Hi there, I was hoping someone could help me identify what is harming my newly-planted Bok Choy?

I planted some seedlings three weeks ago, and the leaves up until now have seemed firm and healthy. Some of the leaves touching the ground went yellow and wilted away, but the ones on top were thriving.

Then, a few days ago I noticed these grey patches about 10mm in diameter. They seemed to be minor and only affected two leaves on my bok choy, and one on my broccoli.

This morning, I checked on them before work and everything was fine but to my horror, when I got home in the afternoon I noticed one of the formerly healthy leaves had this long, trailing yellow stain on it!

What is it? The closest thing I could find is powdery mildew, but it doesn't look the same to these untrained eyes. Can anyone help identify the problem and what, if anything I need to do about it?

Thanks,

John

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