I acquired Red Volunteer late August of this year. It looked great until the rust showed up in my garden beds in September. It promptly proved to me that it is rust susceptible. As far as a rating goes, I'd say probably a 3.0. I have others that were susceptible to rust but were more on the 5.0 scale, this one was not as bad as those. But ... yes! It is definitely rust susceptible.
I think the number ratings are confusing. Rust resistance ratings can go from 1.0 to 5.0. Rust Susceptible ratings can go from 1.0 to 5.0. At some point it would seem that the ratings overlap each other. Like a 2.5 - 3.0 rating. If a daylily has a 2.5 rust resistance rating ... it gets rust. If a rust susceptible daylily has a 2.5 rating .... it gets rust. So how do those numbers really help you?????
