Pomaceous Fruits 



65 



With the flow of sap in the spring these infested hmbs 

 exude a milky fluid teeming with the bacteria and thus be- 

 come centers of reproduction. It has been demonstrated that 

 several species of insects can carrj^ infection, but it is not 

 probable that they are really important agents of transmis- 

 sion."^ Two additional forms of this disease, according to the 

 part affected, are recognized: (1) blossom-blight, (2) body- 

 bhght and canker. 



Neither of these differs essentially from twig-blight, 

 though body-blight or canker is especially worthy of men- 

 tion on account of its pecul- 

 iar destructiveness. This 

 form occurs where infection 

 is led by a spur, shoot, or 

 sucker, or where the germ 

 enters through a wound 

 into the body or main hmbs 

 of the tree. The result is a 

 canker more or less cir- 

 cular, which may even 

 girdle the tree or branch, 

 and which in any event 

 causes serious injury. 



The susceptibility of the tree is largely influenced by ex- 

 ternal conditions. In general, anything that leads to rapid 

 growth, resulting in tender shoots, favors the development of 

 the disease. There is also much difference in resistance 

 offered by different varieties. The Anjou, Kieffer, An- 

 gouleme, and Seckel possess more resistance than the Bartlett, 

 Clapp, or Flemish Beauty. 



Based on the facts as stated above, the following sugges- 

 tions for avoiding the blight were deduced, and tested by 

 Waite. 



1. Pruning in winter when the tree is dormant promotes 

 growth and favors blight. Withholding the pruning knife, 

 which may not otherwise be best for the tree, will reduce 

 this tendency. 



Fig. 31. — Fire-blight showing exu- 

 date from bark, much enlarged. 

 This teems with the causal bac- 

 teria. After Whetzel. 



