£eb id 1^24 
441 
Physiological Studies on Flag Smut of Wheat 
no infection occurred in 277 plants when inoculation was made after 
the coleoptile was broken, it would seem that in the greenhouse, at least, 
shoot infection was almost negligible. 
RELATION OF CUTTING BACK INOCULATED PLANTS TO DEVELOPMENT 
OF SMUT 
In addition to the possible infection of shoots during the process of 
tillering, there is another type of shoot infection which evidently comes 
from within the plant and is the result of the original seedling infection. 
In the spring of 1921 inoculated plants of Bunyip, Cedar, and Come¬ 
back, grown in the greenhouse at Arlington Experiment Farm, appar¬ 
ently were free from the disease. Each plant had about four culms, 
all of which were yellow and well headed at the time of note taking. 
As these varieties had been practically free from the disease in the pre¬ 
vious season, and as other infected varieties needed immediate atten¬ 
tion, they were allowed to mature. However, the plants were watered 
each day, and small secondary shoots soon began to develop at the base 
of the mature culms. On later examination several of these shoots were 
found to be infected, even though the mature plants had shown no infec¬ 
tion. The occurrence of the disease in these small, secondary shoots, 
while the headed culms of the same plants remained smut-free, possibly 
may be explained in two different ways. First, it may be assumed, as 
McAlpine has suggested, that they were infected by spores present in 
the soil at the time of emergence; or, second, that they were infected 
from mycelia already in the plant, which, for some reason, had not been 
able to form spores in the older leaf tissue. 
Greenhouse experiments were conducted in 1921-22 to determine the 
possible source of infection in such secondary shoots and to study the 
effect of cutting back the plants at various stages of growth on the 
production of diseased secondary shoots. The results of the experiments 
in which seedlings had been inoculated immediately following rupture of 
the coleoptile and later, during tillering, have already been given. There 
was no infection of any shoot when the inoculations were made just after 
the coleoptile had been broken. Two plants out of a total of 95 were 
infected when inoculations were made after the seedlings were about two 
weeks old. 
Again, 50 uninoculated seedlings of Harvest Queen were cut to the 
level of the ground immediately after the coleoptile became ruptured. 
The cut surface of each seedling and the soil around it were heavily 
inoculated with flag-smut spores. No infection developed in any of 
these plants. In later experiments no new cases of infection have been 
noted where inoculation of the soil and plants was delayed until the 
latter had emerged from the soil. 
None of the soil in which the cut-back plants were grown was inocu¬ 
lated with flag-smut spores. The spores were sown with the seed and, 
therefore, were about 2 inches under ground. Thus there were no spores 
near the surface of the soil excepting those which might have been 
carried up through the soil by the plumule during its growth. 
The crowns of wheat plants grown in the greenhouse were practically 
on the surface of the soil, so that often the secondary shoots never came 
in contact with the soil. The shoots arising from the base of the culms 
often were pressed closely to the culms. In cases of this kind the basal 
leaf sheaths of the culm incased both the secondary shoots and the lower 
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