The patterns are caused by papaya ringspot virus type-W, which despite its name only infects plants such as watermelon, cucumber and courgettes.

While it can stunt growth and reduce yield, the patterns on the skin are, as you say, attractive, and in some parts of the world they sell for higher prices. The virus is classified as a potyvirus and like most plant viruses has a single-stranded RNA genome.

Meriel Jones, Port Sunlight, Merseyside, UK