylinder the same height. Add in the displacement of soil and you may have conditions where the soil has become exhausted. Frequent weak fertilizing would be good and a larger pot with room for corms and roots to expand will be necessary. Bugs may have a role, but it's probably best to look to nutrient, humidity and consistent clean moisture first.