I learned something new in my garden this year. All of my zucchini plants (11 of them!) had several zucchini at a time that were half-grown with one end (the blossom end) rotting off. When two other people mentioned to me that they had the same thing happening, I immediately got on the computer to look it up.
It turns out that Blossom End Rot is caused by a calcium deficiency in the soil. So, once the plants are growing and the ends of the zucchini are rotting, it’s too late to add calcium to the soil. At that point, you can simply pluck the blossoms off the small zucchini and they won’t develop rot. You can also spray epsom salts dissolved in water on the leaves to try and draw calcium up into the plant, but if you don’t have enough calcium in your soil to begin with, that won’t help. I tried that and it didn’t make any difference on my plants.
But once you know you have calcium-deficient soil, you can add calcium the following year when you are planting your seeds or starts. Next year I’m going to try adding bonemeal and see how my plants do. People also add things like eggshells or powdered milk, but I’m going to try organic bone meal. Apparently this can happen to any fruiting vegetable, like tomatoes. I’ve never had a problem with my tomatoes rotting, just my zucchini, and they are planted in the same soil just a few feet apart.
It can also be caused by uneven watering….too much and then too little. If the watering is not consistent, the plant may not be able to take up calcium when it’s growing.
Here’s a good, short article about it with some solutions listed: http://www.gardeningknowhow.com/problems/squash-blossom-end-rot-causes-and-treatment.htm
Good luck you squash growers!