Viability of Ur edinios pores 
13 
Table 2. — Length of time after being exposed to the various 
temperatures and relative humidities that the urediniospores 
of Puccinia graminis tritici Form III were capable of pro- 
d ucing in fection. 
Approximate 
relative humidity 
25° C. 
20° C. 
15° C. 
10° C. 
5° C. 
Per cent 
Weeks 
Weeks 
Weeks 
Weeks 
Weeks 
100.0 
0 
1 
0 
1 
0 
89.9 
0 
0 
0 
0 
1 
80.5 
0 
1 
1 
2 
2 
70.4 
0 
1 
2 
16 
16 
60.7 
2 
0 
14 
14 
16 
49.0 
2 
1 
15 
16 
16 
38.0 
0 
1 
14 
14 
15 
29.5 
0 
1 
1 
3 
4 
21.5 
0 
0 
1 
2 
4 
10.5 
0 
0 
1 
0 
0 
0.0 
0 
1 
1 
5 
0 
percentage of germination decreased from week to week, the 
germ tubes became shorter, narrower, and the protoplasm was not 
as dense as in normal germ tubes. Further, when such inoculum 
was used, hypersensitive flecks were produced on the leaves, indi- 
cating that, while the spores were capable of germinating and 
sending a germ tube into the tissues, the resistance offered by the 
host cells was greater than the fungus could overcome. Judging 
from the data obtained, we can assume that a point is reached 
where the urediniospores are so attenuated under certain envi- 
ronmental conditions that while they are still able to germinate 
they are unable to produce infection. 
DISCUSSION 
The problem of the viability of the urediniospores of stem 
rust under field conditions is a question of their ability to over- 
winter in the North or oversummer in the South. The consensus of 
opinion is that the urediniospores of Puccinia graminis tritici do 
not overwinter in the North. This is borne out by the results ob- 
tained by the writer during the past two winters in field experi- 
ments conducted at Lincoln. Rust epidemics were started in the 
fall and urediniospores were found that germinated readily as far 
into the winter as January,, but after this time it was exceedingly 
