428 PLANTS AND MAN 



and life cycle. They are all parasitic, and have as their most 

 common hosts members of the Grass Family, although they are 

 known to attack many other herbaceous seed plants and some 

 ferns. The smuts are all internal parasites, growing within the 

 tissues of the host plants (fig. 259). In the majority of smuts in- 

 fection is seed borne, the pathogen occurring either as a spore on 

 the surface of the seed, as is the case in the loose and covered 

 smuts of oats, stinking smut of wheat, and covered smut of barley; 

 or as a resting mycelium within the seed, a condition found to 

 exist in loose smut of wheat, brown loose smut of barley, and the 

 loose and covered smuts of oats. In the case of the corn and onion 

 smuts, infection results from soil or wind borne spores. Control 

 measures must be applied in the light of this knowledge of infec- 

 tion sources and can be effected by cultural practices such as 

 proper spacing in planting, rotation of crops, and by the use of 

 resistant corn and onion varieties. In instances where spores are 

 borne on the seeds, surface disinfection of seed by means of 

 chemicals is effective; this method is of no avail in the case of 

 internal seed borne smut mycelium, which must be killed by 

 heat, since the seed embryo can tolerate a higher temperature 

 without injury than can the fungus mycelium. 



Corn smut causes an estimated annual reduction in yield 

 amounting to 3-5% of the total corn crops of the United States. 

 Infection takes place early in the growing season by means of 

 wind borne spores which are able to penetrate the host plant 

 only by way of new tissues. Danger of infection is generally past 

 after the plant is over eighteen inches high. The pathogen grows 

 within the plant tissues, remaining near the spot where infection 

 entered and stimulating the plant to produce the large galls or 

 tumors so characteristic of smuts. Ultimately the mycelium in 

 these galls becomes segmented and heavy walled, giving the 

 whole mass a black, dusty appearance. These heavy- walled 

 mycelial segments are known as chlamydospores, and function 

 as overwintering structures in the soil or plant refuse until the 

 next growing season, when they germinate to form the basidium 

 stage which carries the infection to new corn plants. Control, 

 as indicated above, is accomplished by cultural practices. 



