I have no immediate solution but I would suggest a soil test right away. Calcium deficiency will surely cause blossom end rot. But it takes a while to get the calcium distributed throughout the soil and digestible for plants. I try to do organic gardening which means being pro-active for the next year. By adding the needed calcium for next years heavy feeders I eliminate blossom end rot. Just make sure you are not overwatering. I mulch about everything especially squash. I water less frequently and eliminate problems associated with overwatering. I manually squash the squash bugs and use homemade organic spray to deter them. Anyone else want to weigh in?
It’s already hit 100 a time or two in Texas so the garden needs the every other day or so watering. Everything else is happy with the watering. I’ve hand squished some of the squash bugs but haven’t seen any since the Sevin dust but now that I’ve said it they’ll be baaaack. I haven’t had too much trouble with any other plants getting chewed on - a few of the mustards and a little on a broccoli so that’s not bad (knock on wood). There had been lots of bees when the squashes (yellow crookneck, zuke, sweet dumpling and delicata) were blooming but only I saw one earlier this week so at one time it seemed they were happy.
BTW, we’re in the country so we get our water from our own well that’s fed from the aquifer up in the hills and from the river we’re on. There shouldn’t be much chemical fertilizer run-off since there’s only one small farm that just grows hay. All the other land is natural except for a half dozen houses up from us. I’m sure there is some mixture of underground water coming from the river but it has an “excellent” water rating even with the lawn chemicals and boating activity on it. Bottom line, the squash are getting about the best water they could wish for.