Something has gone wrong with my sweetcorn cobs grown on the allotment...
Dotty Howse, Sutton Bonington, Nottinghamshire
Stefan says: I’m surprised yours is the first question I’ve had recently about this problem, as it’s essentially a disease of hot summers. It’s sweetcorn smut, a fungal infection that first achieved prominence in Britain in the hot summers of 1975 and 1976.
The black powder comprises the spores of the fungus. The infection doesn’t spread through the plant but is restricted to the cobs. These spores persist in the soil and in crop debris and although some varieties of sweetcorn are said to be partially resistant, this cannot provide a solution because there are several strains of the causal fungi.
I’m afraid all I can advise you to do is destroy the plants and crop debris and try to avoid growing sweetcorn on the same land for at least the next five years.