As a member of the Cucurbita family, zucchini is susceptible to a variety of bacteria and fungus diseases. Among the most common are powdery mildew and bacterial wilt. Plant disease problems are most common in hot and humid weather, and especially towards late august as the plant is aging and weakens. Promoting a strong, healthy plant and fungicide treatment will help avoid these problems. Treat with fungicides at the first sign of problems.
Some people wait until the fruit becomes a monster.
While definitely still edible, it is tougher and the skin may need to be peeled. Zucchini is not a hardy plant. It is susceptible to frost in the spring and summer. They are also very susceptible to insects and disease. These prolific producers overcome these threats to produce a bountiful crop.