Last summer I had a terrible outbreak of rust late in the season but don't have a clue as to where it came from. It was in our lawn, in our flower beds, everywhere it seemed on the side of the house that gets only morning sun. This also happens to be the side of the house that the beds have the least amended soil, which means less drainage and more standing water for longer periods of time. I know that moisture,poor air circulation, and walking around in it goes a long way in spreading rust once it presents its ugly face. Thankfully, our winters here in Ohio are severe enough to eradicate it. I haven't seen any signs of it this season.
Strangely, some years I get it to some degree and other years not at all. If I get it at all, it is usually always on that side of the house and it is always late in the season, when rainfall is limited and I have to supplement water with the hose. I've not seen it on any plants that I've ever purchased since living here but then they are always planted in my beds that have the best growing condition, such as full sun and rich, loose soil that drains well. I've only treated for rust once as it seemed a waste of time and money. It didn't magically make it go away and our winters will kill it at the end of the season more thoroughly anyway.
I'm seriously thinking that one of the reasons I didn't get it this year is because I have watered less often and in the mornings instead of the evenings, giving ample time for drainage and evaporation to occur before nightfall. I've noticed less occurrence this season of white powdery mildew on some of my other perennials that seem to get it every year as well as other minor lawn diseases that we seem to get every year in certain places of our lawn. This season we have only used the sprinkler on our lawn early in the morning rather than late in the day and only twice a week (if there is no rainfall) for about a half hour or so before moving the sprinkler to another spot. Well, there WAS one morning that I forgot I had the sprinkler on and 3 hours later remembered it.
Becki