Getting iscsiadm to use CHAP for discovery is tricky. There's two parts:
First, iscsiadm creates a bunch of configuration in /var/lib/iscsi. In your case, after running with -m discovery, you should have the file:
/var/lib/iscsi/sendtargets/192.168.53.200,3260/st_config
Edit that file to change authmethod to CHAP and add your username and password:
discovery.sendtargets.auth.authmethod = CHAP
discovery.sendtargets.auth.username =
discovery.sendtargets.auth.password =
Then, use "-m discoverydb" instead of "-m discover" to make use of the stored configuration. Include "--discover" on your command line:
iscsiadm -m discoverydb -p 192.168.53.200 -t st --discover
Take care to use discoverydb -- if you do "-m discover" it'll just blow away your changes to that st_config file, and you'll have to edit it again.