A plotholder has sent in this picture of what has happened to his sweetcorn this year. It is called smut. It is the result of a fungal disease caused by Ustilago maydis. In some cultures the deformed kernels are a delicacy similar to mushrooms in flavour but for most it means trimming the top part as the rest remains edible. The fungus may over winter in the soil so it best to leave it 3 years before replanting corn. There are resistant varieties but the changing climate may well be causing it to become more common.