I have two plants of zucchini planted side by side with 4' in between. One is producing fruit although the fruit has one inch at blossom end that is skinny, then it grows out normal. However, the other plant seems to get fruit started but when it is 3 inches or so, it turns yellow and stops growing. I have also other squash plants that do not seem to have a problem, including one zucchini italiano largo about 6' away that is producing like crazy.