He did a thorough investigation on "Puccinia graminis", the pathogen of rust of wheat, rye and other grains. He noticed that "P. graminis" produced reddish summer spores called "urediospores", and dark winter spores called "teleutospores". He inoculated sporidia from the winter spores of the wheat rust on the leaves of the "common barberry" ("Berberis vulgaris"). The sporidia germinated and led to the formation of aecia with yellow spores, the familiar symptoms of infection on the barberry. De Bary then inoculated aecidiospores on moisture-retaining slides and then transferred them to the leaves of seedling of rye plants. In time, he observed the reddish summer spores appearing in the leaves. The sporidia from the winter spores germinated, but only on barberry. De Bary clearly demonstrated that "P. graminis" required different hosts during the different stages of its development (a phenomenon he called "heteroecism" in contrast to "autoecism", when development takes place only in one host). De Bary’s discovery explained why the eradication of the barberry plants had long been practiced as a control for rust.