skin
itchy
red, scaly
allergic
diaper (AmE), nappy (BrE)
heat
poison ivy (esp. AmE)
nettle (usually nettlerash)

have
break out in, come out in, develop, get
When I'm stressed I break out in a rash.

cause
Some pesticides can cause rashes and burns.

appear, come out, develop, spread
The rash most commonly appears on the back.

rash on
He has a slight rash on his chest.

Rash is used with these nouns:
decision, promise