CEREAL PESTS PARASITIC FUNGI 517 



The presence of this smut in the field may be detected 

 by the fact that infected plants are usually shorter than 

 healthy plants, and the smutted panicles, instead of 

 spreading and drooping, are considerably shortened, erect, 

 and more or less constricted. Examining the infected 

 panicle closely, it is seen that instead of each flower giving 

 rise to a normal oat kernel, the entire flower and even 

 glumes are converted into a mass of loose, easily distrib- 

 uted smut (Fig. 159 6). Usually all panicles of a stool 

 are diseased. Occasionally the upper portions of certain 

 panicles are sound while the lower portions are entirely 

 destroyed by the fungus. The loose smut mass is found 

 on microscopic examination to consist of myriads of brown 

 resting-spores, which continue to be viable for at least 

 7 years. When the smutted grain is sown, if not treated 

 with a good fungicide, the conditions which favor the 

 germination of the seed also favor the development of 

 the fungus. Infection takes place during the seedling 

 stage of the host, and apparently at no other time, so in 

 this respect the fungus is similar to wheat bunt. 



On germination the spores produce a germ tube which divides 

 into 3 or 4 cells, each of which develops a single conidium. These 

 conidia germinate and produce narrow, elongated germ tubes, 

 which, if near or lying upon the oat seedling, will infect it. Within 

 the host the parasite attempts to grow in proportion to the vigor of 

 the former. In vigorous plants the mycelium is perhaps most abun- 

 dant at the nodes, and it appears that in some instances the rapidly 

 growing tissue may actually grow away from the invading fungus. 

 However, once established in the tissues of the growing point, it is 

 hardly possible for the host to escape its enemy. A microscopic 

 examination of the immature ovaries of an infected plant will show 

 that instead of being filled with normal host tissue they contain a 

 mass of fungus mycelium more or less knotted and of indefinitely 

 appearing structure. Later they will be found filled with resting- 

 spores. 



