Nov. 3, 1923 
Helminthosporium Disease of Wheat 
201 
at 20 0 averaged 6.2, while at the extreme soil temperatures only two 
tillers per plant were formed. At temperatures between 20° Cand both 
higher and lower extremes a gradual decrease in number of tillers 
was noted. 
Under certain conditions with wheat the soil temperature seems to 
influence the ultimate position of the crown and permanent root system 
with respect to the seed and soil surface. At high temperatures the 
crown tends to be developed near the surface of the ground, or, in other 
words, a long subcoronal intemode is formed; whereas, at low soil 
temperatures, the crown tends to form low or at the seed. Intergrading 
relations of these structures develop at the intervening temperatures. 
Just how important soil temperature is in connection with this modifi¬ 
cation is not known. It seems apparent, however, that other factors 
may, under certain conditions, completely obscure the temperature in¬ 
fluence, for the writer has occasionally observed plants in well-controlled 
temperature experiments which did not conform to the above observa¬ 
tion. Certain varieties also do not seem to respond in this way. 
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Fig. i.—G raph showing summaries of Helminthosporium sativum infection ratings on underground parts of 
wheat and barley seedlings grown in soil at different temperatures, as shown in Tables II, III, and IV, 
and average diameters in mm., in five experiments, of mycelial colonies of the same parasite grown in 
artificial culture at various temperatures. 
The lower temperatures (16 0 to 20° C.) tend to favor the development 
of the roots as compared with the tops. The optimum temperature for 
root development of barley and wheat on a basis of dry weight seems to 
be about 6° lower than that for top development during the periods of 
the recorded experiments. Dickson (4) considers that the optimum soil 
temperatures for the various host responses recorded is about 4 0 higher 
for Marquis wheat than for Turkey. While there is a slight indication 
in this research that this relation holds between Marquis and Harvest 
Queen, the evidence is not sufficiently striking to warrant a definite 
statement at this time. 
Disease development. —The results of disease development at the 
several soil temperatures are tabulated in Tables II, III, and IV. The 
average data from all the experiments are tabulated at the end of each 
of these tables and are shown graphically in figure 1. From the several 
tabulations it will be noted that while there has been a slight shifting of 
the optimum soil temperature for disease occurrence in the several 
experiments, this shifting has been within rather restricted limits. 
