6 io 
Journal of Agricultural Research 
Vol. XXIV, No f 
no new spores were produced thereon. Soon after the fall rains began, 
however, uredinia began to appear at the ends of the old stripes where 
the mycelium had survived. The rust developed and spread rapidly 
until by October 15 ah abundance of uredinial infection could be found 
upon the plants of this species in this locality. This same method of 
surviving the dry summer months was noted in the same place during 
the summer of 1918 (PI. I), and upon Hordeiim nodosum, Bromus mar- 
ginatus, Elymus glaucus, Elymus canadensis L., and Hordeum juhatum in 
the rust nursery. 
Waterhouse (jo) has r^iported that in the hot dry summers'of certain 
parts of Australia, Puccinia triiicina Erikss. survives the dry period on 
volunteer wheat. 
It seems evident, then, that one of the determining factors for an 
epidefnic of stripe rust in any given year must be the weather conditions 
during the summer and fall of the previous year. If the precipitation 
during these months is insufiScieht to keep the grass hosts of P. glumarum 
and volunteer wheat more or le^s green in order to tide the fungus over 
this rather critical period, very little infection material will be present 
to start an epidemic upon winter wheat. It seems to the writer that the 
months from July to November in each year may Well be considered the 
critical period for the fiingus in regions where the winters are more or 
less mild. The meteorological conditions in the spring would need to 
be such as to favor the spread Of the ftmgus which had survived the 
winter months. However, if fall-^wn Wheat becomes well infected 
in the fall, the early spring conditions ate usually more or less favorable 
fot subsequent spread. This, Of course, would apply Only to winter 
wheat. The Weather conditions in the spring would undoubtedly be 
the determining factor in the’ infection Of Spring wheat. 
RESISTANCE OE UREDINIOSPORES TO DRYING 
It has been shown above that uredinial mycelium of stripe rust may 
survive the dry summer months in the tissues of the host. It seemed also 
important to learn the length of time in which urediniospores might 
remain viable under various conditions. Eriksson and Henning (3) 
found that the urediniospores of this rust were difficult to germinate 
unless they had been chilled to the freezing point or lower. The writer 
has had no difficulty in securing good germination of fresh urediniospores 
at any time. A number of trials were made to ascertain how long 
urediniospores would remain viable when-kept at room temperature in 
herbarium packets. Table I gives the results of several of these trials. 
Tabi^E I. —Percentage of initial and final germination of urediniospores of stripe rust, 
collected from different hosts f kept in paper packets at room temperature 
Host. 
Date collected. 
Initial 
semiina- 
tion. 
Date of final count. 
Final 
sermina- 
tion. 
Total 
storage. 
Wheat. 
Do. 
Agropyron dasystachyum. 
Elymus condensaius . 
Dec. i, 1917 
Nov. 27,1917 
Aug. 28,1916 
Sept. 5,1916 
Per Cent 
80 
90 
95 
85 
Jan. 12,1918 
Jan. 19,1918 
Oct. 24,1916 
Per cent. 
I 
Trace. 
I 
Trace. 
Days. 
43 
$6 
S8 
49 
