FOOT-ROT DISEASES OF WHEAT IN AMERICA 95 
organism in the laboratory for eight months, during which time 
practically no germination took place. During this period a high 
percentage of the spores germinated when removed from the water 
-and placed on agar, and not until the end of the eighth month did 
a marked decrease in viability become apparent. 
According to Stevens (67) Helminthosporium. sativum tends to 
produce new races (“saltations”) when grown on artificial media. 
He states that the variant or saltating portions of the colonies fre- 
quently show morphological characters which differ widely from 
the original colony. He says, further, that these variants retain their 
characters and sometimes resaltate when transferred. Christensen 
(13) states that he has found at least four physiological forms of 
I. sativum. These forms are based on cuitural and morphological 
differences primarily, and although he says that the four strains 
showed some differences in pathogenicity when applied to a sus- 
ceptible host, his evidence does not seem to warrant a definite state- 
ment concerning genetic differences in virulence between the strains 
he studied. In connection with Christensen’s work it would be of in- 
terest to know whether he found “saltation” in his cultures of H. 
sativum. His Plate 5 gives a suggestion of differentiating sectors 
in the colonies of his physiologic Forms I and III, similar to the 
sectors shown by Stevens (67), but there seem to be no recorded 
observations in Christensen’s paper concerning the occurrence of 
“saltation ” among the strains he studied. 
CONDITIONS INFLUENCING THE ARR Maumee OF HELMINTHOSPORIUM 
Data have been published previously (45) on the influence of soil 
temperature and moisture on the development of seedling infection 
and early stages of foot-rot. In general, these results show that 
fairly high soil temperatures (28° to 32° C.) and high soil moistures 
(55 to 65 per cent moisture-holding capacity) favor infection. 
These studies indicated that the optimum temperature for infection 
of seedlings and young plants of Marquis wheat and Hanna barley 
is near 28°, whereas for Harvest Queen wheat the optimum is near 
32° C. The general summary of the results obtained in the soil- 
temperature studies is represented by the curve in Figure 2. The 
results of a combined soil-temperature and moisture experiment in- 
dicate that the temperature optimum is fairly stable when soil 
moisture is varied. However, when soil temperature was varied the 
moisture optimum seemed to vary. At high soil temperatures high 
soil moistures seemed most favorable for infection, whereas at low 
soil temperatures infection seemed to increase at lower soil moistures. 
Extremely low soil moistures were unfavorable for infection at all 
temperatures, and at extremely high and low temperatures the 
moisture curves for infection were very irregular. While this com- 
bined experiment was preliminary in nature, the results obtained 
were consistent both within themselves and with the data obtained 
in the independent temperature and moisture experiments. A pre- 
liminary experiment in which alternating soil temperatures were 
employed gave essentially the same amount of infection as an ex- 
periment conducted at the same time but in which a constant tem- 
perature equivalent to the mean of the fluctuating temperatures was 
used, 
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