A most remarkable circumstance in the life history of the wheat rust is the fact that the mycelium produced by the sporidium can live only in barberry leaves, and it follows that if no barberry bushes are in the neighbourhood the sporidia finally perish. Those which happen to lodge on a barberry bush germinate immediately, producing a mycelium that enters the barberry leaf and grows within its tissues. Very soon the fungus produces a new kind of spores on the barberry leaves. These are called æcidiospores. They are formed in long chains in little fringed cups, or æcidia, which appear in groups on the lower side of the leaf (Fig. [283]). These orange or yellow æcidia are termed cluster-cups. In Fig. [284] is shown a cross-section of one of the cups, outlining the long chains of spores, and the mycelium in the tissues.
Fig. 283.—Leaf of Barberry with Cluster-cups.
Fig. 284.—Section through a Cluster-cup on Barberry Leaf.
The æcidiospores are formed in the spring, and after they have been set free, some of them lodge on wheat or other grasses, where they germinate immediately. The germ-tube enters the leaf through a stomate, whence it spreads among the cells of the wheat plant. In summer one-celled reddish uredospores (“blight spores,” red-rust stage) are produced in a manner similar to the teleutospores. These are capable of germinating immediately, and serve to disseminate the fungus during the summer on other wheat plants or grasses. Late in the season, teleutospores are again produced, completing the life cycle of the plant.
Many rusts besides Puccinia graminis produce different spore forms on different plants. The phenomenon is called heterœcism, and was first shown to exist in the wheat rust. Curiously enough, the peasants of Europe had observed and asserted that barberry bushes cause wheat to blight long before science explained the relation between the cluster-cups on barberry and the rust on wheat. The true relation was actually demonstrated, as has since been done for many other rusts on their respective hosts, by sowing the æcidiospores on healthy wheat plants and thus producing the rust. The cedar apple is another rust, producing the curious swellings often found on the branches of red cedar trees. In the spring the teleutospores ooze out from the “apple” in brownish yellow masses. It has been found that these attack various fruit trees, producing æcidia on their leaves. Fig. [285] explains how a parasitic fungus works.
Fig. 285.—How a Parasitic Fungus works. Anthracnose on a bean pod entering the bean beneath. (Whetzel.)