Before anyone can answer, we'd need to know this: are the cukes planted outside or are they growing in containers inside?

that means they need Nitrogen.

We also need to point out that yellowing leaves can mean many different things. The most common cause is too much water.

But you "add nitrogen" by adding fertilizer, by feeding them. The type/brand of fertilizer you use is your choice.

Dave

thanks, they are outside in the veggie garden. It's only the lower leaves so maybe they are okay.

Yellow lower older leaves = too much water. Tiny new growth leaves on top that are yellow = (maybe but not always) not enough nitrogen. ;)

Dave