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

Can anyone help? I have a feeling these plants aren't going to make it through the winter.

Also, we need more info because the only thing you've mentioned is yellow spot.

At the very least, tell us about the following
the soil and what you did before planting.
how do you decide when to water?
when you water, by what method and how long do you do so?
Do you fertilize? If so, with what?
D you see any critters? If so, please describe.