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Any apple growers here? HELLLLP. This is my first time cultivating apple trees. Granny smiths. I started these over the fall, and they are now about 16" tall. The newer leaves are curling, and eventually start browning on the edges, some are even showing brown spots on the leaf body. General cultivation says it could aphids or powdery mildew...not one bug to be found, and zero signs of PM. Google has been no help. I supplemented some of my hydro nutrients...micro and macro elements, in case of a deficiency. No change. Ph is a decent 6.1 to 6.8. I even flusbed them with Hydroguard in case of root rot. Water is sufficient, without being too much. What am I missing?

Are you saying you are growing them hydroponically? I didn’t know you could do that.

Maybe some trees could thrive in hydroponic culture, but apples? My understanding about general tree/woody plant development is that they establish symbiotic relationship with soil microbes as they grow. Some trees can’t grow without these essential friends in the rhizosphere. Some are not so fussy.

I agree with Applestar. There is a strong relationship between the root systems of trees with soil born microbes.
Most plants grown by hydroponics are short lived plants.. Grown for their annual crop. Toms, straws,etc.
I suspect that even these would suffer if you tried to continue with them in further years.
In fact strawberry growers who I know who use hydroponics for their crops, all move these plants to a soil base after two croppings. These being Autumn and spring croppings.

These are in soil. I used some of my specialized hydroponics nutrients on them while chasing possible deficiencies. I washed off the roots on one of the worst affected...they are not slimy and don't smell like rot. But, they are a dark tan to brown in color, rather than the white to tan I am used to seeing. These have been bottom watered to reduce the gnat population. I am now leaning towards root damage by overwatering. Poor things are starving for nutrients Sound plausible?

I could not see the size of the individual pots. A tree sapling 16" tall should be in a container no smaller than 12" in diameter, preferably larger, with good drainage. No pot saucers. They also need to be moved outside as soon as possible. Trees are not indoor plants.

Good luck.

I agree lenaree it does sound like water logging. Give them a chance to recover.
Tree can be quite tough and all the time there’s some leaf there they will keep going.
You may well find that if they get through this tough time, they may suffer from a few deficiency problems. Magnesium...Nitrogen...etc.
Just be patient.