I've noticed in past years that when my cucumber vines get about 3 feet
long, SOMETHING happens to the growing tip. It gets kind of brown up one
side of the stem. The tip curls over and then dies.
Eventually the tip-less vine makes one or more side branch and the vine
grows on but sometimes the new tips die off too.
Today I noticed the first tips starting to be affected for this year. So
what does that? I've never seen any visible pest. I haven't used a
magnifying glass or anything.

Steve in the Adirondacks