Why Does My Zucchini Fruit Have Sunken or Mouldy Spots?

When zucchini fruit develops sunken, discoloured, or fuzzy mouldy spots — distinct from the calcium-related blossom end rot — you are looking at a fungal fruit rot. Several fungi attack zucchini fruit in damp, humid conditions, and while the names differ, the causes and the cure are broadly the same: moisture, contact with wet soil, and fungal spores. Let me help you identify what is rotting your fruit and how to stop it.

The common fruit rots

A few rots show up on zucchini fruit. Belly rot, caused by a soil fungus, produces a tan to brown, sunken, water-soaked patch on the underside of fruit resting on damp soil, which can develop a fungal crust. Grey mould (botrytis) causes a soft brown rot often starting at the blossom end or a wound, covered in characteristic fuzzy grey spores in humid weather. Phytophthora and other soft rots cause mushy, collapsing lesions, sometimes with white fungal growth. They differ in detail, but all share a love of moisture and an entry point — a spent flower, a wound, or contact with wet ground.

Why they appear

These fungal rots thrive in warm, wet, humid conditions and where fruit and foliage stay damp. The classic risk factors are fruit lying on wet soil, dense plant cover trapping humidity, overhead watering that wets everything, crowded plantings with poor airflow, and clinging dead flowers that rot onto the fruit tips. Wounds from insects, handling, or cracks give the fungi an easy way in. So an outbreak usually follows a spell of warm rainy weather on a crowded, damp plant.

How to manage and prevent them

The strategy is the same regardless of which rot you have: keep things drier and remove sources of infection. Pick off spent flowers from developing fruit, especially in damp weather, so they cannot rot onto the tips. Keep fruit off bare wet soil using a tile, slate, or thick dry mulch beneath it. Improve airflow by removing some lower and inner leaves and spacing plants generously, so fruit and foliage dry quickly. Water at the base in the morning, never overhead. Remove and bin any rotting fruit immediately — do not leave it on the plant or soil where it sheds spores. Clear all debris at season's end and rotate crops, since these fungi overwinter in the soil.

The takeaway

Sunken or mouldy spots on zucchini fruit are fungal rots driven by moisture and contact with wet soil or debris. You manage them by keeping fruit and foliage dry, lifting fruit off the ground, removing spent flowers and any rotting fruit promptly, and improving airflow — and you prevent them with good spacing, dry-leaf watering, mulch barriers, and end-of-season clean-up. Stay ahead of the damp, and your zucchini will develop clean and edible right through to harvest.