294 Minnesota Plant Diseases. 



are produced in yeast fashion. This is an important fea- 

 ture, for the fungus may thus continue vigorous in such 

 places as manures for an indefinite length of time. Any 

 of these spores, when placed under favorable conditions, can 

 send out a small germ tube which, when it comes into contact 

 with the seedling of an oat plant, will pierce the sheath of the 

 seedling and make its way to the little mound of growing tissue 

 at the tip of the stem. The fungus branches here freely and es- 

 tablishes itself, as a well developed mycelium, between the cells 

 of the host plant. There is no external mark by which such a 

 plant can be distinguished from a healthy one until the forma- 

 tion of the grain. The attacked plant appears perfectly normal. 

 The fungus in the growing point keeps pace with the latter in 

 its growth. The fungus threads disappear in the older tissues 

 so that the mycelium can usually only be found in the region of 

 the growing point. When the oat stem branches the fungus es- 

 tablishes itself in the growing point of the branch as well as in 

 the growing point of the main stem and this accounts for the 

 fact that usually all heads of an attacked plant are smutted. 

 When the head commences to form, the fungus invades every 

 flower and in the organs of the latter it forms its smut spores in 

 great abundance. It is not until this period that the fungus 

 comes into evidence. Every grain is thus attacked and filled 

 with the smut spore powder. 



One well known method of prevention of oat smut is the hot 

 water treatment. This has in general been replaced in recent 

 years by the formalin method. Both of these treatments are de- 

 scribed under Steeps in Chapter XV. By both of them the 

 smut spores which cling to the grains are killed off, while the 

 grains themselves are not injured. Infection in oats is dependent 

 on the bringing together of germinating smut spores and seed- 

 ling oat plants, and the destruction of the smut spores attached 

 to the grain very considerably lessens the danger of infection, 

 as it is from-'these spores that infection generally takes place. 

 The smut spores also germinate most readily at about 50 degrees 

 Fahrenheit or about the out-of-doors spring temperature, and 

 their ability to germinate decreases with the rising temperature. 

 For this reason late sowing is sometimes recommended. This is 

 however entirely unnecessary when the above methods of pre- 

 vention are used. 



