Diseases and Pests of the Apple and Their Control 219 



over on mummied fruit and in cankers in the bark. 

 These sources of infection should be removed when the 

 disease is bad. 



Blister-canker . (Nummularia discreta) . 



This canker, sometimes spoken of as the Illinois blister- 

 canker, is the most destructive apple disease in the Middle 

 West. It is particularly prevalent in the Illinois, Ozark 

 and Missouri River regions where it has caused heavy loss 

 in trees. The fungus gains entrance strictly through 

 wounds in the roots or branches. The initial cankers 

 produced, by the disease vary in size from 2 to 18 inches 

 in length and from y 2 to 6 inches in width. Their ap- 

 pearance very often resembles that of cankers produced 

 by winter-injury, sun-scald, blight or collar-rot, except 

 that blister-canker affects not only the bark, but also the 

 wood. The bark becomes shrunken and assumes a darker 

 color and the cankers increase in size as long as the branch - 

 remains alive. The disease is spread by means of spores 

 and infected wood. 



Unfortunately, the most important varieties of the Mid- 

 dle West are the most susceptible to this disease, namely 

 Ben Davis and Gano. The Delicious, though not widely 

 grown in these regions, is also susceptible. Oldenburg, 

 Wealthy, Winesap and Jonathan are more resistant. 

 Whole trees often appear to die in a single season although 

 as a matter of fact the infection has probably been present 

 for several seasons, but has been invisible from the outside. 

 The disease makes rapid progress in drought years. 



Treatment must be preventive rather than curative. 

 Once infected, a tree can be saved only by the removal of 

 the infected wood. Fortunately, vigorous well-cared-for 



