118 SEVENTEENTH REPORT. 



cracking is due no doubt to the rapid drying and slirinking which 

 takes place after the bark is killed. As the healthy portions of the 

 limb increase in girth, the rift between the diseased strip and the 

 sound bark becomes more pronounced. "S'ery soon a definite lesion 

 is formed and the healthy portions are walled otf by a wound 

 callus. No doubt the inability of the fungus to penetrate this 

 wound callus brings about the restriction of its advance around 

 the limb. 



The Avoolly ai>hi8. ^cliizoneura lanifjcra, attacks this wound callus 

 very severely, often lining the entire margin of the canker with a 

 white cottony layer. As a result of the stimulation of the i)un(tures 

 of this insect, the wound callus grows three or four times the nor- 

 mal thickness and swellings and knobs occur all along it. The net 

 result of the aphis attack is to aggravate greatly the cankered con- 

 dition. The a]>his is a secondary factor increasing the severity of 

 the disease. It may be noted that the woolly aphis is connnonly 

 found upon the tender growing parts of the apple, and wound 

 callus about any wound is subject to attack. The canker of apple 

 ( aused by Nectria galUgena is similarly aggravated by this same 

 insect. 



The fruiting bodies of the causal organism, are to be found on 

 all stages of the cankers. Where the dead bark still clings to the wood 

 the fruiting bodies, seen as small globose or flattened masses about 

 14 of «i niillimeter across, are present in small numbers embedded or 

 half submerged in the rotted bark. They are most connnon. how- 

 ever, in series or clumps upon the decorticated wood. 



This canker differs from the common i^phaernpsifi cajiker in many 

 ways. The rather rapid decortication of the wood, the tendency 

 to kill the bark in long strips along the limbs and the rectangular 

 checking of the dead bark are distinguishing characteristics. 

 (Jirdling by lhe fungus has never been found ujion older limbs 

 although it may take ])lace on young ones, (lirdliug accomiKinied 

 by a swelling (»f the diseased tissue is a connnon characteristic of 

 the Sphacropsis canker. 



MOUl'IIOLOOY OF THE CAUSAL OUOAMSM. 



The fruiting bodies of the fungus consist of single-celled |»ycni- 

 dia. These pyciiidia are tyjdcally globose, bec(uning flattened or 

 (lejiressed when old. Although they usually stand distinct from 

 each other, two may grow together, but in this case the wall ol i-ach 

 is distinct, hence the association can not be considered as an evi- 

 dence of stroma formation. Sometimes pycnidia are fonnd wliicli 



