438 
Journal of Agricultural Research 
Vol. XXVII, No. 7 
The data show that only one variety, Bobs, became infected at 6° to 
12° C. and that the highest percentage infection for each variety occurred 
at 21.5 0 to 23.5°. The fact that no infection developed in any of the 
five varieties when grown at 25 0 to 26.6° is probably due either to the 
accelerated growth of the wheat plants at this higher temperature, 
which enabled them to escape infection, or to the inhibition of spore 
germination, or to both effects. 
Even at the optimum temperature all of the percentages of infection 
are relatively low for such susceptible varieties. In the varietal resistance 
experiments for 1921-22, herein described, in which the inoculated seed 
was sown in the soil, the percentages of infection in these same varieties 
were 86.7 for Bobs, 80.8 for Hard Federation, 88.5 for Harvest Queen, 
97.8 for Little Club, and 97.4 for White Federation. 
A preliminary experiment on the effect of controlled soil temperatures 
on infection was conducted by W. H. Tisdale and R. W. Leukel in the 
greenhouse at Arlington Experiment Farm in the spring of 1923. An 
equal amount of soil was placed in each of 12 galvanized cans. Two cans 
were set in water in each of six tanks held at io°, 15°, 20°, 24 0 , 28°, and 
32 0 C., respectively. Twenty seeds of Bobs wheat were sown in each 
can on March 26. The seeds were spaced at regular intervals in a 
circle, 1.5 inches from the border of the can, and covered with about 1 
inch of soil. No smut developed in any of the cans except the ones in 
which the soil temperature was held at 20°. In these cans 5, or 12.5 per 
cent, of the 40 plants became infected with flag smut. 
STAGE OF GROWTH AS RELATED TO INFECTION 
In the case of seedling infection it is stated by McAlpine (9) that 
infection occurs before the plant emerges from the soil. Soon after 
emergence the coleoptile, or sheath, which completely invests the plumule, 
is broken, and the latter appears. In order to determine the stages of 
seedling growth at which infection could occur, the following experiments 
were conducted. Seed of Bobs, Hard Federation, Harvest Queen, Little 
Club, and White Federation, the same varieties used in the temperature 
experiments, were placed between moist blotters, at 18 0 to 20° C. in a 
seed-germination chamber on December 6, 1921. When these seeds were 
at three definite stages of germination—namely, (1) testa, or seed coat, 
just broken; (2) plumule about 1 to 1.5 cm. long, and three strong roots 
developed; and (3) coleoptile just broken, those in each stage were di¬ 
vided into two lots. In addition, dry, ungerminated seed was divided into 
two lots. Both lots were inoculated and placed in clean soil in the green¬ 
house December 9 to 16, 1921. One lot of each was inoculated by rolling 
the seed or seedling in dry spores and the other by immersing in a spore 
suspension. This suspension consisted of spores soaked in tap water for 
at least two days. The spores in suspension were not necessarily germi¬ 
nating. It was thought that if the time between the transplanting and 
emergence of the seedlings was very short the dry spores might not have 
time to germinate and infect the seedlings before the latter came through 
the ground. On the other hand, if the spores were thoroughly soaked 
they could germinate in a very short time. Unfortunately, the emergence 
dates for these seedlings were not recorded, although these data would 
have been valuable in correlating rate of growth and infection. Table 
VII contains the results for the two lots of each variety inoculated with 
the soaked and dry spores, respectively, at the three stages of growth 
described above. 
