Question
fruit is rotting before they mature

Answer
A couple things could be going wrong.  Getting too much moisture on the fruit or there is not enough air circulation around the plant.  Once the fruit forms if it gets too wet or touches wet ground they may rot.  The best way to prevent this is to mulch under the plants with a bit of straw, grass clippings, or even newspaper may work to keep the plants from getting wet.  Water at the base of the plant versus overhead is another way to prevent excess moisture getting on the fruit.  

The other issue could be poor air circulation, if the plants are really close together and as the leaves get large they can prevent air from getting around the fruit.  Make sure you give zucchini and squash plants are least 3 feet square to grow well.  You can cut some of the large leaves around the fruit as well.

It is probably a combination of both issues.  Remove all the rotted fruit, water just at the base of the plant and trim some of the large leaves and you should still get some healthy fruit.

I hope this helps, let me know if you have any further questions.

Catherine
www.your-vegetable-gardening-helper.com