14 BULLETIN 629, IT. S. DEPARTMENT OF AGRICULTURE. 
variety with the purpose of obtaining a resistant variety through 
hybridization. 
Xone of the seedlings of the 23 varieties belonging to the Aveiw 
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 
Arena 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 beino- o^rown in the Southern States. Thev 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 TThite 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 than when more mature. It is more likely, however, that 
some of the failures to get normal or heavy 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 H-enn., and (b) 
