HEAT CANKER OF FLAX. 
13 
ably no canker would have occurred in this plat if the high tempera- 
ture had not been preceded bv a protracted period of wet. cloudy 
weather. This caused a somewhat succulent growth and also com- 
pacted the soil. In plat J the lack of severe injury probably was 
due to the fact that the plants were growing under hot, dry conditions 
and were not as succulent as the plants in the other plats in which 
canker occurred. 
Table 9. — Soil temperatures and severity of heat-canker injuvj produced on young fiax 
plants at Fargo, N. Dak., on the four hottest days during June and July, 1920. 
Number 
of days 
after 
plants 
emerged. 
Date can- 
kering 
occurred. 
Maximum soil tem- 
peratures when can- 
ker occurred C^C). 
Average 
percent- 
age of 
oank». 
Plat. 
Thermo- 
graph 
record at 
a depth of 
half an 
inch. 
Approx- 
imate soil- 
surface 
tempera- 
ture. 
Plat G 
8 
14 
4 
9 
June 13 ; 49.0 
July 3 • 47.5 
July 11 50.0 
July 2S 49.0 
5.5.2 
53.7 
56.2 
.55.2 
23.3 
PlatH 
Trace. 
Plat I 
Plat J 
4.7 
Trace. 
In 1921 the investigations were continued in a similar manner both 
at Fargo and at Mandan, X. Dak. However, as will be seen in 
Table 8 and Figure 4, high soil temperatures did not occur until late 
in June, and therefore little, if any, flax canker developed before 
that time. Likewise, following the high soil temperatures in the last 
days of June and early July, but little canker developed, even though 
the soil-surface temperatures were sufEicieiitly high to expect the 
production of canker in the late sowings. Xo doubt the lack of 
canker production under these conditions is chargeable to a com- 
bination of several factors, both from the standpoint of the condi- 
tion of the young plants on the one hand and their environment 
on the other. Plants which had grown during the hot, dry weather 
of the latter part of June and early July were apparently not suffi- 
ciently succulent to be susceptible to canker injury from the high 
soil-surface temperatures. The soil throughout this period also was 
dry and unusually mellow, without any crust, and this also may have 
had some effect on the conditions governing the production of canker. 
It may be added that when the surface soil is mellow little air pock- 
ets tend to form immediately about the plants, in contrast to the 
close contact of the soil surface with the plant stems which occurs when 
the fine soil surface is compacted by rains and a thin crust is formed. 
The soil crust caused by rains brings the surface soil in immediate 
contact with the tender surfaces of the succulent vounc:: fiax stems. 
Injury results when such surface layers in immediate contact with 
the tender livmg tissues reach the high temperatures. This surface 
