Feb. 16, 1924 
Flag Smut of Wheat 
47i 
The stage of growth of the host at this period is of great importance. 
McAlpine (29) also has pointed out that environmental conditions which 
favor a prolongation of this critical period are generally conducive to 
heavier infections. In discussing resistance to bunt, he mentions that 
differences in susceptibility of the wheat varieties may sometimes be 
correlated with differences in rapidity of their seedling growth. 
In preliminary experiments with dry spores of Urocystis tritici , the 
writer inoculated wheat seedlings of several varieties of wheat known 
to be susceptible to flag smut (Federation, Marshall’s No. 3, and Can¬ 
berra) and sowed them in moist soil at from 18 0 to 23 0 C., but in no 
instance was infection noted when the coleoptiles were more than 4 mm. 
in length at inoculation. 
As a result of subsequent investigations, it is highly probable that 
seedlings in later stages of growth may become infected when inoculated 
with germinating spores and kept in a suitable environment. 
AMOUNT OF INOCULUM 
Several investigators have called attention to the relationship between 
the amount of inoculum employed and the subsequent development of 
the disease. In this connection, McAlpine (29) has recorded the results 
of inoculation experiments with bunt spores. When inoculum was 
applied at the rate of 1 bunt ball per 5 kernels, there was from 79 to 81 
per cent of infection; when applied at the rate of 1 ball per 100 grains, 
there was from 56 to 58 per cent of infection. 
Heald (15) has indicated a quantitative relationship between the 
spore load in seed wheat and the percentage of st inkin g smut produced 
in the crop. Using artificially smutted seed, a load of 36,000 to 150,000 
spores of Tilletia tritici was necessary to produce maximum infection. 
He suggests that either multiple infection occurs or that there is a chemical 
mass effect due to the number of spores. 
There are some points of similarity between infection of the wheat 
seedling by the flag smut organism and infection by the bunt organism. 
Infection may result from inoculum on the seed or from inoculum present 
in the soil. The germ tubes or infection hyphae penetrate the tissues of 
the coleoptile and establish parasitic relationships within the young 
growing tissues of the host. 
In pathogenicity studies with Urocystis tritici , infection frequently has 
been difficult to secure by means of dusting the spores on the grain, even 
though large amounts of inoculum were used, but when the young 
coleoptiles were inoculated with several platinum loopsful of germinating 
spores and the seedlings subsequently incubated under favorable condi¬ 
tions, almost complete infection resulted. Hence, a few spores germi¬ 
nating under suitable environmental conditions may be as effective in 
producing heavy infection as a much larger number in which the per¬ 
centage of germination is relatively low. 
In flag-smut pathogenicity experiments in the past, dry spores have 
always been used as a source of inoculum on account of the very capricious 
germination of spores. In laboratory tests it has been difficult to germi¬ 
nate the spores, but they appeared to germinate somewhat more readily 
under certain conditions existing in the soil. At other times, however, 
it appeared as if the spores did not germinate readily even under appar¬ 
ently favorable soil conditions. This fact seems to constitute one of the 
reasons why the flag-smut organism is such an important pathogene 
74026—24 - 4 
