H Nebraska Experiment Station Research Bulletin 22 
difficult to find urediniospores, and what few were found did not 
germinate. 
Infections in the late fall on winter wheat are usually con- 
fined to the lower and outer leaves and these are the first leaves 
which dry up under adverse environmental conditions. Thus, the 
use of detached leaves in the experiment reported was the nearest 
approach possible, under artificial conditions, to what actually 
occurs in the field. The results, while they do not touch the whole 
of the overwintering question, do show that down to certain 
temperatures the urediniospores are viable for long periods of 
time at the prevailing relative humidities encountered in the field 
in the fall in the winter wheat belt. Experiments will be carried 
out this coming year with temperatures below freezing. The effect 
of alternate freezing and thawing on the viability of uredinio- 
spores will also be determined. 
The results show clearly that at constant temperatures below 
the mean temperatures prevailing during the summer months in 
the South the urediniospores are not viable for any great length 
of time at any relative humidity. At first thought one would say 
that the results show that oversummering could not take place 
in the South. Further, the mean temperatures prevalent in the 
South during the summer months are near the maximum at which 
germination of the urediniospores can take place. Where, then, 
do the urediniospores that produce infection in the fall on winter 
wheat in the South come from? 
The mortality of the free urediniospores must be ex- 
tremely high at all times. The rate of mortality would naturally 
vary with the environmental factors encountered. Further, a 
definite set of conditions must be at hand for infection to take 
place. These conditions are not always present, so that the death 
rate of the viable spores would further be increased. Thus, 
it is safe to assume that only a small proportion of the countless 
number of urediniospores produced actually infect the host, 
except during the short periods when the conditions for successful 
infection are at hand. 
It should be distinctly understood that the generalizations 
offered above are based on the results obtained with only one 
biologic form. Will other biologic forms of stem rust react in 
the same way, or will they behave differently to stated tempera- 
tures and relative humidities by having a lower or higher per- 
centage of germination and a shorter or longer period of viabil- 
ity? To determine these points, it is the purpose of the writer to 
continue this work with other biologic forms of stem rust. 
