62 DISEASES OF TROPICAL PLANTS 



CH. 



The species of this order may be spread in various 

 ways ; in some cases they cling to the seed, are planted, 

 grow, and penetrate the young plant ; in some cases 

 the spores are carried by wind, insects, etc., to other 

 host plants which are in bloom, grow and penetrate the 

 developing seed, in which they remain dormant until 

 the following season, or they may be carried over in 

 the old decaying plants which are left on the field. 



The order includes two families, the Ustilaginaceae 

 and the Tilletiaceae. Under the first family are several 

 genera, of which we will consider but one, Ustilago. 



U. avenae, (Pers.) Jens., or the loose smut of the 

 oats, is the most common and most destructive. It is 

 as widely distributed as the oats themselves. The 

 flowering plants are frequently thoroughly infested by 

 the mycelia, which give rise to masses of black spores 

 within the flowering glumes. These spores are scattered 

 by the wind, and possibly to some extent by insects. 

 They are also thoroughly mixed with the grain at 

 harvest time. When no precautions are taken to 

 destroy them they germinate with the germinating 

 grain, and thus cause the infection of the young plants. 



U. tritici, (Pers.) Jens., is the loose smut of the 

 wheat, and is also very widely distributed. The life- 

 history of this organism is practically the same as that 

 of U. avenae, but it is also claimed that the maturing 

 grain may become infected, and that the organism may 

 thus be carried over from season to season by mycelium 

 within the grain, 



U. zeae, (Beckm.) Ung., of the corn (Fig. 44) is 

 the cause of the large, very unsightly growths on the 

 ears, tassels, and leaves of the plant. The spores live 

 in the soil and in the old and decaying stalks, and 

 infect the young and growing plants (see p. 95). 



U. sacchari, Rabenb., of cane. 



U. sorghi, (Link) Pass., of corn and kaffir corn. 



U. segetum, (Bull) Dittm., of cereals. 



In the family Tilletiaceae we will consider two 

 genera, Tilletia and Urocystis. 



