Mar. 17, 1923 Influence of Soil Temperature on Seedling-Blight 867 
high enough to start immediate germination. This was followed in 8 
days by a low temperature period of about 10 days’ duration in which 
the temperature went considerably below the critical temperature for 
blighting. 
The second planting was made during the low-temperature period, but 
did not begin germinating until the soil temperature had risen to about 
16° C. While this planting was subjected to low soil temperatures 
later in the germination period, the soil temperatures did not go as low 
as those following the previous planting, and the depression occurred 
later in the period of seedling development. As a result, the stand was 
reduced only 13 per cent. The last planting showed only a small degree 
of blighting in two or three of the most susceptible strains. 
The field temperature experiments with com in general showed results 
similar to those obtained in the greenhouse. The field plantings, however, 
indicated a slightly lower critical temperature for blighting than that 
shown in the soil temperature tanks in the greenhouse. 
Fig. 15.—Graph showing critical soil temperature for 
the development of seedling-blight of wheat. The 
heavy horizontal line at 12 0 C. represents the critical 
soil temperature for the development of blight in 
the soil-temperature tanks in the greenhouse. The 
rise of the daily mean soil temperature to and above 
the i2° C. line resulted in the development of seedling- 
blight. 
DISCUSSION OF RESULTS 
While the influence of soil tem¬ 
perature in the development of 
seedling-blight was operative 
over a wide range of tempera¬ 
tures, there was, however, in both 
wheat and com a very clearly 
defined critical temperature for 
blighting. Soil temperatures 
below 12 0 C., for instance, pre¬ 
vented blighting of the wheat 
seedlings, whereas soil tempera¬ 
tures at 12 0 or above produced a 
large amount of blight. The 
critical soil temperature, there¬ 
fore, for the seedling-blight of 
wheat was always at 12 0 C., 
whether a constant soil temperature or the mean of a fluctuating field 
soil temperature. The critical soil temperature is represented graphically 
in Figures 5 and 6 for the constant soil temperature series and in Figure 
15 for one instance in the field series. Other cases equally striking could 
be chosen from periodic seedings of either the spring or the winter wheat. 
The straight line at 12 0 C. in figure 15 represents the approximate 
critical soil temperature for seedling-blight of wheat. The first seeding 
germinated and emerged when the soil temperature curve was well 
below this line, with no blight as a result. The second sowing developed 
in a warmer soil with a mean temperature still below the 12 0 line, yet 
approaching it. In many cases the temperature during a portion of 
the day was above this line. The blight appeared on just a few of the 
seedlings. The last seedings shown developed in a soil having a mean 
temperature of 12 0 or above, and as a result 53 per cent of the plants 
were blighted. The mean soil temperature in fluctuating field tempera¬ 
tures corresponded very closely with the constant soil temperatures in 
the greenhouse experiments. 
The seedling-blight of com, both in greenhouse tank series and in 
field series, developed at 20° to 24 0 C. and below, while above 24 0 no 
