PEAR DISEASES 



327 



and a uniform browning of all the foliage simultaneously. 

 Cankers most frequently surround the base of a spur (Fig. 85), 

 watersprout, or small limb (Figs. 87 and 88). 



Cause. 



Fire-blight is caused by the bacterial pathogene Bacillus 

 amylovorus. This pathogene passes its entire life-history 

 within the tissues of the liv- 

 ing host, except, of course, 

 during dissemination from 

 one place to another. It 

 cannot survive long, even in 

 the dead parts of the plant 

 attacked. The organism 

 passes the winter in an in- 

 active condition in the tissues 

 along the margin of the blight- 

 cankers both on larger limbs 

 and on twigs; such lesions are 

 sometimes called hold-over 

 cankers (Figs. 87 and 88). 

 In the spring the bacteria be- 

 come active, multiply rapidly, 

 and spread into adjoining 

 healthy tissues. Great num- 

 bers of them ooze forth in 

 sticky masses from the len- 

 ticels (Fig. 89). This ooze 

 is the source of trouble for 

 the season; it is visited by 

 wasps, bees, flies, beetles, bugs, aphids, curculios and leaf- 

 hoppers, any or all of which insects may carry the bacteria 

 to the opening blossoms, to the tender growing-tips of 

 twigs, or to wounds in the bark. Pruning tools are also 

 agents in transmitting the bacteria. In these various in- 



FIG. 88. Hold-over canker. 



