184 



Bulletin 236. 



a crack where, in drying, the diseased tissue has separated from the 



heakhy bark (Fig. 52). The older 

 cankers are brown, somewhat 

 darker than the healthy bark. They 

 are distinctly sunken. The surface 

 is smooth, never checked or rough- 

 ened or beset with pustules or pim- 

 ples, except in the old cankers, 

 where, after a time, rot fungi gain 

 entrance and, thriving in the already 

 dead tissues, produce their fruit 

 bodies on the surface. The progress 

 of the spreading canker depends 

 largely on the continuation of fa- 

 vorable weather conditions, which 

 seem to be a humid atmosphere and 

 cloudy days. With the return of 

 bright sunny weather, the active 

 spread of the canker is checked ab- 

 ruptly, often to be resumed again 

 with the return of favorable condi- 

 tions. This checking and renewing 

 of activity sometimes result in large 

 cankers with concentrically ar- 

 ranged cracks within the cankered 

 area (Fig. 53). This renewal of 

 activity may take place during the 

 same season or the canker may par- 

 tially heal over to spread anew the 

 following year (Fig. 54). A large 

 percentage of the cankers are active 

 during but one season. There are 

 always some, however, in which the disease is perennial, living through 

 the winter to l)ccome active again the following spring, spreading and 

 enlarging the original limits of the cankered area (Fig. 55). The dis- 

 eased bark is usually killed by the wood, to which it clings tenaciously the 

 first season. It gradually decays, however, and falls out. leaving the wood 

 bare and exposed (Fig. 54). In small cankers the cone of diseased bark 

 may be quickly forced out by the rapidly formed calluses, which heal and 

 close the canker wound (Fig. 56). In some cases the canker is superficial, 

 never reaching the cabium except, perhaps, in a limited area at the point 

 of infection. Such wounds heal quickly beneath the dead bark, which 

 clings to the tree as a sort of scab (Fig. 57). 



Fig. 53. — Concentric cracks in the can- 

 kered area, indicating two periods of 

 activity during the seasoji due to 

 abrupt changes in the weather con- 

 ditions. 



