APPLE DISEASES 



145 



bark sinks in definite areas ; these spots are dark, cracked, and 

 have ragged margins (Fig. 39). These characters give rise to 

 the name rough-bark. Some of the lesions spread uninter- 

 ruptedly until the whole branch is involved. Small branches 

 are thus occasionally killed as a 

 result of girdling. Other affected 

 areas are halted and the wounds 

 are occluded. As a result of the 

 above abnormalities the foliage as- 

 sumes an unhealthy aspect; it be- 

 comes pale and cannot function 

 properly. The leaves are never 

 directly affected under natural con- 

 ditions. 



Cause. 



Only recently has it been shown 

 that the rough bark of the Yellow 

 Newtown is a disease and not a 

 normal characteristic. The trouble 

 is now known to be due to the 

 fungus Phomopsis Mali. It gains 

 entrance to the bark tissues through 

 wounds in the earlier part of the 

 growing-season. While the fungus 

 can be induced under artificial con- 

 ditions to develop on the fruits and 

 foliage of the apple, yet the lesions FlG - 39, Rough-bark canker 



5> of apple. 



on these organs are not regarded as 



economic phases of the disease. Shortly after the fungus 

 attacks the bark, pycnidia develop from the mycelium. From 

 these fruiting pustules spores ooze forth in cream-colored 

 tendrils. A moisture-period of some duration is essential to 

 this process. These spores are capable of initiating new lesions 

 on the bark. The fungus hibernates in cankers which have been 



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