Yes the EM rash is a sign/symptom of Lyme Disease.
"As traditionally described, the first symptom associated with LD is a dermal reaction at the site of the tick bite called Erythema Migrans (EM). "
jemsekspecialty.com/erythema-migrans/"Fewer than 50% of patients with Lyme disease recall any rash. Although the erythema migrans (EM) or “bull’s-eye” rash is considered classic, it is not the most common dermatologic manifestation of early-localized Lyme infection. Atypical forms of this rash are seen far more commonly.
It is important to know that the EM rash is pathognomonic of Lyme disease and requires no further verification prior to starting an appropriate course of antibiotic therapy."
www.ilads.org/lyme/about-lyme.php" Erythema migrans (EM) is diagnostic of Bb infection, but is present in fewer than half."
www.lymenet.org/BurrGuide200810.pdfYou get a bullseye rash - treatment should ensue.