Rust is a very common fungal problem on leeks causing orange streaks on the foliage with raised pustules containing spores that are easily dispersed on the air. In very severe cases rust can even cause the leaves to yellow and wither, dying back prematurely and stunting growth. Rust attacks tend to be most prevalent from midsummer to autumn and seem to be encouraged by warm humid conditions. There is little that you can do for your current crop but you can still use your leeks - simply cut away the infected areas before you cook them. For future crops here are a few preventative measures that are well worth taking.
Plant hygiene is the key. Destroy any infected plant debris and burn it or remove it from the site - dont put it in the compost heap. Try to practice crop rotation on your plot and choose rust resistant leek varieties. Increase the spacing between plants and make sure that there is good drainage to improve ventilation and reduce humidity. Avoid using too much high nitrogen fertiliser as nitrate rich soil causes lots of lush growth which is particularly susceptible to rust. Potassium rich fertilisers Chempak® High Potash Feed - Formula 4 will help to counter this problem. Finally, keep an eye out for early signs of infection and as soon as they appear. An old exhibitors trick is to smear Vaseline on the infected areas. This will seal the spores in and prevent them from spreading to the rest of your crop.