HEAT CANKER OF FLAX. 



13 



ably no canker would have occurred in this plat if the high tempera- 

 ture had not been preceded by 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 injury produced on young flax 

 plants at Fargo, N. Dak., on the four hottest days during June and July, 1920. 



Plat. 



Plat G. 

 PlatH 

 Plat I. 

 Plat J. 



Number 



of days 



after 



plants 



emersed. 



Date can- 

 kering 

 occurred. 



Maximum soil tem- 

 peratures when can- 

 ker occurred (° C.) 



Thermo- 



record at 



a depth of 



half an 



inch. 



June 13 

 July 3 

 July 11 

 July 28 j 



49.0 

 47.5 

 50.0 

 49.0 



Approx- 

 imate soil- 

 surface 

 tempera- 

 ture. 



55.2 

 53.7 

 56.2 

 55.2 



Average 

 percent- 

 age of 

 cajoker. 



23.3 

 Trace. 



4.7 

 Trace. 



In 1921 the investigations were continued in a similar manner both 

 at Fargo and at Mandan, N. 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 sufficiently high to expect the 

 production of canker in the late sowings. No 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 young flax stems. 

 Injury results when such surface layers in immediate contact with 

 the tender living tissues reach the high temperatures. This surface 



