Photomicrograph of Aecium (Cluster Cup) of Puccinia graminis
The life cycle of a rust fungus is often very complicated and the complete cycle of a rust may require a number of different spore types on two different host plants. The disease known as 'Stem Rust of Wheat' caused by the fungus Puccinia graminis has some stages on wheat and other stages on a broad-leaved shrub called barberry (Berberis vulgaris). This micrograph is a section through a so-called cluster cup (=aecium) of the fungus. Cluster cups are produced on the underside of the barberry leaves. To see a macro view of cluster cups click here. The spores are produced in chains and as they mature they are released and drop out of the cup. They are windborne and infect wheat plants within many miles down wind of the infected barberry plants. The role of the barberry in the cycle was recognized early and a barberry eradication program was invoked to eliminate all the Berberis vulgaris plants growing in the US and Canada and thus break the cycle. It didn't work! Why?
Many years ago the ornamental Red Barberry was a common hedge plant or shrub in Southern Ontario. You can still find it in many gardens of homes built thirty years or more ago. It was supposedly immune to the Stem Rust of Wheat fungus. One day, however, a plant pathologist found a rust pustule on ornamental barberry and the shrub was, therefore, banned as an ornamental. So, while not uncommon around the older gardens in the province, you can't buy it at a nursery.
Note: I now see that horticultural varieties of barberry are now (2008) available for purchase et horticultural outlets.