464 
Journal of Agricultural Research 
Vol. XXIV, No.« 
.THE MODE OF inheritance OF RUST RESISTANCE 
The Fj plants grown in the field during the summer of 1919 were not 
infected, since no rust developed in the field where they were grown. 
Consequently no determinations of resistance or susceptibility could be 
made under field conditions. 
The P3 plants were extremely susceptible when grown in the field under 
an epidemic produced artificially by inoculation with several biologic 
forms. The Marquis control plants, which are completely susceptible to 
all of the forms used in producing the epidemic, had an average rust 
infection of 87.7 per cent. The hybrids growing under the same con¬ 
ditions had an average rust infection of 80 per cent. This high suscep¬ 
tibility of the hybrids growing in the field was to be expected, because 
both the Marquis parent and the Kanred parent are susceptible to some 
of the forms used in producing this epidemic. These results very clearly 
demonstrate how a general field epidemic may fail to differentiate the 
segregation for resistance and susceptibility in the progeny from a given 
cross. The most accurate and reliable method of determining the 
resistance and susceptibility of hybrid progeny to any given number of 
biologic forms is to grow the plants under controlled conditions and to 
inoculate them with single known biologic forms. 
The plants of 10 families from each of the first six classes, and 5 fami¬ 
lies from the seventh class, for growth habit (fig. i), were tested for their 
reaction to Biologic Form I. This form is one which has been carried 
in pure culture since 1916 and has remained constant in its reaction on 
various host plants throughout this period. Of the 65 families tested^ 
23 were pure for resistance to this form of rust, 10 were susceptible, and 
32 were heterozygous. The ratios are not very significant when taken 
from such a small number of families. There is a numerical ratio of 23 
resistant families, 32 heterozygous, and 10 susceptible. The ratio of 
homozygous to heterozygous families is a very close approximation to 
the expected i :i ratio. The inoculation results are given in Table IV.^ 
Table IV. —The reaction of Marquis and Kanred, and various families of the from a 
cross between Kanred and Marquis, to Biologic Form I of stem rust ^ 
Class. 
Homozygous for 
resistwce. 
Heterozygous for resistance. 
Homozygous for 
susceptibility. 
Number of 
families. 
Number of 
individ¬ 
uals. 
Number of 
families- 
Number of 
individ¬ 
uals 
resistant. 
Number of 
individ¬ 
uals sus¬ 
ceptible. 
Number of 
families. 
Number of 
individ¬ 
uals- 
3 
102 
4 
96 
26 
3 
82 
3 
75 
7 
184 
54 
0 
o 
3 . 
3 
108 
4 
102 
35 
3 
88 
4 . 
I 
21 
8 
239 
59 
I 
26 
4 
III 
4 
89 
28 
2 
73 
6. 
6 
140 
3 
65 
18 
I 
24 
7 -. 
3 
50 
2 
23 
2 
0 
0 
Total. 
23 
607 
32 . 
798 
222 
10 
293 
Marquis. 
0 
0 
0 
83 
Kanred. 
83 
0 
0 
0 
o 
1A total of 2,086 individual plants were inoculated. The resistant hybrid plants were just as free from 
lesions as the resistant Kanred parent. The susceptible hybrid plants were completely susceptible, pro¬ 
ducing large, vigorous, and confluent uredinia. This type of infection was like that obtained on thfr 
Marquis parent (PI. 2). 
