That's generally N deficiency.

Dan

oh, sorry, it's a crookneck squash, not a cuke.

N deficiency? I fertilized with Gardentone in the planting hole and with granules (you can see) and a time or two with diluted MG or Neptune's Harvest fish fertilizer. Yikes, they need more?

That is one of the oldest leaves on the plant. The first that will show yellowing from excessive watering. IME Nitrogen deficiency will normally show up in new growth, new leaves first.

As any plants grows/ages it is normal for the oldest leaves to begin to slowly yellow and die off but in this case it isn't a very old plant. Have you had lots of rain lately? If so then that would explain it. If not, then you may be over-watering the plant.

From the pic that sure looks like a small container for a crookneck.

Dave

Yes, we have had a lot of rain and I dare not complain after the past few years of severe drought.

It's a 3 gallon container with one plant in it.

Any suggestions? Just let it be? Cover it from rain when I can so it dries out? I don't have any in-ground space to put it, unfortunately.

That plant doesn't need fertilizer. It looks great. As Dave said it could be too much water causing the yellow in the oldest leaf. Let it dry a little then stick it in a larger pot with some more potting mix. I wouldn't use anything smaller than a 5 gallon container. I might even cut some 55 gallon barrels in half for cucs and squash this year.