June 15, 1921 
Varietal Susceptibility of Beans to Rust 
393 
been noted repeatedly. In order to have more definite data on this 
point two plants each of 72 varieties were inoculated on March 29, 1920, 
and were held under similar conditions. Thirty-seven of the varieties 
produced uredinia only, 3 both uredinia and flecks, 27 flecks only, and 
5 neither sori nor flecks. The number of days elapsing before the ure- 
dinial “anlage” were visible varied from 6 to 10 on different varieties, 
and the time of sporulation from 10 to 15 days. The “anlage” were 
visible on 6 varieties on the sixth day, on 9 varieties on the seventh day, 
on 22 varieties on the eighth day, on 1 on the ninth day, and on 2 on the 
tenth day. Sporulation ensued on 6 varieties on the tenth day, on 30 
on the eleventh day, on 2 on the twelfth day, on 1 on the fourteenth, and 
on 1 on the fifteenth. The number of days between the appearance of 
the “anlage” and sporulation varied from two to five, the mean being 
three days. As a general rule the sorus develops more rapidly on the 
more susceptible varieties than on the more resistant. The flecks on the 
resistant varieties were visible, as a rule, sooner than the “ anlage” on the 
susceptible varieties. Flecks were observed on 9 varieties on the fifth 
day, on 16 on the sixth day, on 4 on the seventh, and on 1 on the eighth. 
RELATIVE SUSCEPTIBILITY OF BEAN VARIETIES 
In our previous publication (4) a number of varieties of beans were 
classified according to their susceptibility to rust under field conditions. 
Four classes were recognized—rust-free, rust-proof, rust-enduring, and 
rust-susceptible. In this work we have attempted to obtain a more 
definite expression of susceptibility by using a susceptible variety as a 
standard and expressing the relative susceptibility of other varieties in 
terms of that standard. A standard dosage is employed and the relative 
susceptibility of a variety is indicated by the percentage of infection 
obtained, modified by the amount of flecking, the size of the sorus, and 
the length of the incubation period. 
The determination of the degree of infection is based on the assump¬ 
tion that all varieties receive approximately the same number of uredin- 
iospores per unit area of leaf surface exposed as the standard. The 
resultant infection is then determined by count and is calculated as 
average number of infections per square inch. The number of sori 
obtained on Tennessee Green Pod is then rated 100 per cent, and the 
number of infections on the other varieties in the test is reduced to 
percentage of the standard. The percentage of infection is expressed 
as total infection (both sori and flecks), fertile infection (sori only), and 
abortive infection (flecks). Total infection and fertile infection are in 
terms of the standard, but abortive infection relates only to the variety 
in question and is the percentage of the total infections aborted on that 
variety. The details of this method are illustrated in Tables I and II. 
Table III is a summary of the experiments and shows the degree of 
infection obtained on the 64 varieties tested. 
