Rotting Watermelons

Hi everyone,

Does anyone know why our watermelons are rotting?  They have developed a black surface at one end and then take over the whole thing.  Not all of them are suffering this ailment, so I'd like to know what is causing this to prevent it on the other ones.

They are two different kinds of watermelon.  One is 'sugar babies' and the other is 'orange glow'.  Any ideas?

Bob