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 themargin 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 ae ee ee Oe 
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- 
