How do apples spoil?
The apple is wrapped in the relatively impervious cuticle. The calyx and the stem ends however contain a significantly higher abundance and diversity of microbes than the smooth skin of the fruit. These structures trap moisture and nutrients, creating a perfect micro-environment for microbes to thrive.
The major route in is through the calyx. This star-shaped opening at the bottom of the fruit is the remnant of the original apple blossom. Some apple varieties (like Red Delicious and Granny Smith) even have an open calyx tube that extends into the apple's core. This provides a direct, unrestricted pathway for fungal spores and bacteria to enter the apple.
Even when the calyx tube appears closed, it retains moisture and traps significant microbial communities.
Fungi frequently colonise the delicate flower petals early in the season and then travel inwards from the calyx.
Whilst bacteria penetrate into the calyx cavity itself, they are rarely recovered from the deep inner flesh or seeds of the apple.
The vast majority of the microbes are harmless or beneficial. Organically grown apples feature a more diverse, balanced microbiome that contains helpful bacteria like Lactobacillus.
Apples grown under modern pesticide spray regimes however may harbour fewer beneficial microbes and a higher prevalence of plant-health affecting ones.
Fungi causing core rots like
Botrytis cinerea and Sclerotinia sclerotiorum can cause tissue decay and discoloration.
Black Rot is caused by the fungal pathogen Botryosphaeria obtusa, which often infects the calyx end, appearing as brown and black concentric rings.
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