You might want to think about downy mildew. Wet with low temps fosters that. But it's a little early in the season to be seeing that, depending on where you are. Warmer, dryer weather will often naturally fix it.

you might want to think that those were the top leaves when you transplanted ONE WEEK AGO ... and due to interference of the roots to pump water.. they sunburned a bit ...

i dont get this jumping to disease and malnutrition issues within days or weeks of tranaplanting ..

and btw.. were they properly hardened off before planting in the sun.. did they come from a greenhouse.. or inside your house???

and if bigbox.. who the heck knows what happened when they traveled 500 miles to the store ...

relax ... a little leaf damage means nothing.. look to new current gorwth to see if anything is actually currently going on ...

ken

ps: zone is the prediction of the coldest day of winter .. it means nothing the rest of the year.. plz include a big city name for us .. it really helps.. in every post ... and btw.. z6 is about half the US from new england to the rockies.. so city really helps...

Downy mildew is actually quite common in seedlings. Plants are understood to be especially susceptible early in the growth cycle when roots are just developing. There are DM-resistant varieties of cukes. Maybe yours is? The variety that most often infects cukes is, however, not very treatable with fungicides, but you can try. In particular, copper or Neem. Infection doesn't have a lot to do with transplanting. If the plants were outside after germination, they'd be exposed.

Looks like the stem was damaged.