38 
BULLETIN 878, U. S. DEPAET^IENT OE AGRICULTURE. 
When rust occurs on the varieties grown at field stations the per- 
centage of infection usually is recorded, This is estimated by exam- 
ining the wheat plants and comparing them with the standard scale 
shown in figure 2. On this scale 100 per cent infection means that 37 
per cent of the surface of the culms is covered by rust pustules. An 
infection of 10 per cent or more is sufficient to cause a noticeable re- 
duction in yield, while 100 per cent infection of black stem rust usuallv 
results in a total failure of the crop. The quality of the wheat also is 
injured by rust. 
Data on stem-rust infection have been obtained at seven stations 
for one to five years, with a total of 20 station years for some varieties. 
Stem rust was recorded in 1914 at Highmore. S. Dak., and Akron, 
Colo. : in 1915, at Dickinson. X. Dak., Xewell, S. Dak., Archer. Wyo., 
and Akron, Colo.: in 1916, at Williston. Dickinson, and Mandan, X. 
Dak., Highmore and Xewell. S. Dak., and Archer, Wyo.: in 1917. at 
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A. 5 per cent. B. 10 per cent. C. 25 per cent. D . 40 per cent. E, 6o per cent. F. 100 per cent. 
Fig. 2.— Scale for estimating rust, illustrating six degrees of rustiness used in estimating the percentage of 
stem-rust infection. The shaded spots represent rust, and the figures represent approximately the rust 
percentages computed on the basis of the maximum of surfaces covered by rust as shown in the 100 per 
cent figure (F). Figure F in the diagram represents 37 per cent of actual rust-covered surface and is 
arbitrarily selected as 100 per cent. The other percentages are in terms of figure F. 
Highmore. S. Dak.: in 1918, at Mandan. X. Dak.. Highmore. S. Dak., 
and Archer. Wyo.: in 1919. at Mandan. X. D.. Highmore. S. Dak., 
Archer, Wyo.. and Akron. Colo. Stem rust has not occurred at Moc- 
casin. Mont., in the seven years from 1913 to 1919, or at Havre. Mont., 
or Sheridan, Wyo.. during the periods of the experiments. Rust 
notes were not recorded on the varieties at North Platte. Nebr. The 
average percentages of stem-rust infection on 30 varieties of wheat in 
comparison with Marquis are shown in Table XVII. 
Marquis wheat shows an average stem-rust infection of 17.7 per 
cent in 20 station years. All important commercial varieties of com- 
mon spring wheat show a higher percentage of rust infection than 
Marquis. A few early varieties of common wheat show less rust, 
but most of the later varieties of common wheat show a higher in- 
fection. Haynes Bluestem. a late-maturing variety, shows the 
highest percentage of infection, averaging 31.6 per cent in 19 station 
vears. None of the varieties of common wheat shown here is really 
