Q.zucchini
The plants look healthy. Blossoms form, as does the zucchini. When about 3 inches in length, the zucchini turns brown. How can this be prevented?

This sounds like it could be either Botrytis blight or Bacterial wilt.
These links have more information.
https://www.gardeningknowhow.com/edible/vegetables/zucchini/solutions-for-common-problems-growing-zucchini.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/signs-and-treatments-of-squash-wilt.htm
https://www.gardeningknowhow.com/plant-problems/disease/treating-botrytis-blight.htm