16 BULLETIN" 1120, U, S. DEPARTMENT OF AGRICULTURE. 



susceptibility to injury before the excessive heat occurs. Investi- 

 gations along these lines are being continued by the Office of Cereal 

 Investigations in cooperation with the North Dakota Agricultural 

 Experiment Station. Tentative suggestions for lessening this injury 

 are (1) higher rates of seeding, (2) earlier dates of seeding, and (3) drill- 

 ing north and south rather than east and west. In this connection it 

 is well to remember that the beginning of these intermittent periods 

 of overheating in this section usually occurs in the first or second 

 week of June. 



SUMMARY. 



Investigations have shown that the cause of the heat-canker type of 

 flax injury is nonparasitic in its nature. 



It occurs somewhat uniformly each year in the northern Great 

 Plains area and causes a marked loss in flax production. 



The cortex of the stem is killed at the surface of the ground. Sooner 

 or later the cankered plants topple over. Young cankered plants die 

 at once, while those that are a little older may remain alive for days 

 or weeks, as long as the vascular systems function. Stems of the older 

 cankered plants usually enlarge just above the injury, and sometimes 

 also just below it. The result is a girdling of the plants at the soil 

 line. 



In these experiments and observations flax was cankered only 

 during and immediately following very hot days. Flax plants when 

 more than 4 inches high are only slightly susceptible. Flax plants 

 which have developed under hot, dry conditions are less susceptible to 

 injury from high soil-surface temperatures than more succulent 

 plants. 



Flax plants which are grown in a soil having a shallow surface 

 mulch over a firm seed bed are less readily injured than those grown 

 in a soil in which the surface layer has been compacted into a crust 

 by rains. Plants shaded by a vertical strip of canvas 10 inches high 

 were not cankered, while many unshaded plants in the same row were 

 cankered. Thinly sown flax was cankered more than thickly sown 

 flax. 



Flax sown with cereals as nurse crops was cankered comparatively 

 little, and flax with weeds was cankered less than flax free from 

 weeds. 



Evidence indicates that heat canker of flax results from a combina- 

 tion of succulence in the young flax plants and high temperatures of 

 the surface soil in immediate contact with the succulent stem tissues. 



Killing the cortex of young flax plants by artificial heat produced 

 typical heat canker. 



Promising control measures seem to be thicker seeding and early 

 sowing, and possibly drilling rows north and south instead of east and 

 west may prove helpful in lessening the severity of canker injury. 



