HEAT CANKER OE ELAX. 3 



or light relations; but under the Michigan conditions few of the 

 plants toppled over, and in the moist latter half of the season there 

 was almost complete recovery. Anthracnose canker seems to be 

 rather rare in the United States during some years, and when present 

 its damage is confined almost entirely to young seedlings. 



HEAT CANKER OF FLAX, A NONPARASITIC TYPE. 



This type of flax canker, according to continuous studies since 

 1916 reported by the writers in 1920 {18), causes severe losses and 

 occurs to about the same extent each year in the semiarid flax- 

 producing section of the United States. It is most evident during 

 the latter haK of June and the first half of July. The outside por- 

 tion of the stem is killed at or near the surface of the ground when 

 the plants are comparatively young (PL II, A and B ; PI. Ill, A and B) . 



Generally speaking, if the injury occurs when the plants are less than 

 3 inches in height the tissues collapse at the point of injury and the 

 plants wither and die (PL III, A and B ; PL IV, A to D) . If the injury 

 occurs somewhat later, when the plants are 3 to 5 inches in height, 

 only the cortex is killed, allowing the plants to topple over, but 

 usually to remain alive for days or weeks because of the uninjured 

 vascular systems within (PL II, A and B ; PL V, A to C) . Only in 

 rare instances are plants more than 5 inches in height injured in this 

 way. Numerous more mature specimens of heat-cankered flax can 

 be found, but in such cases growth continues after the initial injury. 

 Enlargement of the stem occurs just above and sometimes just 

 below the injury (PL II, A and B; PL V, A to C). In most cankered 

 plants the stem is severed, sooner or later, at the point of girdling 

 by the winds or by the disintegration of the remaining tissues, due 

 to the action of saprophytic organisms. Otherwise the plant dies 

 when the starving roots can no longer support the increasing needs 

 of the aerial portion. For this type of canker the name "heat canker 

 of flax" is suggested. 



In order to avoid confusion, it may be well to state here that the 

 nonparasitic flax canker discussed in this paper does not include a 

 type which occurs late in the season in the driest districts of the 

 northern Great Plains area. Following drought the base of the 

 stem becomes very woody, dry, and brittle, with little tensile 

 strength, and is snapped off by the wind. This trouble also seems 

 to be nonparasitic in its nature. 



CAUSE OF HEAT CANKER. 



ISOLATIONS AND PATHOGENICITY EXPERIMENTS. 



Hundreds of isolations from cankered specimens of this type 

 resulted sometimes in the growth of no organisms at all and never 

 in the constant association of any one organism with the disease. 



