Spots appear suddenly on Amur and JM

These spots just showed up on my Amur maple bonsai and on one of my young Japanese Maples.
The plants are at least 10’ apart from each other in my rooftop garden (I live in a 3-story loft condo), the spots can be seen front and back and no plants around either affected plant show anything. The leaves affected are only about 20% of the plants and seem grouped together on the plant. They appeared quite rapidly as I did not see these spots 2-3 days ago, at least as I remember and I visit my bonsai at least 2-3x a day during these shelter-in-place times. Fungus? Other thoughts? It does not rub off so seems not just on the surface. And as mentioned the spots show through both sides.

looks like a fungal issue to me. I would monitor the watering carefully, allowing the plants to dry out a bit. Would also cut off the affected leaves leaving half the petiole!
Try to move plants to gain better air movement between the plants!
Treat with a fungal spray, Neem Oil, Mancozeb, Daconil, for example!

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Thanks Frank. My roof garden gets a decent wind in the afternoons so plenty of air movement and they aren’t close to each other. I will try to pull back on wetting the plants down though. And will remove the affected leaves. Please refresh my memory as to why leaving half the petiole?

It is my habit to leave a portion, stops me from cutting too close to the branch accidentally and damaging the latent bud at the base of the petiole. Just habit I guess.
Perhaps more important in defoliation or partial defoliation when you are counting on a second flush of new leaves.

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Thank you. :slightly_smiling_face:

We just had some rain and high humidity - in some cases that’s all it takes for fungus to take hold. When watering, avoid wetting the foliage of trees that are susceptible to fungus this time of year (e.g. trees that already have spots, maples with leaves that haven’t hardened off, live oaks, etc.)

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