212 Bulletin 236. 



13. Secondary Factors in the Destructiveness of the Disease 



I have already made casual references to certain things and con- 

 ditions that may strongly aggravate the effects of canker attacks. My 

 own observations along this line, while of a very general nature, indicate 

 that this phase of the subject is one of extreme importance; for upon 

 these secondary factors may depend not only the very general appearance 

 of the disease in a given locality but also the subsequent effects of the 

 same. 



Those agents which seem most frequently to aggravate canker attacks 

 are : winter injury of the trees, cultivation, fertilizing, and attacks of 

 secondary organisms fike rot fungi, etc. The first three of these result in 

 a predisposition of the host to attacks of the bacteria, while the fourth, 

 by taking advantage of the wounds made by the canker, may complete the 

 destruction already begun. A summary of my observations on each of 

 these factors follows : 



Winter injury. — Anything that reduces the general vitality of the 

 tree tends to render it more susceptible to attacks of the bacteria. I have 

 already referred to the apparent effects of low temperature in relation to 

 this disease in the Hudson River region. A long growing season during 

 1902, with excessive rains followed by a sudden and extreme fall of 

 temperature early in December, is referred to by growers in that section 

 as the beginning of the injury to their orchards. The winter that followed 

 w^as a severe one with sudden and severe changes of temperature during 

 the early days of the spring of 1903. Many trees failed to leaf out and 

 large cankers were now observed on limbs an 1 bodies of dead and dying 

 trees. The general conclusion at once prevailed that these dead spots 

 were the direct results of these weather conditions. Careful questioning, 

 however, brought out the fact that previous to 1902 certain growers had 

 observed these cankers on the bodies of their trees and some few trees 

 had died, apparently from those injuries of the bark. It is also pertinent 

 to point out at this place, that the rainy weather of 1902, which resulted 

 in an excessive and long continued growth of tender tissues, afforded 

 conditions most favorable to the infestation and development of the 

 canker bacteria. It seems very reasonable to suppose that many of the 

 cankers appeared during the summer and autumn of that year. That they 

 M^ere not generally observed by the grower until 1903 is probably ex- 

 plained by the fact that the effects of their attack did not become evident 

 in the foliage of the trees until that season. My own observations on 

 cankers with whose entire development from the beginning I have been 

 acquainted, show that rarely do they cause the death of the affected limb 

 or tree the first season. More than that, no evidence of their presence 

 is to be detected in the appearance of the foliage. I am, therefore, of the 



