This is nearly always caused by sudden wetting of the soil after a prolonged period of relative drought causing the plant's tissues expand too rapidly as it sucks up the water. Cracks may become infected by moulds and will reduce storability.
Vertical cracks in roots such as parsnips, carrots and beetroot
Splits in the skin of tomatoes
Hollow spaces on the insides of potatoes and onions
Prevention & control
- Keep plants well-watered during dry spells - a mulch will help to prevent the soil drying out
- Keep plants growing strongly as stop and start growth can also cause this problem
- In tomatoes, harvest in good time (and ripen indoors if necessary) to avoid the cooler, wetter conditions in the autumn