Bottle gourd leaves are green but veins are yellow

This is happening on 1 of 3 Bottle gourd plants. All three plants are growing very vigorously and producing gourds. This seems to have started on older leaves and working its way up leaves on the main vine of that one plant. The base of the vine looks OK, no damage.

Probably a slight nitrogen deficiency. I wouldn’t worry too much, probably just due to fast growth.