Nov. 3, 1923 
Helminthosporium Disease of Wheat 
205 
In the results from experiment 2 in Table II, experiment 1 in Table III, 
and experiment 1 in Table IV, it will be noted that the plants grown 
at 8° and 12 0 C. were not removed at the same time as those grown 
at the higher temperatures. They were removed at later dates for 
the purpose of getting som6 idea of the influence of time on the develop¬ 
ment of the disease. The data recorded in the above table show that 
time is an important factor, as evidenced by the sharp rise in the disease 
curve at 8° and 12 0 , in contrast with the depression of the curves 
at the low temperature end of the experiments, where plants grown 
at all temperatures are removed and examined at the same time. These 
results are in line with natural expectations. 
In experiments 3A and 3B with Harvest Queen wheat 105,000 and 
6,575 conidia of the parasite, respectively, per cc. of water were used to 
inoculate the seed before sowing. The results of this experiment show 
clearly that the amount of inoculum greatly influences the disease de¬ 
velopment. In this experiment the greatest amount of disease occurred 
where the greatest number of conidia were used. 
It will be noted in figure 1 that the disease curve for Marquis wheat is 
considerably higher than those for Harvest Queen wheat and barley, 
except below 16 0 C. for barley. This relation is explained for the present 
on the basis of varietal susceptibility. In all of the work done by the 
writer to date, Marquis wheat has shown higher susceptibility than 
barley or the other varieties of wheat used. The indications are that 
the varieties of barley used develop a greater amount of Helmintho¬ 
sporium infection at low soil temperatures than is the case with wheat; 
and Marquis (spring) wheat seems to show the same tendency as compared 
with Harvest Queen (winter) wheat. While these relations seem to be 
tied up with specific and varietal differences, such a general explanation 
falls far short of completely satisfying the many questions which come to 
the mind of the experimenter. It is hoped that more satisfactory ex¬ 
planations for some of these results may develop from research now 
under way. 
EXPERIMENTS AT ALTERNATING TEMPERATURES 
Experimental Methods 
As far as known, all of the controlled soil temperature studies on plant 
disease development thus far have had to do with “constant” tempera¬ 
tures. While such temperatures are a means of obtaining very valuable 
data which may be analyzed readily, it is recognized that under no circum¬ 
stances in nature is the plant or the disease-producing organism submitted 
to a constant soil temperature for any length of time. Naturally this 
may lead some to inquire as to the actual value of constant temperature 
results as an aid in interpreting the reaction of disease to variable tempera¬ 
tures under field conditions. We are inclined to assume that the average 
daily soil temperature over a given period will produce practically the 
same results as a constant soil temperature equivalent to the mean for 
such a variable. In the case of potato scab this conception seems to 
hold, as is evidenced by the field experiment and observations on soil 
temperature cited by Jones, McKinney, and Fellows (<£)*; but, as far as 
known, no controlled experiment has been carried out to determine this 
point. In view of this fact, it was decided to devise a simple, controlled 
experiment to determine the relation of variable and constant soil tem¬ 
peratures in connection with the Helminthosporium disease. 
