I have found that the bigger and the healthier the plant is in general, the less rust it will have. If you have a struggling SF, and it gets rust, I would hardly say it was prone to rust, until you could get the plant bigger and then see what happens. I too, have rust on different plants, different years. The weather also plays a big role.
Last year was the year I sprayed round up on all my plants, and had to cut them all down to the ground. I think it sure helped the plants, Im going to do some cutting back this year as well.
