14 BULLETIN 629, U. S. DEPARTMENT OF AGRICULTURE. 



variety with the purpose of obtaining a resistant variety through 

 hybridization. 



None of the seedlings of the 23 varieties belonging to the Avena 

 sterilis group showed any resistance to the stem rust, and in only 

 three of the varieties did the plants inoculated at heading time give 

 any evidence of resistance to this rust. It is entirely safe to conclude 

 that all of these varieties are quite susceptible to stem rust, and the 

 Avena sterilis group probably will offer little in the way of resistance 

 to stem rust that is of value to the plant breeder. 



Of these 23 varieties 16 show some degree of resistance to crown 

 rust» Certain strains were strikingly resistant in both the seedling 

 stage and at heading time, and from the clear-cut evidences of re- 

 sistance there can be no doubt of the presence of resistance to crown \ 

 rust in varieties of this group. 



These varieties which are actually resistant to crown rust, if found 

 to be high in yield, should replace some of the " rustproof " types 

 now being grown in the Southern States. They may be of use also 

 in the breeding of rust- resistant varieties for culture in other sections 

 of the country. 



Table I shows that there are many more cases of resistance to the 

 crown rust than to the stem rust. This is especially true of the work 

 on seedlings, where none of the varieties tested except White Tar- 

 tarian and Ruakura Rustproof showed any resistance to stem rust. 



In the studies of both rusts, more apparent cases of resistance are 

 recorded from the inoculations made on the plants at the time of 

 heading. This may be due to the fact that plants are more susceptible 

 as seedlings tha»n when more mature. It is more likely, however, that 

 some of the failures to get normal or heaA r y infection were due to the 

 fact that it was more difficult to wet thoroughly, and hence inoculate 

 heavily, the upper leaf blades and sheaths than the young seedling 

 leaves. 



The use of the word " immune " is avoided, for in the forms studied 

 none were observed in which very distinct evidences of infection did 

 not appear. The words " resistant " and " resistance " are used only 

 in a relative sense and refer to that condition in which normal 

 urediniospore production by the fungus was either prevented or 

 seriously interfered with. As Stakman (13) has pointed out, the 

 quality which is called resistance may actually be, in the extreme 

 sense, susceptibility or hypersensitiveness. It amounts to " commer- 

 cial resistance," using that expression to describe a variety which will 

 suffer less severe damage in the field than some others. 



SUMMARY AND CONCLUSIONS. 



(1) Two distinct rusts of oats are common in the United States: 

 (a) Stem rust, Puccinia graminis avenae Erikss. and Henn., and (b.) 



