THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 307 



Ustilaginales 



The Ustilaginales are Basidiomycetes in which the basidium is produced 

 from a chlamydospore. It is either transversely septate and bears the basi- 

 diospores laterally, or unicellular and bears the basidiospores terminally. 

 The chlamydospores are thick-walled, spherical spores which are formed 

 as swellings of the hyphae within the tissues of the host. 



The Ustilaginales include the Smuts or Brands and constitute important 

 diseases of cereals. They live as obligate parasites, and infection takes place 

 usually either through the flowers or at a ver\' early stage of the seedling. In 

 some genera asexual conidia are also formed. 



Compared with the Urediniales the Smuts have a simple life-cycle. They 

 have as a rule only two types of spores, the chlamydospores and the basi- 

 diospores. The latter have been shown to be of ( + ) and ( - ) strains, and 

 chlamydospores are only produced, and the life-cycle completed, after a 

 union of mycelia of ( - ) and ( - ) 

 strains. They must therefore be 

 classed as heterothallic. 



We shall consider the genus Lsti- 

 lago, but rather than select a single type 

 species it is preferable to consider to- 

 gether the five species which are im- 

 portant diseases of economic cereal 

 crops. 



Ustilago (Smuts of Cereals) 



The crops attacked are Oats, Barley 

 and Wheat, all of which when affected 

 by these parasites show certain obvious 

 and ver\' similar characteristics. From 

 flowering time onwards some of the 

 ears appear blackened and shrivelled, 

 the grain being replaced by a loose, 

 dark-brown powder which is com- 

 posed of the chlamydospores of the 

 Fungus (Fig. 301). Despite the fact 

 that infection actually occurs at an 

 early stage in the development of 



the grass it does not make its visible F 10. 301.— Ustilago avenae attacking the 



* . . flowers 01 Oats, 



appearance until the flowering stage, 



when it is seen that the whole tissue of the flower is abortive. Two types 



are recognized. The one which is called the Loose Smut develops a 



blackish powder which is exposed at flowering time and is blown aw^ay by the 



wind. The other is termed Covered Smut, and in this type the spores remain 



in the grain and are only liberated by the breaking of the w^all of the grain 



which generally occurs during threshing. 



