Feb. 16,1934 
Physiological Studies on Flag Smut of Wheat 
429 
Over 1,600 culms were examined to determine the frequency of the occur¬ 
rence of sori on the lower leaves of the culms. Sori were found on all 
the leaves on 16.7 per cent of the infected culms; on the second leaf and 
all leaves above on 48.7 per cent of the infected culms; on the third leaf 
and all above on 25.3 per cent; on the fourth and those above on 6.7 
per cent; on the fifth and those above on 2.4 per cent; and on the sixth 
and any above on 0.2 per cent. When infection was noted on the first 
and second leaves there usually was just a trace of the disease, the infec¬ 
tion increasing in severity from the third leaf on up to the apical leaf. 
The diseased leaves generally were shorter and narrower than the cor¬ 
responding leaves of healthy culms. The leaves of sound and of infected 
culms from the same infected plants of 12 varieties were measured and, 
even in these cases, the diseased leaves were consistently shorter and nar¬ 
rower than the corresponding leaves on the sound culms of the same 
plant. 
CAUSAL ORGANISM 
Flag smut of wheat is caused by a fungus, Urocystis tritici Kcke., 
belonging to the family Tilletiaceae Tul. It was reported by McAlpine 
(9) as occurring in Australia as early as 1868 and determined by Wolff 
(16) in 1873 to b e U. occulta (Wall.) Rab., the species which occurs on 
rye ( Secale cereale). In 1877 Komicke (7), on examining specimens of flag 
smut from Australia, separated it from U. occulta upon morphological 
characters alone and named the new species U. tritici . However, the 
name was not adopted, and Saccardo (13, p. 515) gave this name as a 
possible synonym for U . occulta , and Sydow and Butler (14, p. 427), 
McAlpine (8), and Hori (5) still reported this fungus as U. occulta . 
McAlpine (9) conducted cross-inoculation experiments with Urocystis 
from wheat and rye in 1907 and 1908. He concluded that “ the Flag smut 
of wheat and rye are not mutually infective, and therefore the name given 
to Flag smut of wheat by Koernicke in 1877, who received specimens from 
R. Schomburgk in South Australia, should be retained, viz., Urocystis 
tritici 
Urocystis tritici is not known to infect plants of any genus other than 
Triticum, though several genera have been found to be infected with 
other species of Urocystis. In 1921, plants of redtop were received which 
were infected with Urocystis. During the winter of 1921-22 two lots 
of seed of redtop, Pal timothy, St. John rye (C. I. 130), and Little Club 
wheat (C. I. 4066) were inoculated, one lot with U. tritici and the other 
with U . occulta. These lots of seed were sown on December 1, 1921, in 
the greenhouse at Rosslyn, Va. There was insufficient spore material 
from the infected redtop plants to inoculate similar lots of seed. U . tritici 
infected all, or 100 per cent, of the wheat plants, but did not infect any 
of the rye, timothy, or redtop plants. U. occulta infected 7.1 per cent 
of the rye plants, but did not infect any of the wheat, timothy, or redtop 
plants. 
The spores of flag smut are either solitary or held together in groups 
of two to several, and the whole completely invested by a layer of small 
sterile cells (PI. 2, A). Each spore of a spore ball may germinate, 
though usually only one or two actually do so (PI. 2, E, G). How¬ 
ever, spore balls with three and even four germinating spores are fre¬ 
quently noted (PI. 2, H). On germination, the spore sends out a 
promycelium (PI. 2, B) with a whorl of one to several sporidia at the 
apex (PI. 2, C, D). These sporidia do not become separated from the 
