104 



DISEASES OF ECONOMIC PLANTS 



Fig. 43. — The blight showing exudate from 

 bark, much enlarged. This teems with the 

 causal bacteria. After Whetzel. 



With the flow of sap in the spring these infested linibs 

 become centers of reproduction. From these a milky fluid 

 teeming with the bacteria exudes. This attracts insects 

 which then carry the infection to other twigs and to flowers. 

 Two additional forms of this disease, according to the part 



affected, are recognized: 

 (1) flower blight, (2) 

 body blight and canker. 

 Neither of these differs 

 essentially from the twig 

 blight, though the body 

 blight or canker is es- 

 pecially worthy of men- 

 tion »)n account of its 

 peciMar destructiveness. 

 Thiaform occurs where 

 the disease is led by a spur, shoot, or ^cker into the cam- 

 bium under the bark of the body or main limbs of the tree, 

 or where primary infection occurs thrAigh a woimd so as 

 to lead to disease in such parts. Thl result of such in- 

 fection is a diseased area more or less cffcular, 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 

 external conditions. In general, anything which leads to 

 rapid growth, resulting in tender shoots, favors the devel- 

 opment of the disease. There is also much difference 

 in resistance offered by different varieties. The Anjou, 

 Kieffer, Angouleme, and Seckel possess more resistance 

 than the Bartlett, Clapp, or Flemish Beauty^ 

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



