During our evening walk, we came across a (maple) leaf with white/black patches, presumably a fungal infection.

I got a thin sample from the epidermis using tape and looked at it under the foldscope. Here are some fields of view of what presumably looks like the infectious agent. It looks like something with a dense nucleus in the center and 4-6 appendages emanating from it.

Image 2

I also looked at the whole leaf under the microscope. It seems wherever there are black/white patches, the pathogenic agents seem to be condensed.

Image 3

On the suggestion of Manu and Laks, I also imaged parts of the leaf that were not infected to make sure these structures are not condensed trichomes. I did not see them in uninfected parts of the leaf.

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Image 4

To be doubly sure, I went back to the same plant and took another leaf that did not have any white/black patches of infection as better control.

Again, I did not see the structures that I saw in the infected parts of the previous leaf.

Image 6

I also imaged the impression of the uninfected part of the leaf under the microscope as I did for the infected one in Image 2. I did not see any signs of the appendages-like structure, only debris etc.

So, it doesn’t look like condensed trichomes as per the images, and I will try to follow up on Laks’ suggestions to dig into the giant book of fungus to identify this tiny creature that is infecting the benign maple leaf.