June 15,1921 
Varietal Susceptibility of Beans to Rust 
389 
Table II .—Summary of experiment No. 2Q. Percentage of total infection and fertile 
infection in terms of Tennessee Green Pod , which is taken as the standard and rated 
100 per cent 
Tennessee 
Green Pod. 
McCaslan. 
Improved 
Goddard. 
Dwarf 
Black. 
Total number of sori on 20 leaves. 
Total number of flecks on 20 leaves.... 
Total number of infect^ns on 20 leaves 
Total area in square inches. 
3,7i2 
3, 7 1 * * * * * * * * * * 12 
116. 34 
5,33i 
5,33i 
140. 79 
2,411 
2,411 
234. 97 
668 
452 
1,120 
106. 04 
Number of sori per square inch. 
Number of flecks per square inch. 
Number of infections per square inch 
31. 91 37. 87 
31.91 37.87 
10. 26 
10. 26 
6. 30 
4. 26 
10. 56 
Percentage of total infection.... 
Percentage of fertile infection. . 
Percentage of abortive infection 
100.00 118.68 
100.00 118.68 
32. 15 
100. 00 
33- 09 
19. 74 
40. 36 
1 Calculated from total infection 011 each variety separately. 
Following inoculation the plants are removed to a moist chamber 
large enough to hold all 40 plants. They are kept here in a humid 
atmosphere for 24 hours and are then set on the greenhouse bench. 
The incubation period varied within limits of 8 to 14 days during the 
tests, the average time from inoculation to the rupture of the sorus 
being 10 days. The sori are visible two or more days prior to sporula- 
tion. The final data on infection are secured 7 days after sporulation 
begins. This permits a full development of the initial infection on all 
varieties but comes early enough to exclude secondary sori which develop 
around the primary sorus in the more susceptible varieties (PI. 71, B). 
At this time the primordial leaves are removed from the plants and the 
number of infections (both sori and flecks) are determined. The typical 
infection obtained on susceptible varieties is shown on a leaf of Dutch 
Case Knife in Plate 70, A, and abortive infection or flecking on a leaf of 
Improved Goddard in Plate 70, B. 
A photographic retouching frame has been found a great aid in the 
counting of infections. The leaves are placed on a glass plate in this frame 
before an incandescent lamp, and by this lighting it is possible to deter¬ 
mine the number of infections on both leaf surfaces at the same time, 
thus preventing the duplication which would result from separate counts. 
Infection takes place with equal readiness on both upper and lower sur¬ 
faces, but with the method of inoculation used the greater number develop 
on the upper surface. In many cases the sorus breaks through on both 
surfaces. Before the count is taken the leaf surfaces are brushed with a 
camePs-hair brush to remove dirt and superfluous spore powder. Fol¬ 
lowing the count the area of the leaf is determined by means of a plani- 
meter, and the number of infections is reduced to a unit area basis in all 
varieties. 
