I had been having trouble with blossom end rot on zucchini and took your suggestion to use calcium rich spray feeding and also shared this idea with my daughter. After using the calcium spray, the zucchini perked up and leaves stood more uproot in my and my daughter's cases both. However, within 2 or 3 days after using the calcium, the zucchini had wilted very badly, blossoms stopped forming all together, the plants became very, very ill and wilted shriveled up and dried out, with one plant totally dying. I am very reluctant to ever use calcium rich spray again due to this. I'm sure you can see why. What happened? Perhaps, the problem had been with inconsistent watering rather than a calcium deficiency? (Although the plants have been watered what I considered to be very regularly) It has been exceptionally hot and dry here this summer in SW Missouri. The plants had been planted square foot gardening style with 1/3 vermiculite, 1/3 peat moss, and 1/3 compost. Even some overspray with the calcium had a terrible effect on a few nearby cosmos. What did I do wrong? What happened?
Squash Blossom End Rot
Certified GKH Gardening Expert
What may have happened was that you burned the plants. When applying any kind of nutrient, it is best to do so after the plants have been well watered and on a cloudy day or in the early evening. Thirsty plants will suck up any moisture that comes their way, even if that moisture contains concentrated amounts of nutrients. You can kind of think of it like a human overdosing on vitamins - good in small amounts, not so good in large amounts. Also, many nutrient sprays can react strongly in the sunlight. Applying on cloudy days or at dusk will help prevent this.
I am sorry to hear that your plants are not doing well. I hope they recover.