220 The Commercial Apple Industry 



trees are very much less susceptible to the disease than 

 slow-growing neglected individuals. As stated above, in- 

 fection always enters through wounds and for this reason 

 all pruning and other wounds should be disinfected and 

 covered. White lead and oil or liquid asphaltum are rec- 

 ommended as suitable coverings for wounds. A mixture 

 of two-thirds coal-tar and one-third creosote serves not only 

 as a covering, but also as a disinfectant. Cheap grades of 

 these materials are adequate. 



Heavy pruning encourages rather than checks the 

 disease and for this reason should be avoided in regions 

 where blister-canker is serious. Preventive measures may 

 be summed up as follows: (1) Avoid heavy pruning; 

 (2) disinfect and paint all wounds; (3) remove all infec- 

 tions from diseased trees; (4) select resistant varieties; 

 (5) keep trees vigorous. 



Collar-rot. 



The seriousness of collar-rot has become apparent in 

 many commercial apple orchards and the cause of this 

 injury has been the subject of considerable study. Cer- 

 tain varieties such as Grimes, the trunk wood of which 

 does not harden fully for the winter and which for that 

 reason is more subject to injuries from alternate freezing 

 and thawing, seem more susceptible to collar-rot than 

 others. Collar-rot on Grimes is also sometimes attributed 

 to fire-blight. The connection of certain fungi have been 

 associated with collar-rot injury, but at the present time 

 there is no authentic publication which describes the iso- 

 lation of any particular fungus responsible for the injury 

 for which specified treatment is recommended. Trees in 



