Nov 3, 1923 
Helminthosporium Disease of Wheat 
199 
Soil inoculations were made by sprinkling or spraying a spore sus¬ 
pension over all the soil used in a complete series. This soil afterwards 
was thoroughly mixed and put in the pots before the seeds were sown. 
This method insured uniformity of the inoculum throughout all the pots 
in a series. In all cases the control or uninoculated pots were sown be¬ 
fore working with the inoculum for the inoculated pots. 
In all of the experiments, only enough inoculum was used to produce 
a moderate amount of infection on the underground parts. This was 
done in order that the temperature influence might be determined 
more accurately. In no case was there sufficient inoculum to produce 
a marked killing of the plants. In the soil-temperature studies on. 
potato scab (8) it was found that heavy inoculation tended to produce 
undue flattening of the temperature and disease curve, and this same 
condition seems to hold with the Helminthosporium disease. The 
exact temperature optimum tends to be obscured when' an excess of 
organism is present. 
In determining the comparative influences of the several soil tempera¬ 
tures in any one series the amount of disease produced was taken as a 
basis. As pointed out in the work with potato scab (<?), it is not adequate 
to use alone either the number of infected individuals or the degree of 
infection as the sole index for the amount of disease. 
In the case of the data from the greenhouse experiments the extent 
of disease is expressed as an infection rating, which represents the per¬ 
centage of the total number of plants which were infected and also the 
degree of infection. 
In recording the extent of disease, the plants were separated into five 
classes according to the degree of infection, and each plant was given a 
numerical rating, as shown in Table I. 
Table I.— Classes , degrees of infection , and numerical ratings used in rating diseased and 
healthy wheat 
Class. 
Degree of infection on the underground parts. 
Numerical 
rating. 
1 
None. 
2 
Very slight.. 
•75 
I. OO 
3 
Slight. 
4 
Moderate... 
2 * OO 
5 
Abundant. 
1 OO 
J* w 
The classes are described as follows: (1) No signs of infection, as evi¬ 
denced by the absence of any lesions on the underground parts; (2) very 
slight infection, as evidenced by small lesions on the coleoptile; (3) slight 
infection, as evidenced by small lesions on the coleoptile or sheaths in ex¬ 
cess of (2); (4) moderate infection, as evidenced by the partial or almost 
complete rotting of the coleoptile, with a few lesions on lower leaf sheaths 
or roots; (5) abundant infection, as evidenced by a complete rotting of the 
coleoptile and numerous lesions on the subcrown intemode 4 , lower leaf 
sheaths or roots. 
In most of the experiments herein cited relatively slight root infection 
occurred. Whether this is due to a difference in resistance between the 
4 The term subcrown intemode is here used to apply to the elongated structure of the wheat plant which, 
under certain conditions, develops between the germinated seed and the crown. In wheat and barley this 
structure is covered by the coleoptile. 
