Its underside.

Would need a much closer picture to ID aphids or the black spots.for sure but my best guess based on the pic is mold spores and over-watering.

The edges of the leaves and the yellowing color indicates root rot from soil kept too wet and it sure appears wet in the pic. Overly wet soil without adequate air circulation in the area generates mold growths. Peat pots are notorious for causing over-water issues.

But knowing your location would also help - that's why a space is provided for that info. :) If that is snow outside the window then not only is it too early for cukes to be seeded but it is too cool by far and it could easily be too cool by that window.

Dave

I agree with Dave. Those darn peat pots kill more plants.

According to your profile info, you are in Canada, which means it would be too early and too cold to have cucumbers growing. And I also agree that the peat pots are a terrible invention.

I'm in Vancouver, Canada so (thankfully) that is not snow, just my patio siding. It has been sunny and close to 20 degrees celcius as of late.

They are indeed in peat pots - and here I thought they were a great idea! "No transplant shock" they said.. Oy vey.

I'll take another picture when I get home later but I think they are mold spots and not bugs, comparing to some pictures on google. If it is indeed mold, am I screwed? Can you save a plant from mold? I read cucumbers can be direct-sown when the weather is warmer, so should I try that?

Plants can be saved from molds if it doesn't progress too far. Molds only grow in damp stagnant air conditions. So you need to dry the soil and pot out substantially and cut way back on the watering. Most important improve the air circulation. Use a small fan to increase the air circulation all around them. You may lose those leaves but the new growth should be ok.

Some also suggest lightly misting the undersides of the leaves with a 1:10 diluted of hydrogen peroxide in water. I have no personal experience with that.

But bottom line, yes, cucumbers will always do better when direct seeded out doors at the proper time.

As for the peat pots, if you keep using them then despite what the manufacturer says, be sure to strip them off the root ball prior to transplanting.

Dave

Here's an update. They look a bit better now now that they have more air circulation and less watering. The peat pots actually got a chance to dry lately. Here's a closer look at the possible mold. Probably mold, yes?

A look at the curl and yellowing. The new leaf looks green and happy at least.