I have a controller extension and I get the unknown property error on page load.

public with sharing class ExtensionController{  
       public ApexPages.StandardController standardCon;    
       public ExtensionController(ApexPages.StandardController con){  
          standardCon = con;  
       }    
       public void populateRelAccFields(){  
          Opportunity opp = (Opportunity) standardCon.getRecord();  
          opp.Account = [select Phone, Type from Account where id =: opp.AccountId];  
       } 
    }