Why Is My Zucchini Rotting on the Plant?

When zucchini rot while still attached to the plant — going soft, mushy and sometimes fuzzy with mould — it is disheartening, but the cause is usually a fungal rot taking hold where the fruit stays damp. This is different from blossom end rot, which is a calcium-and-water disorder; here a real fungus is at work, often entering through the dying flower or where fruit touches wet ground. The good news is that simple changes greatly reduce it. Let me explain.

Flower-end fungal rot

A very common version on zucchini begins at the blossom end, where the old flower clings to the tip of the developing fruit. In damp, humid conditions, that withering flower stays wet and starts to rot, and the fungus — often a grey mould — spreads from the decaying flower back into the fruit, turning the tip soft, brown and fuzzy. You will often see the fuzzy grey or white mould on the old flower and the rotting end. This is why zucchini rot is worse in wet, humid weather and on plants with poor airflow, where the flowers and fruit stay damp.

Fruit resting on wet soil

Zucchini fruit that develops lying on damp ground is vulnerable to soft rots from soil-borne fungi and bacteria, which enter through the skin where it stays wet against the soil. The underside goes soft and discoloured, then the rot spreads. Crowded plants, heavy unbroken leaf cover trapping moisture, and overhead watering that keeps everything wet all encourage these rots. Any wound in the fruit — from an insect, a crack, or handling — also gives the rot an entry point.

How to stop it

The strategy is to keep things drier and remove sources of infection. Pick off the spent flowers from the tips of developing fruit once pollination is done, especially in damp weather, so they cannot rot onto the fruit. Improve airflow by removing some of the lower and inner leaves and spacing plants well, so flowers and fruit dry quickly. Water at the base of the plant in the morning, never overhead, so foliage and fruit are not left wet. Where you can, keep developing fruit off bare wet soil — a piece of slate, a tile, or a thick mulch under the fruit helps.

Remove rot promptly

As soon as you find a rotting zucchini, pick it off and bin it — do not leave it on the plant or drop it on the soil, where it sheds spores and spreads the rot to healthy fruit. Keeping the plant clear of rotting material, combined with better airflow and dry-leaf watering, usually brings fruit rot under control quickly. At the end of the season, clear away all debris so the fungi do not overwinter. Stay on top of the spent flowers and the damp, and your zucchini will develop clean and sound to harvest.