42 



Diseases of Economic Plants 



The Lowell, Isham, Smith Cider, Yellow Transparent, 

 Jonathan, and Willow Twig are especially susceptible 

 varieties, while Celestia, Buckingham, Mammoth Black 

 Twig, White Winter Pearmain, Winesap, and Ben Davis 

 are resistant, the last almost en- 

 tii-ely so. In general the crab vari- 

 eties are more susceptible than 

 others. 



' ^ Black-rot, canker ^^^ ^^ {Physalo- 

 spora cydonice Arn., Sphceropsis) . — 

 In many respects this disease closely 

 resembles the bitter-rot, particu- 

 larly in that it appears both as rot 

 of the fruit and as cankers upon 

 the limbs. In some instances it 

 also appears as a twig blight and 

 as a leaf spot. 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 sweUings, 

 with the bark rough, and black. 

 In other cases the black, decayed 

 wood from which the bark has dis- 

 appeared is exposed. This cankerous infection sometimes ex- 

 tends for nearly a meter along the branch, and occasionally 

 girdles it. When occurring upon the trunk or main branches 

 it is called black-rot body-blight. 



The Tolman Sweet is said to be quite resistant to canker, 

 while Twenty Ounce is very susceptible. Four other va- 

 rieties range in susceptibility as follows: Baldwin, Wagener, 

 Greening, King. 



Black-rot twig-blight somewhat resembles fire-blight with 



Fig. 17. — Apple black-rot 

 canker showing numer- 

 ous pycnidia. After 

 Whetzel. 



