CERTAIN ELEMENTS IN NURSERY PRACTICE 193 



blackened appearance of the young leaf tufts and the 

 blossoms, from which the disease extends rapidly into the fruit- 

 spurs. 



The wilted and brown or dead appearance of the stem and 

 foliage is the characteristic symptom of the disease in the 

 twigs (Fig. 210). There is gen- 

 erally a viscid milk-white sub- 

 stance exuding in small drops 

 on the surface of the twig or the 

 petioles, which later becomes ox- 

 idized into a dark brown gum. 

 The leaves on the affected twigs 

 shrivel and turn brown or 

 black, and resemble foliage that 

 has been killed by frost. 



The blight bacteria often 

 work down the twigs into the 

 trunk of the tree and within a 

 short time may extend into the 

 roots. 



Cause. Fire-blight is a bac- 

 terial disease, caused by Bacillud 

 amylowrus. The bacteria live 

 over winter in diseased areas 

 in the bark, known as cankers. 



In the spring, a gummy ex- FIG. 210. Fire-blight. The dead 



Udation laden with the germs s jj rive ^d leaves above the branch 



& . show the presence of the disease. 



oozes from the cankers and this 



attracts many kinds of insects, as honey-bees, wasps and flies, 

 which carry the bacteria to the blossoms. Other insects, as 

 the tarnished plant-bug, leaf-hoppers and aphids, also spread 

 the blight. While visiting blighted tissues the insect becomes 

 smeared with the gummy exudate and carries the bacteria to 

 the tender twigs. In sucking the sap from the twigs the insect 



