Feb. x6, 1924 
Flag Smut of Wheat 
473 
content within which germination of the spores may occur most readily. 
However, the fact that the spores can germinate and produce sporidia 
when entirely submerged in water would indicate that there is not 
necessarily any upper limit such as has been indicated for the oat-smut 
organism described by Jones (22). Hence, a special study was not 
made of this factor under soil conditions. 
RELATION TO TEMPERATURE 
The writer (35) has stated elsewhere that the optimum temperature 
for germination of the spores of Urocystis tritici is between 18 and 24 0 C. 
It was observed that spores presoaked in water for six days at 20° 
germinated most readily at 24 0 , but that spores exposed to constant 
temperature only, germinated most readily at 18 0 . 
From field observations and the results of laboratory studies it appeared 
that soil temperatures might have an important influence on the develop¬ 
ment of flag smut. The following experiments, therefore, were made 
in a series of soil-temperature tanks. 
The containers were of galvanized iron, about 18 inches deep and 8 
inches in diameter. Each held approximately 17 kgm. of clay-loam 
soil, the moisture content of which had been adjusted to 17.5 per 
cent on a dry-soil basis. The surface of the soil was about 1 inch below 
the level of the water in the tank, thus making it possible to maintain 
a fairly uniform temperature throughout the container. 
After the first adjustment for moisture, water was applied through a 
central tube, which extended to a small crock at the base of the container. 
The soil was not sterilized, because it was known that it did not contain 
spores of the flag smut organism. A range of four temperatures was 
used in each series. By means of suitable adjustment, it was found 
possible to maintain the series fairly constantly within the following 
ranges: 14 0 to 15 0 C., 19 0 to 21 0 , 24 0 to 26°, and 29 0 to 31 0 . Although 
the air temperature of the above-ground portions of the plant ranged 
from 15 0 to 29 0 , it remained fairly constant at 22 0 . Thus the soil tem¬ 
perature could have only a direct effect on the process of entrance and 
infection, although it is possible that there also may be an indirect effect 
on the parasite due to changes in the physiological condition of the 
host induced by high soil temperatures. 
Canberra, an Australian wheat susceptible to flag smut, was used 
throughout the test. The seed was treated three minutes in a per 
cent solution of copper sulphate and, because of the low percentage of 
germination, only germinating seeds were planted. From 20 to 24 such 
seedlings were sown in each container at a depth of from one-half to 
three-quarters of an inch. Four containers were used at each tempera¬ 
ture, a single container being used for each test. The spores used for 
inoculation were approximately six months old and were known to ger¬ 
minate promptly in laboratory tests. 
In the first series (pots 1, 5, 9, and 13), the seedlings were inoculated 
with spores which had been presoaked on the surface of distilled water 
for three days at 20° C. Pots 2, 6, 10, and 14 contained seedlings inocu¬ 
lated with dry spores. Spores which had commenced to germinate were 
used to inoculate the seedlings in pots 3. 7, 11, and 15. In each of these 
instances the seedlings were inoculated and then sown immediately at the 
respective temperatures. The final series (pots 4, 8, 12, and 16) con¬ 
tained seedlings inoculated with germinating spores and sown at 15°. 
