That's because the infection of daylilies with this particular fungus is new to the United States, having arrived from Asia or South America probably sometime in 2000. I do not pretend to be an expert on the subject but am passing along information I've heard from other people. Some of those who have shown interest in researching this disease include Pat Henley, Patrick Guidry, Dave Talbott, Sue Bergeron, John Baron, Patrick Stamile, George Lawrence and many others. Rust is a term applied to certain types of fungi that look like rust in varying degrees. There are over 1,000 varieties of rust that affect many different plant species.
Daylily rust is the particular fungal pathogen 'Puccinia hemerocallidis.
It spreads by spores. Leaves turn brown as the rust kills them. It is new to the United States. However, recent reports from daylily growers seem to be downgrading the threat. Some even think that rust will turn out to be less of a threat than leaf-streak.