I hope you dont mind a SB responding to your question as well. 

I usually chose to apply disease and Dot after purge first, then WS-debuff and then STR. The reason for this is, that people usually purge after having lost a good portion of their health. Hence a double Healcharge/pot is likely to follow, so disease has the first priority. Dot for the damage.  WS - Debuff and Str is then following in that order, as it reduces incoming damage only, after Purge. 
If the purge happens early on for whatever reason, I debuff with WS and Str first, then apply disease and Dot in that order. 
I hope that helps you.
Kindest regards.