Every year, I grow a zucchini plant, and every year a few of the zucchini stop growing once they reach a few inches in length, then turn yellow from the blossom end:
Left to themselves, they will start to turn soft and shrivel, again starting at the blossom end. I generally pick and dispose of them once I notice them.
Other than a few zucchini that don't develop properly, the plants themselves are generally healthy and usually have healthy ones forming at the same time.
What causes this, and what can I do to prevent it?