STUDIES OF THE ETIOLOGY AND CONTROL OF 
BLISTER CANKER ON APPLE TREES 
By J. Ralph Cooper 
INTRODUCTION 
The blister canker caused by Nummularia discreta Tul. is 
by far the most destructive disease of apple trees found in the 
United States. Serious damage due to this disease was first 
reported in Illinois in 1902. Since that time the disease has 
been reported to cause much damage in all apple-producing 
sections east of the Rocky Mountains. In Nebraska the dis- 
ease is so prevalent that it is practically impossible to find an 
orchard free from it, and in many instances whole orchards 
have been destroyed thru its attacks. 
Because of the rapid dissemination and the destructive 
nature of blister canker, the writer in the fall of 1912 began 
a series of experiments in an attempt to find some means of 
controlling it. It was soon found that little progress could 
be made in this direction without a thoro knowledge of the 
etiology of the disease. Accordingly this phase of the work 
was taken up in 1914 and both phases continued to date. 
DISTRIBUTION 
Our present knowledge of Nummularia discreta indicates 
that it causes a serious disease only of various species of 
Malus. However, it has been reported to cause cankers on 
Amelanchier, Gleditsia, Sorbus, Cercis, Ulmus, and Mag- 
nolia. The writer has succeeded in inoculating artificially 
both Pyrus communis and Amygdalus persica. Altho no 
characteristic cankers have appeared on either of these, micro- 
scopic examination has showTi an abundance of invading 
hyphae thru the discolored area of the wood. 
Since 1902, when it was reported as a destructive disease 
in orchards in Illinois, it has been reported in every apple- 
growing section east of the Rocky Mountains. It is especially 
The writer is under great oblig-ations to Dr. E. Mead Wilcox and Prof. 
R. F. Howard for valuable suggestions and encouragement, to Dr. Florence 
A. McCormick for advice and assistance in histology and microtechniQue, 
to Grove M. Porter and Emmett B. Catterson for assistance in maRing in- 
oculations, and to Miss Edna Beaty for assistance in assembling- and check- 
ing data. 
