the original collections of smut ol)tained from Missouri were 

 used for inoculation. These two \arieties are grown (juite 

 extensiveh' as winter oats in the southern part of the United 

 States. In recent \ears the \ariety Fulghum particularK- has 

 been grown b\' the agronomists, and selections (jhtained which 

 are well adapted to the southern area of the spring oats region. 

 Some of these selections ha\e shown a high agronomic \alue. 

 One of the strong points in connection with the \ariet\' h ulghum 

 has been its resistance to smut. However, we ha\'e recently 

 disco\-ered that there is a race of Ustilago Avenae which causes 

 se\'ere infections on Fulghum, as well as some other \arieties. 

 A distinct race of this same smut has been demonstrated to 

 occur on the Red Rustproof. Consequently, these \arieties 

 can not be considered as resistant to all races of smut, but only 

 to certain ones. 



There is a large number of other races of oat smuts differen- 

 tiated on the basis of their reaction to different varieties. Simi- 

 larly, there is a number of races of the covered smut of barley. 



These physiologic races are recognized by their capacity for 

 infecting known varieties of the host. Their behavior is definite 

 and specific. A given variety may possess complete resistance 

 to one race of smut and at the same time be highly susceptible 

 to another. Consequently, in the study of the inheritance of 

 smut resistance, we must use definite specialized races of smuts 

 as well as pure lines of the host varieties. 



When known races of smut as w^ell as pure lines of the hosts 

 are used, quite definite results on the inheritance of smut 

 resistance are obtained. A number of crosses between Black 

 Mesdag, very resistant, and Hull-less, very susceptible to 

 both the Missouri races of loose and covered smut of oats, 

 have been studied. In the second generation, out of 465 

 plants inoculated with loose smut, 107 (23 percent) have been 

 infected. In a similar series with the covered smut 196 plants 

 were inoculated and 40 (20.4 per cent) infected. Crosses be- 

 tween such varieties as Early Gothland and Hull-less, in which 

 both varieties are susceptible to loose smut, while Early 

 Gothland is resistant to covered smut, have yielded interest- 

 ing results. The F2 plants inoculated with loose smut have 

 practically all been infected, while of the 94 plants inoculated 

 with covered smut, 37 (39.3 per cent) have been infected. 



63 



