I have an Opal apple sapling that I've been growing since she was a seed: only for the joy of watching a plant grow; not with any intent that she become fruit-bearing.
She's spent her whole life in pots (changed when I feel she was outgrowing one) and I've always kept the pot inside a mesh tent for fear of critters. Her current pot is conical: 12" top diameter, 9" bottom diameter, 11" tall. She is currently 18" tall from the ground.
I felt that she was strong, growing and healthy for well over a year: thick growth of well-formed, deep green leaves. Over the past two months, though, she's been developing a worsening symptoms on her leaves: at first a few odd brownish dots that I brushed off as the "plant equivalent of birthmarks or natural imperfections". But then the leaves started growing with odd deformities: holes of varying sizes, and sometimes large percentages of the leaf outright missing.
The only apple-related potential match I could find on Gardening Stack Exchange was this one: Apple Tree Leaf Growth/Disease? Cedar-Apple Rust? -- but those pictures don't exactly match mine: that apple tree's leaves discoloration was yellow/orange and uniformly circular, whereas my leave's discoloration is brown (the color of a dried leaf), and has irregular boundaries. Also the other post didn't show any leaf deformities.
I've wondered whether a change in the sapling's watering was the onset: until about two months ago, I'd always watered her with the same filtered drinking water that we humans drink in our household. When we went on vacation two months ago, I set her up with a drip irrigation line (it looks like this product: https://www.rainbird.com/products/micro-bubblers). I don't know the specifics of our sprinkler water quality; it's certainly not the filtered-for-humans water that she grew up with, but it's kept several plants and trees in our yard alive for many years.
She lives in south Orange County, California: I believe we are "BSh (Hot semi-arid)" per the Koppen climate types. Earlier in my sapling's life I used to bring her outside in the daytime for sunshine, then bring her inside at night; over the past two months, I've left her outside all day long, partly because she's getting too big for me to move her frequently, partly because I felt (nothing more than a gut feeling) that she was mature enough to transition to a fully outdoors lifestyle.
That is my sapling's history, and my thoughts at relevant changes in her recent history. My plea to the gardening community is: can anyone please help identify what the problem is, and any cures for my poor apple sapling's ailment? Is this a disease, a harmful critter, or unsuitable weather?
I've scrutinized the sapling as best as I could, and as far as I can tell, there are no insects or other critters visible to the naked eye on the plant or in the soil.
I thought this was perhaps noteworthy: as I mentioned above, my little plant was strong and healthy her first year of life; now interestingly, the leaves from that healthy phase of her life are completely untouched: they remain unblemished, a healthy deep green, and without the "munched on" appearance of the leaves in the pictures above. If critters were munching on these leaves, surely they would be indiscriminate, and the eating would occur everywhere, all up and down the plant, no?