Question
MY TOMATOES AND ZUCHINNI PLANTS HAVE GROWN INTO HEALTHY PLANTS BUT ONCE THE FRUIT FORMING PHASE STARTS , AND AFTER, THE FOLLOWING HAPPENS:   THE TOMATOE LEAVES TURN YELLOW AND DRY UP; THE TOMATOES BRANCHES WILT AND DRY UP; THE ZUCHINNI FRUIT FORMS TO A SIZE OF THREE OR FOUR INCHES AND THEN START SHRIVELING UP AND ROT AT THE ENDS. BOTH ARE PRODUCING FRUIT ON OTHER PARTS OF THE PLANTS.

Answer
Henry,
I'm not sure what the last sentence refers to, but I will attempt to address the body of the post.  If this is happening to both types of plants, it has to be either the soil or the water, and I'm thinking soil.  It sounds like too much fertilizer or plant food.
Add some plain top soil around the plants, as deeply as you can without disturbing the roots.  Sometimes that will dilute the strength of the fertilizer.
Kind Regards,
Kindred
www.eingedi.us