I'm growing melons for the first time this year. I planted seeds in mid-May, and they had been doing well when I left for work yesterday. When I came back, one of the arms on the biggest plant was mostly dead, and the center stem and first ~2" of all the arms were dry and shrivelled. From what I found online, I assumed squash borer beetle, but now I don't know. I looked closely at the base of the stem, and didn't see any "sawdust" piles, or evidence of any bug in the stem. Should I be actively digging into the stem, looking for something? What else could it be? It's been 90+ F, so I've been watering daily. I planted them on a decomposed granite slope, but dug out a 2' trench and replaced it with planting mix from a local nursery. It's still very well-drained soil, so I don't know if it's possible to overwater.
Any advice? Thanks very much.