Hi all fellow bonsai enthousiasts!

I am relative new in the field. I started 4-5 years ago and trying to develop my bonsai trees from seeds, like the ones attached here.
The previous years when the young trees were 1-3 years old i was fighting mainly with powdery mildew, but no big issues there.

This year there are different issues.
1) The little hazel grows vigorously but it has brown spots on the leaves. Do you think its some fungi disease? I did partially defoliate it but i think its irrelevant.
The hazel:



2) The two other pictures are from a young oak. The first spring growth became very "papery", brittle leaves with mild yellow coloration. Now at the second phase of growth during summer, new growth appeared which I think shows the initial stage of the same symptoms. You can see the early yellow-ish spots being formed. Eventhough these are week-old leaves. This seems more like a deficiency in some element but cant figure out if indeed the case or what.

Oak spring growth:


Oak summer growth



Both trees are always outside.

You help is very much appreciated!

Thanks!

Look normal plants to me, leaves do die. Why did you defoliate on such small plants? That is something that is only done on far more mature trees not young plants like these.

I was thinking the same. Don't fiddle with them too much. Just plant them in the ground and let them grow freely for 5-10 years. Cut back heavy, let recover for a few seasons. Repeat if you want thicker trunk or pot them up.

Growing bonsai from seed in small pots do make great bonsai. But then you are not growing them for yourself, you are growing them for your granschildren. They will just be sticks in a pot for decades to come. If defoliated every year it is centuries.

If you want to practice and learn bonsai techniques now, just get yourself more mature materials to work on.

I appreciate your advices and I know that it will take much longer. I am not in hurry
As i wrote i didnt defoliate completely just partially to gradually train them.

My question was specific on the diseases.
The oak might look ok for now, but i have another one exactly same age same stage yet the spring leaves are perfectly fine. so there has to be something wrong with this one. Leaves feel very brittle and a bit wrinkled.

Partial defoliation does not add anything to the training of the trees. No quality will be gained from it for the final result whatsoever. Don't do partial or full defoliation on trees that are not in show preparation stage.

Scorched leafs can have many reasons. Old leafs soon to be shedded. Too much sun too sudden. Pests etc. But the most common amongst beginners is underwatering.

Thank you Tropfrog, underwatering might be problem.
Have a look at the same leaf some days later. Do you still think its water related?

I have not provided any thoughts on what I think is the reason and will not do it now eigther. Simply because it is impossible based on a photo.

I would just cut the leaf off and focus on correct care. Not worrying about one single leaf.

its almost all leaves on that tree. But ok thank you anyway for your time.

Have you checked for insects? Look under the leaves, I would not be worried if it was mine though, in such a young plant. Water when needed, so when the soil is almost dry, and unless rain is relentless for hours wait until it stops before watering otherwise you will get wet.

Your oak has a fungal infection. Defoliation & treatment with a fungicide can help.
However, peak growing season is behind us, and I do not defoliate after summer solstice anymore.

Keep an eye on it in spring and see how it develops. If you live in a place where you can get systemic fungicide, consider using it.