Why am I getting Rust on my leeks?
It's caused by spores of the leek rust fungus which are blown in by wind. As the particular rust which affects leeks spends its whole time on living plants and doesn't survive on dead leaves, it's likely it was blown from another infected plant in the surrounding area. Leeks are one of the few veg that can be growing at any time of the year, so there's always a host around somewhere. I had it on my leeks about 10 years ago but never before or since.