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 
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 
onthe bark. The fungus hibernates in cankers which have been 
L 
Fia. 39.— Rough-bark canker 
of apple. 
