I'm in San Diego, CA (coastal) so its usually always in the mid-70's. Although last week it did get in "scorching" 80's. Not sure if that's too hot for it. Today its 73 degrees F. Most of the lower leaves are gone because I cut them off as soon as they start to yellow, wilt then die. It started with the lower leaves and have moved up the entire plant. Its a lot more pale yellow now than it was a week ago (when I took the photos). Maybe another sign of root issues? It's in one of the larger clay pots. Maybe 12 gallons? I don't know for sure but I don't think the pot is too small. I have larger plants that are in smaller pots that seem to be thriving. I fertilized again about 2 weeks ago (with one of those slow release fertilizer sticks).

Chlorosis?

Well the brutal answer is 'It's dead'. As to why it's dead it's impossible to tell from the picture. It could be underwaterd, overwatered, overheated, frozen, attacked by a fungus or a combination of several factors. What is the mix it is in and has that pot got any holes for drainage? What do the roots look like? (It won't hurt it to pull it up - it's past any suffering.) October seems a strange time to be starting cucumbers in Canada so my suggestion, harsh though it might appear, is to give up for now. Do some reading over the winter and start again at a more appropriate time.

i can't tell you, but mine look the same, so hope we can both get an answer...