DISEASES OF SPECIAL CHOPS 



81 



Buckingham, Mammoth Black Twig, White Winter Pear- 

 main, Winesap, and Ben Davis are resistant, the last al- 

 most entirely so. In general the 

 crab varieties are more suscep- 

 tible than others. 



Black rot, canker (Sphoerop- 

 sis Malorum Peck). — In many 

 respects this disease closely re- 

 sembles the bitter rot, particu- 

 larly in that it appears both as 

 rot of the fruit and as cankers 

 upon the limbs, each of which is 

 almost indistinguishable from 

 bitter rot upon the similar part. 

 In some instances it also appears 

 as a twig blight and as a leaf 

 spot. 



Upon the fruit the spot, though 

 sometimes of a darker hue, is 

 often quite indistinguishable from 

 that of the bitter rot. As seen 

 under the hand lens the coils of 

 exuding spores are black, instead 

 of pink, as is the case with bitter 

 rot. A compound microscope is necessary to bring out 

 other distinguishing characters. 



The limb cankers, first studied by Paddock,' consist of 

 swellings with the bark rough, and black, or in many cases 

 expose the black and decayed wood from which the bark 

 has disappeared. This cankerous infection sometimes 



1 Paddock, W., N.Y. (Geneva) Agr. Exp. Sta. Eul. 163, December, 1899. 

 G 



Fig. 29. — Apple black rot canker 

 (sphaeropsose) showing numer- 

 ous pycnidia. After Whetzel. 



