CHAPTER XIX 

 RUST FUNGI 



Suborder Uredine^. — -Usually in systematic works placed as 

 ORDER UREDINALES. The fungi belonging to this suborder are 

 characterized by basidia which are divided either by transverse or 

 longitudinal septae. In this character, they are contrasted with the 

 EUBASIDII, which have unseptate basidia. Including the rusts this 

 suborder embraces some of the most important disease-producing 

 fungi, the study of which concerns the mycologist. 



The uredineous fungi are those which are strictly parasitic and 

 which in some cases are so specialized, that their growth is confined to 

 the species of a single host. Those fungi in which the different stages 

 of the life cycle are passed on the same host are known as autoecious, 

 while those which grow on two or more hosts are known as heteroecious. 

 The plant on which the final stage is passed is called the final host, 

 while the other plant on which some of the stages occur is designated 

 the alternate host. So speciahzed is the nutrition of the rust fungi, 

 that they never have been grown on culture media off the host 

 plants on which they live. Hence, they are obligate parasites. The 

 myceUum is septate, much-branched, usually ramifying between or in 

 the walls of the cells and sending haustoria into the cell cavities. 

 The reproductive spores are borne in more or less definite clusters, or 

 sori, below the surface of the host, or rarely singly, and the spores are 

 set free by the breaking open of the overlying tissues of the hosts. 



Five different kinds of spores may be found in the uredineous fungi, 

 but they are not all present in every genus (Fig. 64). The final spore 

 form is known as the teliospore, or teleutospore, which determines the 

 name which is to be appHed to the parasite. Such spores are borne 

 in a sorus known as a teUum. When these teliospores germinate, they 

 produce a four-celled promycelium known as a basidium, and this 

 abstricts sporidia, or more properly basidiospores, which are minute, 

 thin-walled spores without surface sculpturings. These are succeeded 

 by spermogonia (spermogonium), which are now called by most 



187 



