DESCRIPTION OF CANKER STAGE. 31 



recommended cutting out all cankers in apple orchards. This prelim- 

 inary circular was followed by a bulletin on the same subject, giving 

 illustrations of the cankers and results of experiments, showing that 

 bitter-rot spores occurred in the canker and that apples could be 

 infected from cankers. 



Investigations as to the relations of the cankers and the bitter rot 

 were begun by the writers two days after Mr. Simpson's discovery. 

 These have been continued up to the present time and will be carried 

 on further. 



DESCRIPTION OF THE CANKER STAGE. 



The cankers found on apple trees in Illinois appear as blackened 

 depressions on apple limbs of various sizes, from last year's fruit 

 spurs to limbs 3 to 4 inches in diameter. Thus far the cankers have 

 not been found on the main trunk. On these limbs rounded or oblong 

 sooty-black sunken spots occur from one to several inches long, which 

 have more or less ragged edges. (See Pis. VII and VIII.) 



The entire bark is killed for a considerable distance back (Pis. VII 

 and VIII), and the dead bark appears cracked and fissured and in some 

 instances broken away. In man}^ cankers regular transverse cracks, 

 caused by the drying out of the bark, are very marked. As the bark 

 dries out it adheres very firmly to the underlying wood. As a result 

 of the decrease in volume of the affected bark and cambium, a 

 marked flattening and final depression take place on the affected limb. 

 Around the dead areas a healing callous layer usually forms (PI. VII, 

 fig. 1; PI. IX, figs. 1 and 4). This starts at the edges of the dead 

 areas and pushes toward the center, frequently lifting the dead bark 

 at the edges. The appearance of this callous layer makes the cankered 

 spots look more and more sunken. It will be noted that most of the 

 cankered spots show the presence, near the center, of a small branch 

 or of a branch stub. There may be some relationship between the 

 formation of the cankered spot and a diseased fruit borne on such a 

 small branch in a previous year. That is, however, a mere conjecture. 



On cross sections of cankers one frequently finds that at its very 

 center the wood has been dead for two years. (See PL VIII, fig. 1.) 

 The small hole in the wood, two rings in, shows where the small branch 

 broke away. This dying and breaking away of the small branch 

 would point to the fact just mentioned, that the canker may sometimes 

 start in the branch. 



The wood of the branch immediately below a cankered spot is dis- 

 colored for a considerable distance toward the center. (See PI. VIII, 

 fig. 1.) The discoloration is brown and resembles that found in many 

 hardwood trees in the region below a wound. The wood cells and 

 medullary rays in the discolored region are filled with a light brown 

 mass, readily soluble in alkalis, which leads one to class it as one of 



