The rash on the scrotum could be anything from benign to something having serious pathology
. It is not possible to comment exactly without examination.However the following information would be useful.
The rash could as well be due to some inherent testicular condition like a testicular lump. The causes include injury, varicocele, cysts, abscess, lipoma, fibroma, benign tumour, cancer, swollen testicle, syphilis, tuberculosis or mumps.