Diagnose: Blossom End Rot on Zucchini

1. The fruit stopped growing because the flower was never pollinated, which is not blossom end rot.
2. Watch whether the fruit is still enlarging, because that single observation splits three different problems apart. A fruit that keeps growing while a dry sunken dark patch spreads at the tip has genuine blossom end rot, caused by water delivery rather than by soil calcium. A small fruit that stops growing, yellows along its whole length and shrivels was never pollinated. A soft fruit carrying fine white whiskers tipped with black has a fungal wet rot instead.

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