Weird white spots on my avocado tree leaves

[ElectroMaster88]

I have a two year old avocado tree.
In the last few months, white spots appeared on the lower leaves of the tree (like in the picture).
The upper leaves do not have those. These spots can be washed away with water but they are always coming back (didn't notice after how much time).
What are those?

 

[Anniekay]

Hard to tell but it looks like the start of powdery mildew which thrives in moist, still air conditions. Since it looks like you planted it close to a building, it may not be getting enough air circulation.

 

[cpp gardener]

They are mineral deposits from your sprinkler water. Nothing to worry about, but they are really big trees and need lots of room unless it’s one of the dwarf varieties.