I don't do anything about greenfly, apart from squishing them off the buds if there are a lot, I leave it to the ladybirds and lacewings, and within a week they've generally cleaned up the lot.
As erica says, hungry and stressed roses are more likely to succumb to black spot and rust, so feed and water them well. I give mine a foliar feed of seaweed solution once a week and that seems to stave off blackspot. It's also good for mildew on stuff like squashes and pulmonaria that are very prone to it.
A cupful of milk (doesn't have to be fresh, if you've some on the turn it's fine) in a watering can of water will also keep away blackspot and mildew. I don't get rust, so no idea if it works on that.