foliage turning brown from the tips

Not sure what is going on here . . . most of my irises are looking great, however, some of them seem to be drying out or turning brown from the tips of the foliage. Is this caused by a specific disease that hits irises? What can be done?

I have one that does the same thing. Wish I knew why. This is only my second year growing iris. My variegated irises have done great but my non-variegated TB has always had foliage problems, turning brown from the tips. But it's flowering just fine. Maybe too much sun? Anyone else have any ideas?

Claire